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Energy flux measurements in a sheltered lemon orchard
Abstract
Orchards and many other land surface types have vegetation that does not completely cover the underlying ground surface. For these land surfaces there are at least two distinct and interacting surface components, the overstorey/canopy and the understorey/ground. Independent measurement of energy fluxes from each component is an important step toward understanding the factors controlling the resultant overhead flux. In this study, we evaluated flux measurement methods in a sheltered lemon orchard where the canopy of the lemon trees covered 39% of the orchard floor. Lemon tree transpiration was measured using the stem heat balance method with branch measurements scaled up to canopy level. These measurements compared well with Penman–Monteith transpiration. For the understorey, the sum of latent and sensible heat fluxes measured by eddy correlation was equal to available energy. Eddy correlation and Bowen ratio methods of flux measurement also compared well in the understorey. However, at the height of the shelters above the orchard we observed an inconsistency between eddy correlation and Bowen ratio methods which suggested that the latter method is unreliable at this location. Flux measurement and interpretation in an orchard can be complicated by the presence of windbreaks or shelters. In particular, the source area for fluxes above a sheltered orchard may include the shelter trees and adjacent orchard blocks. Our orchard block (42 m × 124 m) was surrounded by shelter trees 8 m high and differed from the surrounding blocks in that the understorey was bare rather than covered in grass. This difference enabled us to examine the likely source area contributing to our measurements. Good agreement was found between eddy fluxes measured at 8 m (above the orchard) and the sum of the component fluxes from within the orchard block. Our analysis indicated that the source area for fluxes measured at 8 m is largely within the orchard block studied.
... Many micrometerological experiments have been carried out in orchards, but few with citrus (Kalma & Stanhill, 1972;Kalma & .uchs, 1976;Daamen et al., 1999), and very few with the objective of determining the evapotranspiration (van Bavel et al., 1967;Kalma & Stanhill, 1969;Daamen et al., 1999). .or a sheltered lemon orchard (42m x 124m), Daamen et al. (1999) found that the Bowen ratio (BR) method gave inconsistent estimates when compared with the eddy correlation measurements above the orchard near the height of the shelters. ...
... Many micrometerological experiments have been carried out in orchards, but few with citrus (Kalma & Stanhill, 1972;Kalma & .uchs, 1976;Daamen et al., 1999), and very few with the objective of determining the evapotranspiration (van Bavel et al., 1967;Kalma & Stanhill, 1969;Daamen et al., 1999). .or a sheltered lemon orchard (42m x 124m), Daamen et al. (1999) found that the Bowen ratio (BR) method gave inconsistent estimates when compared with the eddy correlation measurements above the orchard near the height of the shelters. ...
... 1976;Daamen et al., 1999), and very few with the objective of determining the evapotranspiration (van Bavel et al., 1967;Kalma & Stanhill, 1969;Daamen et al., 1999). .or a sheltered lemon orchard (42m x 124m), Daamen et al. (1999) found that the Bowen ratio (BR) method gave inconsistent estimates when compared with the eddy correlation measurements above the orchard near the height of the shelters. Similar results were reported by Braun et al. (2000) for an apple orchard. ...
... One of the advantages of this technique is that all the components of the surface energy budget can be measured at the same time and thus errors can be identified, quantified and corrected by closing the energy balance. A variety of sap flow methods including heat pulse or heat balance sensors could be a very good alternative for determining E p in orchards, but scaling-up of measurements remains an open issue (Allen and Grime, 1995;Daamen et al., 1999). An alternative for determining orchard E p is the simultaneous measurement of ET and evaporation from the soil surface (E s ), which in turn may be measured using microlysimeters (Bonachela et al., 2001) or eddy covariance below the canopy (Baldocchi and Meyers, 1991;Villalobos et al., 2000). ...
... Castel et al. (1987) found orange K c ranging from 0.44 to 0.55 in May, and from 0.67 to 0.79 in August, in plots with higher GC (from 0.54 to 0.71). Daamen et al. (1999), using eddy covariance in a lemon orchard of the same ground cover as this experiment (0.39), measured an average ET of 1.9 mm day À1 in the hot season which is approximately 20% less than the ET measured in this experiment. However, their orchard was wind-sheltered so the resulting decrease in aerodynamic conductance could partially explain this difference. ...
... In this experiment, the evaporation from the soil in dry soil conditions represented 32 and 40% of the total ET in 2000 and 2001, respectively. From the data of Daamen et al. (1999), obtained in a lemon orchard with the same GC as in this study, it is possible to calculate that their E s was 64% of ET. Nevertheless, all their measurements were obtained with wet soil and were therefore taken under conditions of nearly maximum E s . ...
Evaporation of citrus orchards has been widely studied, but differences in methodologies and management conditions have led to conflicting results, mainly due to differences in ground cover and soil evaporation. In this work the contribution of transpiration and soil evaporation has been studied in a drip-irrigated, clean cultivated mandarin (Citrus reticulata Blanco) orchard on a sandy soil in Southern Spain. Evapotranspiration (ET) was measured using eddy covariance while soil evaporation was determined with microlysimeters, during August 2000 and May 2001. Average ET was 2.6mmday−1 in August and 2.1mmday−1 in May. The crop coefficient (Kc) was 0.44 and 0.43 in 2000 and 2001, respectively. The coefficient of transpiration (Kp) was 0.30 in 2000 and 0.25 in 2001. The daily bulk canopy conductance (gc) ranged from 1.2 to 2.2 (average 1.8) mms−1 in 2000 and from 1.2 to 2.7 (average 1.9) mms−1 in 2001. A model of daily canopy conductance as a function of intercepted radiation was derived and applied to calculate the transpiration of orchards with different values of ground cover (GC). The ratio of transpiration over reference ET of mandarin orchards is linearly related to ground cover (Kp=0.7GC). Calculated crop coefficients agree with values suggested by FAO for mature orchards (around 0.65) but are substantially lower than FAO values for young plantations.
... Em seu trabalho, Daamen et al. (1999) ...
... Angelocci (1996), baseado nos trabalhos de Riou et al. (1989), , Buttler (1976) e Thorpe (1978a), valeu-se de relações entre o saldo de radiação do pomar e a radiação solar global para a determinação de Rnf. Daamen et al. (1999), estudando os fluxos de massa e energia em pomar irrigado na Nova Zelândia, estimaram Rnf a partir de medidas do saldo de radiação acima e abaixo do nível das copas das árvores, posicionando os saldo-radiômetros nas linhas de plantio, e assumido que a energia disponível às plantas era dada pela diferença entre eles. ...
... e Pyrus serotina ("Asian pears") e compararam seus resultados com medidas de fluxo de seiva pelo método do balanço de calor encontrando um bom nível d e ajuste entre ambos. Um aspecto interessante deste trabalho foi tanto o uso de dados de medições porométricas no modelo, quanto a estimativa de rf a partir da inversão da equação de Penman-Monteith.Daamen et al. (1999) encontraram resultados razoáveis ao estimarem a transpiração de árvores isoladas de limão, na Nova Zelândia, com o método de P-M, tendo observado superestimativa de 29%, em média, em relação ao fluxo de seiva integrado para períodos de 24 horas. Segundo os autores, esse desvio deveu-se principalmente à forma adotada para determinação da ...
O cafeeiro arábica é extensamente cultivado em regiões tropicais, sendo cultura de grande importância econômica para o Brasil. Atualmente, ele é cultivado em áreas com deficiência hídrica, para as quais a irrigação suplementar é necessária para aumentar a produtividade e a qualidade da bebida. O aumento da eficiência da prática irrigacionista exige em primeiro lugar informação sobre o consumo hídrico do cafezal. Tendo em vista a falta de informações sobre esse consumo e, também, o fato de que os plantios adensados têm tido grande avanço no país, o presente estudo foi realizado com a finalidade de determinar a evapotranspiração de um cafezal Mundo Novo Apuatã e sua partição em transpiração dos cafeeiros e evapotranspiração da entrelinha. O cafezal, localizado em Piracicaba/SP, tinha plantio adensado (2500plantas/ha) e era irrigado por gotejamento. A evapotranspiração do cafezal foi determinada pelo método da razão de Bowen, enquanto que a transpiração foi estimada pelo modelo de Penman-Monteith adaptado, sendo este comparado com medidas de fluxo de seiva pelo método do balanço de calor no caule. Nessa confrontação, verificou-se razoável concordância entre a transpiração diária pelo modelo e o fluxo de seiva, havendo discordância em duas da quatro plantas avaliadas, provavelmente devido à forma de determinação da energia radiante absorvida pelas plantas e à relação entre esta e a área foliar dos cafeeiros, bem como ao. (Continuação)com um coeficiente de cultura global próximo da unidade. Tese (Doutorado).
... First , 1994;Zhang et al., 2008), lemon orchard (Daamen et al., 1999), prairie wetland (Burba and Verma, 2001), red bed (Peacock and Hess, 2004), tall-grass prairie (Bremer and Ham, 1999), temperate steppe (Qiu et al., 2011), desert (Unland et al., 1996) and sandhill ecoregions with sub-irrigated meadows (Billesbach and Arkebauer, 2012). It is worth pointing out that the more widespread application for land-atmosphere energy exchange research currently is the eddy covariance (EC) technique (Baldocchi, 2014). ...
... Já Green et al. (1984) comparando, na Nova Zelândia, a evapotranspiração de pastagem medida em lisímetro de pesagem e obtida através do método da razão de Bowen chegou às seguintes conclusões: os valores medidos e estimados apresentaram boa concordância no total diário evapotranspirado em dias sem chuvas. Ainda, quando os dias chuvosos foram considerados aumentou a discrepância entre os dados.Apesar do largo emprego do método do balanço de energia alguns autores têm relatado problemas com o método(Daamen et al., 1999;Ibanez et al., 1999). O método da razão de Bowen deve ter seu uso limitado ao período diurno(Heilman & Brittin, 1989; Cellier e Olioso, 1993;Kustas et al., 1996). ...
... The development of flux-monitoring procedures including eddy covariance method, the Bowen ratio-energy balance principle, and the large aperture scintillametre, among others, enabled evapotranspiration studies at the orchard-scale for kiwi plantations (McAneney et al., 1992;Judd et al., 1993), lemons (Daamen et al., 1999), olives (Testi et al., 2004;Ezzahar et al., 2007;Cammalleri et al., 2013), citrus (Rana et al., 2005), peaches (Paç o et al., 2006a(Paç o et al., , 2006bQassim et al., 2013), bananas (Tanny et al., 2006), vines (Echeverría et al., 2012) and pineapples (José et al., 2007). In unsheltered northern New Zealand kiwi fruit orchards, evaporation flux is almost equal to the evaporative force (McAneney et al., 1992), but in kiwi orchards protected by fast-growing shelter belt trees, WUE is only weakly dependent on the levels of solar radiation and water dissipation (Judd et al., 1993). ...
Water deficit is the main limiting factor for agricultural production in the dry regions of northern China. Previous studies on water–plant relationships in fruit trees have focused mainly on ecological, physiological and molecular responses to water stress at the leaf or tree-scale; few equivalent studies have been conducted at the ecosystem-scale. In this study, we monitored water vapour exchange and water use efficiency (WUE) in a no-till, 12-year-old peach orchard using an eddy covariance technique. Daily average values of actual evapotranspiration (ETa) and WUE were 2.3 ± 2.1 mm and 0.44 g CO2 kg−1 H2O, respectively, across the monitoring period. Daily changes in WUE at the canopy level were strongly influenced by atmospheric vapour pressure deficit (VPD) during stages rapid plant growth. The rank order of WUE across developmental stages was: fruit post-harvest stage > fruit de-greening and red-colouring stage > flowering period and early fruit enlargement stage. The trends of water dissipation and WUE both had single peaks. During the late period of fruit enlargement, the rate of actual evapotranspiration was very high, reaching a daily maximum value of 7.1 mm d−1. Average daily WUE ranged up to 2.1 g CO2 kg−1 H2O, peaking after fruit harvest. The annual cumulative actual evapotranspiration reached 790.6 mm, with a crop coefficient 1.08. In conclusion, WUE was strongly influenced by VPD in the daytime during peach development, and the key stage of water requirement occurred in the period following the onset of fruit ripening in the orchard.
... The BREB method does have a few well documented limitations, including sensitivity to the accuracy of instruments that measure the air temperature and humidity gradients and energy balance terms, the possibility of discontinuous data when the Bowen ratio approaches −1, the possibility of inconsistency of flux-gradient relationships, and the requirement, common to micrometeorological methods, of adequate fetch to ensure adherence to the assumptions of the method (Perez et al., 1999;Todd et al., 2000). All the same, the BREB method has been applied successfully in wetlands with a homogeneous canopy (Drexler et al., 2004) and many other ecosystems (e.g., Daamen et al., 1999;Dawson, 1996;Domingo et al., 1999;Frank, 2003;Peacock and Hess, 2004;Rohli et al., 2004;Si et al., 2005;Xing et al., 2008;Zhang et al., 2008), including semi-arid steppe (Qiu et al., 2011) and farmland (Todd et al., 2000), as well as a vineyard in an arid desert (Zhang et al., 2008), etc. The BREB method has also been applied successfully to plateau systems (Savage et al., 2009;Staudinger and Rott, 1981;Wang et al., 1996). ...
The Qinghai-Tibet Plateau is a sensitive area of global climate changes, and riparian ecosystems are thought as “hotspots” for climate change adaptation, but little work has been conducted regarding the alpine riparian ecosystem on the Qinghai-Tibet Plateau. We measured evapotranspiration (ET) and surface energy fluxes over the riparian shrub Myricaria squamosa Desv., which is widely distributed on the Qinghai-Tibet Plateau but has not been studied until now. The results indicated that annual ET was 390 mm and 503 mm for the period of 2010 to 2011 and 2011 to 2012, respectively, which was higher than the amount of precipitation during the same period. Cumulative ET was lower than the cumulative reference evapotranspiration during the entire experimental period, whereas ET in August was higher than reference evapotranspiration. ET in the growing season occupied over 80% of annual ET with a maximum daily ET of 7.2 mm d- 1, and the ET in the non-growing season was quite low because of the frozen soil. In general, temperature and net radiation were the key variables controlling daily ET rates for M. squamosa. Annual sensible heat flux (H) consumed 60% of net radiation (Rn) and latent heat flux (LE) 40% during the three years of the study. LE occupied the main part of Rn from July to September. H was the highest in May and June, then sunk in the mid-growing season, and rebounded the other peak at late September and early October. Daily ground heat flux was positive from April to mid-September, and it was an important heat source of land surface in the winter and spring. This study highlighted that as an alpine riparian ecosystem in a semiarid region, ET and surface energy partitioning of the M. squamosa community are strongly affected by the freeze-thaw cycle, groundwater fluctuation, precipitation pulses and soil water content. We speculate that climate warming has a significant impact on ET process and surface energy partitioning of the M. squamosa community by influencing the freeze-thaw cycle and soil water content.
... Green and Moreshet (1979) also reported higher Kc values when evaporative demand was low than when it was high. However, at the orchard level, with mature trees, ETc is not only regulated by the stomatal reaction to the VPD, but also by the aerodynamic canopy resistance (Kalma and Fuchs, 1976;Daamen et al., 1999). ...
The actual evapotranspiration (ETc) of mature 'Valencia' orange trees [Citrus sinensis (L.) Osb.], drip-irrigated and non-irrigated, was calculated using the water balance method, over three years. Annual ETc was 24% higher from irrigated trees that from non irrigated trees (767 and 620 mm year -1, respectively). Maximum monthly average ETc was 3.3 mm day -1 or 80 L tree-1 day-1 (trees were spaced at 6 x 4 m). Generally ETc rate was reduced in January, the month of maximum atmospheric demand, compared with December, even under fully irrigated trees. The average annual value of the crop coefficient (Kc) for irrigated trees was 0.69. Monthly Kc values also showed a clear seasonal trend, with minimum values in summer (0.60), intermediate values in autumn and spring (0.77 and 0.80, respectively) and maximum values in winter (0.87). These values provide a useful base for the design and operation of microirrigation systems, for mature citrus trees in Uruguay.
... A maioria dos trabalhos sobre balanço de energia, baseado na razão Bowen, não contabiliza o fluxo de calor armazenado no dossel da planta (Daamen et al., 1999;Azevedo et al., 2003); o argumento geralmente utilizado é que este componente do balanço de energia é muito pequeno em comparação com os demais fluxos do balanço de energia. Portanto, se espera que, se a planta tiver um dossel bastante desenvolvido, o calor armazenado por ela pode apresentar uma contribuição considerável no cálculo do balanço de energia; por outro lado, também em trabalhos com balanço de energia geralmente não se considera o calor armazenado acima dos fluxímetros; esses sensores devem ser instalados o mais próximo possível da superfície do solo, pois sua finalidade é medir o fluxo de calor que entra e sai do solo; entretanto, por falta de conhecimento os mesmos são comumente instalados a 2 cm de profundidade. ...
Neste estudo se objetivou a avaliação da contribuição do calor armazenado acima das placas de fluxo de calor no solo e no dossel vegetativo sobre os componentes do balanço de energia, no pomar de mangueiras. Os componentes do balanço de energia foram obtidos pelo método do balanço de energia com base na razão de Bowen. O teste t-Student foi usado para avaliar a existência de diferença significativa entre as médias dos componentes do balanço de energia, obtidas mediante as seguintes condições: (i) considerando-se o calor armazenado acima dos fluxímetros e (ii) desconsiderando-se o calor armazenado acima dos fluxímetros, cujos resultados indicaram que: 1) o efeito do calor armazenado no dossel vegetativo é irrelevante quando comparado com os outros componentes do balanço de energia em pomar de mangueiras; 2) o calor armazenado acima das placas de calor no solo não interfere significativamente no cálculo dos fluxos de calor sensível e latente nem na evapotranspiração da mangueira irrigada; 3) o fluxo de calor no solo, corrigido com o calor armazenado acima dos fluxímetros, difere estatisticamente a nível de 5% de probabilidade, daquele obtido sem a correção do fluxo.
... A maioria dos trabalhos sobre balanço de energia baseado na razão Bowen, não contabiliza o fluxo de calor armazenado no dossel da planta (Daamen et al., 1999;Castro Teixeira, 2001;Azevedo et al., 2003). Os sensores de medição de fluxo de calor no solo são comumente instalados a 2 cm de profundidade. ...
RESUMO: O experimento de campo foi conduzido na fazenda "Agropecuária Boa Esperança S.A" (latitude: 9 ° 20' S, longitude: 40 ° 27' O, altitude: 375 m), em Petrolina, PE, num pomar de mangueiras (Mangifera indica, L), var. "Tommy Atkins", irrigado por microaspersão. Objetivou avaliar a contribuição do calor armazenado acima das placas de fluxo de calor no solo e no dossel vegetativo sobre os componentes do balanço de energia. Os componentes do balanço de energia foram obtidos pelo método do balanço de energia baseado na razão de Bowen. Os resultados indicaram que: 1) o efeito do calor armazenado no dossel vegetativo é irrelevante quando comparado aos demais componentes do balanço de energia; 2) o calor armazenado acima das placas de calor no solo não interfere significativamente no cálculo dos fluxos de calor sensível e latente; 3) o fluxo de calor no solo, corrigido com o calor armazenado acima dos fluxímetros, difere estatisticamente daquele obtido sem a correção. ABSTRACT: The field experiment was carried out in "Boa Esperança S.A" farm at Petrolina, PE (latitude: 09 o 20'S; longitude: 40 o 27'W; altitude: 365.5 m) in a mango orchard (Mangifera indica, L), variedade "Tommy Atkins", irrigated by micro-sprinklers. The objective was to investigate the contribution of the heat storage above soil heat flux plate and within canopy upon the energy balance components. Bowen ratio-energy balance method was used for obtaining the energy balance components. The results suggested that: 1) within canopy heat storage is irrelevant as compared to that of the other energy balance terms; 2) heat storage by the above soil heat plates does not influence on sensible and latent heat; 3) soil heat flux, when corrected for the heat storage above soil heat plates differs statistically from that obtained without correction.
... The scaling up of sap flow from the single tree to the field scale requires the analysis of the variability of plant size, to correctly determine the ''mean'' tree. This can be carried out by two different methods: (i) the analysis of spatial variability of plant leaf area (Jara et al., 1998) or (ii) a study of the variability of plant stem diameter (Daamen et al., 1999;Bethenod et al., 2000). The latter method, based on the branch diameter analysis, was adopted in this work. ...
The evapotranspiration (E) of an irrigated citrus (Clementine) orchard in the Mediterranean region was measured throughout a growing season. Transpiration (T) was determined by the sap flow method. Since the measurement took place on several selected branches of the citrus trees, a statistical analysis for determining the “mean tree” has been carried out before the set up of the sap gauges. In this study the daily T measured by the sap flow method has been compared with E measured by the eddy covariance method, with very good results. In addition a model of the citrus E using a Penman–Monteith-type model is presented, where the canopy surface resistance is determined from standard microclimatic variables. The calibration coefficients of the proposed model depend only on the crop and they have general validity with respect to the site.The model's performance was evaluated by comparison with the sap flow data. The results were satisfactory and therefore this simple model allows determination the E of a Clementine orchard grown under a Mediterranean climate.In contrast to reports by the FAO no. 56 paper, and this has practical implications, the crop growth coefficient of this Clementine orchard was not constant throughout the growing season. The highest values occurred during the vegetative stage.
... An increase in H with decreasing T eco is achieved when there is a sufficiently large reduction in r a . In the case of the canopy, heattransfer resistance, r a , is inversely related to the canopy surface area: r a / 1=PAI, where PAI is the plant area index (Daamen et al., 1999), as well as to foliage size and shape and to wind speed (e.g., Jones, 1992;Sellers et al., 1996). ...
Land use and land cover changes greatly influence surface energy balance and consequently climate, and are likely to be associated with the persistent predictions of warming and drying throughout the Mediterranean and other regions. We specifically address the question of how the high radiation load and suppressed latent heat flux, intrinsic to dry regions, interact with land use changes and climate in these environments. We use for this purpose a detailed 6-year (2003-2008) study of the redistribution of the radiation load in an open-canopy pine forest. The results show that compared with the background shrubland, there was a 23.8 W m-2 increase in shortwave radiation load on the forest (to a mean annual net solar radiation of 211 W m-2) associated with a decrease in albedo of 0.1. Surface (skin) temperature in the forest was lower than in the shrubland (by similar to 5 degrees C on average) due to an efficient 'convector effect' and the production of a large sensible heat flux (up to 926 W m-2 in summer), which effectively shifted heat from the canopy to the overlying boundary layer. The cooler forest skin temperature resulted in suppression of upwelling longwave radiation (by 25 W m-2, annual average), further increasing the forest radiation load (mean annual net radiation of 116 and 67 W m-2 for forest and shrubland, respectively). This suppression also resulted in a local 'canopy greenhouse effect', where upwelling longwave radiation from the ground to the canopy was larger than from the canopy to the atmosphere (by up to 150 W m-2 in summer) and was associated with similar to 3 degrees C warming below the canopy. The ability of the dry productive forest to deal with the high radiation load indicates the potential for afforestation in dry areas.
... Theoretical aspects of the BREB method in relation to data exclusion have been examined [1][2][3]. The method has been used for specific purposes to estimate evaporation for different canopy surfaces [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] including grassland, mulch-covered bare soil [19] and open water [20,21]. The method applied requires accurate measurement of air temperature and water vapour pressure differences between two vertical (profile) positions above the surface of interest. ...
The possibility of reliable, reasonably accurate and relatively inexpensive estimates of sensible heat and latent energy fluxes was investigated using a commercial combination thin-film polymer capacitive relative humidity and adjacent temperature sensor instrument. Long-term and unattended water vapour pressure profile difference measurements using low-power combination instruments were compared with those from a cooled dewpoint mirror hygrometer, the latter often used with Bowen ratio energy balance (BREB) systems. An error analysis, based on instrument relative humidity and temperature errors, was applied for various capacitive humidity instrument models. The main disadvantage of a combination capacitive humidity instrument is that two measurements, relative humidity and temperature, are required for estimation of water vapour pressure as opposed to one for a dewpoint hygrometer. In a laboratory experiment using an automated procedure, water vapour pressure differences generated using a reference dewpoint generator were measured using a commercial model (Dew-10) dewpoint hygrometer and a combination capacitive humidity instrument. The laboratory measurement comparisons showed that, potentially, an inexpensive model combination capacitive humidity instrument (CS500 or HMP50), or for improved results a slightly more expensive model (HMP35C or HMP45C), could substitute for the more expensive dewpoint hygrometer. In a field study, in a mesic grassland, the water vapour pressure measurement noise for the combination capacitive humidity instruments was greater than that for the dewpoint hygrometer. The average water vapour pressure profile difference measured using a HMP45C was highly correlated with that from a dewpoint hygrometer with a slope less than unity. Water vapour pressure measurements using the capacitive humidity instruments were not as accurate, compared to those obtained using a dewpoint hygrometer, but the resolution magnitudes for the profile difference measurements were less than the minimum of 0.01 kPa required for BREB measurements when averaged over 20 min. Furthermore, the longer-term capacitive humidity measurements are more reliable and not dependent on a sensor bias adjustment as is the case for the dewpoint hygrometer. A field comparison of CS500 and HMP45C profile water vapour pressure differences yielded a slope of close to unity. However, the CS500 exhibited more variable water vapour pressure measurements mainly due to its increased variation in temperature measurements compared to the HMP45C. Comparisons between 20-min BREB sensible heat fluxes obtained using a HMP45C and a dewpoint hygrometer yielded a slope of almost unity. BREB sensible heat fluxes measured using a HMP45C were reasonably well correlated with those obtained using a surface-layer scintillometer and eddy covariance (slope of 0.9629 and 0.9198 respectively). This reasonable agreement showed that a combination capacitive humidity instrument, with similar relative humidity (RH) and temperature error magnitudes of at most 2% RH and 0.3 °C respectively, and similar measurement time response, would be an adequate and less expensive substitute for a dewpoint hygrometer. Furthermore, a combination capacitive humidity instrument requires no servicing compared to a dewpoint hygrometer which requires a bias adjustment and mirror cleaning each week. These findings make unattended BREB measurements of sensible heat flux and evaporation cheaper and more reliable with the system easier to assemble and service and with reduced instrument power.
... Green and Moreshet (1979) also reported higher Kc values when evaporative demand was low than when it was high. However, at the orchard level, with mature trees, ETc is not only regulated by the stomatal reaction to the VPD, but also by the aerodynamic canopy resistance (Kalma and Fuchs, 1976;Daamen et al., 1999). ...
The actual evapotranspiration (ETc) of mature ¿Valencia¿ orange trees [Citrus sinensis (L.) Osb.], drip-irrigated and non-irrigated, was calculated using the water balance method, over three years. Annual ETc was 24% higher from irrigated trees that from non irrigated trees (767 and 620 mm year-1, respectively). Maximum monthly average ETc was 3.3 mm day-1 or 80 L tree-1 day-1 (trees were spaced at 6 ¥ 4 m). Generally ETc rate was reduced in January, the month of maximum atmospheric demand, compared with December, even under fully irrigated trees. The average annual value of the crop coefficient (Kc) for irrigated trees was 0.69. Monthly Kc values also showed a clear seasonal trend, with minimum values in summer (0.60), intermediate values in autumn and spring (0.77 and 0.80, respectively) and maximum values in winter (0.87). These values provide a useful base for the design and operation of micro-irrigation systems, for mature citrus trees in Uruguay. Se estimó la evapotranspiración real de árboles de naranjo ¿Valencia¿ [Citrus sinensis (L.) Osb.], sin riego y regados por goteo, mediante la metodología del balance hídrico, durante tres años. La ETc fue 24% mayor en los árboles regados que en los de secano (767 y 620 mm anuales, respectivamente). La evapotranspiración media en el mes de máxima demanda fue de 3,3 mm día-1 ó 80 L árbol-1 día-1 (para un marco de plantación de 6 ¥ 4 m, 24 m2). Se repitió sistemáticamente que la tasa de ETc tuvo un descenso en enero, mes de máxima demanda atmosférica, comparado con diciembre, aún en los árboles bien regados. El coeficiente de cultivo (Kc) promedio anual para los árboles regados fue 0,69. Se encontró un claro y sistemático comportamiento estacional, con valores mínimos del Kc en verano (0,60) intermedios en otoño y primavera (0,77 y 0,80, respectivamente) y máximos en el invierno (0,87). Estos valores deberían ser la base para el diseño y la operación de sistemas de riego localizado para explotaciones de cítricos adultos en Uruguay.
... The Monin-Obukhov length, L, was obtained using the gradient Richardson number of each layer, j (Kaimal and Finnigan, 1994; Dyer and Hicks, 1970): The residual of the energy balance (g(Z j /h)) can be plotted against any parameter we choose (for example, the canopy or soil temperature) in order to ascertain the relationship between them. Daamen et al. (1999) used a similar procedure to study the influence of a shelter and Cellier and Brunet (1992) to locate the inertial sublayer. The residual in energy balance g(Z j /h) plotted against the temperature difference (T h ) is between the bare soil surface temperature midway between the rows (T is ) and the soil surface temperature under the pla ...
In some arid land, the irrigated fields are not contiguous and are surrounded by large patches of bare land. During the summer time and rainless season, the solar radiation flux is high and the surface temperature during daylight in the dry bare areas, is much higher than that of the air. The sensible heat generated over these areas may be advected to the irrigated fields. The crops are usually planted in rows and the irrigation systems used (trickle) do not wet the whole surface, the dry bare soil between the rows may develop high soil surface temperatures and lead to convective activity inside the canopy above the bare soil. Advection from the surrounding fields and convective activity inside the canopy affect the layer above the crop. We studied the surface layer above an irrigated tomato field planted in Israel´s Negev desert. The crop was planted in rows, trickle irrigated and the distance between the outer edges of two adjacent rows was 0.36 m at the time of measurement. The gradients in temperature and water vapor pressure were obtained at various heights above the canopy using a Bowen ratio machine. The residual in the energy balance equation was used as a criterion to determine the equilibrium layer. During the morning, unstable conditions prevail, and the equilibrium layer was between Z/h ~ 1.9 and 2.4. In some particular circumstances, in the late morning, the bare soil between the rows reached extremely high temperatures and during conditions with low wind speeds free convection was identified. During these hours the ‘‘residuals’’ of the energy budget to the heights Z/h = 1.5 and 2.4 were significantly different from zero and an extremely large variability was evident for the Z/h = 3.2 layer. Local advection took place during the afternoon resulting in an increase in the stability of the uppermost measured layer and propagated slowly downwards. The equilibrium layer was between Z/h ~ 1.5 to 2.4. The residuals were significantly different from zero for the uppermost layers Z/h = 2.7 and 3.2 during these periods. Our findings suggest that the depth and location of the internal equilibrium layer above trickle irrigated row crop fields surrounded by dry bare areas, vary in response to wind speed and the temperature of the soil in between the rows of the crop. For some time intervals, the computation of fluxes using the conventional flux-gradient approach measurements was not possible.
... The residual of the energy balance (g(Z j /h)) can be plotted against any parameter we choose (for example, the canopy or soil temperature) in order to ascertain the relationship between them. Daamen et al. (1999) used a similar procedure to study the influence of a shelter and Cellier and Brunet (1992) to locate the inertial sublayer. ...
Arid and semi-arid regions are heterogeneous landscapes in which irrigated fields are surrounded by arid areas. The advection of sensible heat flux from dry surfaces is a significant source of energy that has to be taken into consideration when evaluating the evaporation from crops growing in these areas. The basic requirement of most of the common methods for estimating evapotranspiration [Bowen ratio, aerodynamic and Penman-Monteith (PM) equation] is that the horizontal fluxes of sensible and latent heat are negligible when compared to the corresponding vertical fluxes. We carried out measurements above an irrigated tomato field in a desert area. Latent and sensible heat fluxes were measured using a four-level Bowen machine with aspirated psychrometers. Our results indicate that under advective conditions only measurements carried out in the lowest layer are satisfactory for the estimation of latent heat fluxes and that the use of the PM equation with an appropriately parameterized canopy resistance may be preferable.
... The soil projected area of the mango tree canopy (A c ) remained approximately constant throughout the experimental period, around 34 m 2 , corresponding to 85% of the soil surface area available for each plant. For a circular lemon tree canopy, Daamem et al. (1999) found a soil projected area corresponding to 39% of the soil surface area available for the plant. The soil electrical conductivity changed from 0.14 dS m À1 , in the soil layer of 0-20 cm to 4.0 dS m À1 in the soil layer of 180-200 cm, with a soil profile average value of 1.47 dS m À1 . ...
Data of a field experiment carried out at Petrolina-PE, a semi-arid region of northeast Brazil, were used to evaluate the evapotranspiration during the 1999 fruiting cycle of a mango orchard. Sensors for net radiation, two levels dry and wet bulbs air temperature and wind speed were installed above a mango tree in a micrometeorological tower. Soil heat flux sensors were installed at 2cm below soil surface and soil moisture content was measured by sets of tensiometers installed each at 20cm from the surface down to 220cm soil depth. The individual mango tree evapotranspiration was obtained by two methods: Bowen ratio-energy balance (BREB) and soil water balance (SWB). Daily mango orchard evapotranspiration increased slowly from 3.1mm per day at the beginning of the experimental period (middle July) to 4.9mm per day at the maximum growth period of the fruit. Then, it decreased to reach a 4.1mm per day value, approximately at the full maturation fruit. The accumulated mango orchard water consumption for the whole productive cycle was 551.6 and 555.1mm by the soil water and Bowen ratio-energy balance methods, respectively. For the experimental climate conditions, the soil water balance method was shown to be more efficient in the measurement of the mango orchard daily evapotranspiration when compared to the energy balance method, particularly when obtained for time period equal to or greater than 7 days, with a percentage error of estimation of daily crop evapotranspiration around 7%. Also, in the study region, the mango orchard water requirements are not constant throughout the productive cycle. However, it can be estimated by the crop coefficient (Kc) obtained as a function of the days after flowering (DAF) as: Kc=0.36+0.009(DAF)−4×10−5(DAF)2. On average, the percentage error associated to the estimation of the latent heat flux increased from 17% for low to 38% for high atmospheric energy demand with intermediate values, around 25%, for moderate available atmospheric energy.
Leaf area index (LAI) is the main determinant of light interception, and, thus of canopy assimilation and transpiration. For tree orchards, measurements of LAI using direct methods are extremely difficult, expensive, and often not feasible. In this study, the objective is to evaluate two indirect methods: plant canopy analyser (PCA, LAI-2000) and hemispherical photographs for measuring LAI of two orange orchards (Saada1 and Saada2) differing by their ground fraction cover (fc). The evaluation is based on reference data obtained by destructive measurements on limited samples, which allow to calibrate an exponential relationship between the diameter of branches and associated areas of leaves (R2=0.99).
The results we obtain show that the two indirect methods underestimate the reference values of LAI. For the PCA device, the best estimates of LAI are obtained using the five rings for Saada 1 (high fc) and using only four rings for Saada 2 (low fc). For both sites, the hemispherical photographs give accurate estimates of LAI: the relative errors are about 11 and 14% for Saada 1 and Saada 2, respectively. Alternatively, a simple method consisting of calculating LAI as the weighted average of the maximum (LAImax, below the tree) and minimum (LAImin, at the centre of four trees) values using fc as a weighting factor, was successfully tested (R2=0.90).
Data from field experiments conducted in the semiarid climatic conditions of northeast Brazil were used to investigate the energy flux relations and evapotranspiration (ET) of a mango (Mangifera indica L.) orchard. The Bowen ratio-energy balance method was applied during the 1998-1999 fruiting cycles to estimate the energy balance components of the mango orchard, while the FAO Penman-Monteith approach was used for determining the reference evapotranspiration (ETo). Results indicated that latent heat flux density (lambda E) could be obtained, with reasonable precision, as a function of measured net radiation flux density (R-n). The percentage of R-n used as lambda E was higher for the fruit growth and fruit maturation phenological stages, and lower for the flowering and fruit fall stages. For both field campaigns, lambda E was found to be the major component of energy balance, comprising >70% of the available energy. Soil heat flux was always the smaller component, comprising <8%. Daily mean value of ET was higher during the 1998 fruiting cycle than that observed in 1999. Inversely, the ET increased approximately 6% from the 1998 to 1999 fruiting cycle. These results may be used for planning and management of irrigation for mangos grown in similar environmental conditions.
Land surfaces are an assemblage of component surface types, for instance overstory vegetation species, understory vegetation species, and bare soil. Often two or more surface types absorb a significant fraction of the available energy to the land surface as a whole. In these cases the interaction of fluxes from the component surfaces may be important to the total land surface energy balance. We compare three models of land surface energy balance: a Penman-Monteith model; a model with two component surfaces that don't interact (patch model); and a model with interacting component surfaces (Shuttleworth-Wallace model). Data from six published studies are used to invastigate which models best represent a particular land surface taking account of water supply to the component surfaces and overstory canopy architecture. Flux interaction between component surfaces was only found to be important when there was a large difference between the surface resistances (i.e., water availability to the surfaces). Also, all three models were found to estimate the stone land surface energy fluxes (to within 50 W m-2) when both surface resistances were >300 s m-1. The ratio of (aerodynamic resistance between the canopy air space and the reference height) to (mean component surface boundary layer resistance) was useful for indicating the level of interaction between component surfaces.
Characterising aerodynamic properties of agricultural screens covering crops is important for the estimation of resistance to exchanges of mass and momentum in the crop. In this study a small shade net, used to investigate crop response to reduced radiation, was extended at a height of 3m (just above the tree tops) in a citrus orchard. The net was 2m wide and 9m long. A wind/temperature mast was installed between the shaded area and an adjacent control (unshaded) area enabling a comparison between the wind speed and temperature profiles for wind blowing over the canopy and over the net with data from the same mast. The aerodynamic properties of the wind profile, namely the friction velocity, the roughness length, the zero-plane displacement, and the resistance are calculated and presented with respect to the stability of the surface layer above the orchard. The analysis is limited to the transient state since the wind does not reach equilibrium over the relatively small net. It is shown that under neutral and stable conditions the shade net acts to inhibit turbulence, to reduce the roughness length, to displace the wind profile upwards, just above the net and to increase the aerodynamic resistance. However, as the boundary layer above the net becomes thermally unstable the friction velocity and the roughness length increase and the zero-plane displacement decreases relative to their values under stable conditions. The shade net has a strong influence on the wind within the foliage where it reduces the wind speed by about 40% as compared to the foliage when unshaded.
It is reported a simple approach to transform daily values of grass net (all-wave) radiation (Rn, MJm−2day−1), as measured over standard grass surface at meteorological stations, into whole tree canopy net radiation (A, MJtree−1day−1). The revolving Whirligig device [McNaughton, K.G., Green, S.R., Black, T.A., Tynam, B.R., Edwards, W.R.N., 1992. Direct measurement of net radiation and photosynthetically active radiation absorbed by a single tree. Agric. For. Meteorol. 62, 87–107] describing a sphere about the tree measured A in five trees of different species (walnut, dwarf apple, normal apple, olives and citrus), with leaf area LA varying from 8.65 to 40m2. For each tree, A and Rn were linearly related (A=bRn), as previously reported elsewhere, but it was found that the slope of such regression was also a linear function of LA or, b=0.303 (±0.032) LA. Consequently, the hypothesis that total daily tree canopy net radiation per unit leaf area is linearly related to grass net radiation could not be rejected after 86 days of measurements in five locations, and the empirical relationship is A=0.303 (±0.032) RnLA (R2=0.9306).
An orchard carbon fixation between 5-11 μmol m-2 s -1 was recorded through the eddy covariance technique during the summer for a table grape in southern Italy. The total daily ecosystem exchange was equal to 2.07 g C/m2/day in 2001 and 1.68 in 2002, which was warmer. The latent heat of evapotranspiration was 40% Rn in 2001 and about 30% in 2002. Eddy covariance allowed estimation of the water use efficiency (WUE) of the vineyard, which averaged 0.0012 in 2001 and 0.0015 in 2002 at mid-day. WUE, determined at the canopy scale, is very important in those environments where water is a limiting factor and a correct irrigation schedule is necessary.
Data of a field experiment carried out in Petrolina-PE, Brazil were used to evaluate the evapotranspiration of the fruits productive cycle of a mango orchard. Sensors of net radiation, two levels dry and wet bulbs temperature and windspeed were installed above a mango tree in a micrometeorological tower. Soil heat flux sensors were installed at 5 and 15cm bellow soil surface and soil humidity was measured by batteries of tensiometers installed at each 15cm from the surface down to 200cm soil depth. The individual mango tree evapotranspiration was obtained by two methods: Bowen ratio energy balance and soil water balance. Daily evapotranspoiration increased slowly from 2.4 mm/day at the beginning of the experimental period (June, 29) to 7.9 mm/day at the fruits growth period (Octuber, 11). Then, decreased to reach approximately 3.5 mm/day at fruits full maturity (November, 15). The accumulative trees water consumption for the whole productive cycle was 612mm and 643mm by the soil water and energy balance methods, respectively. For the experimental conditions, the soil water balance showed to be more efficient in the determination of the mango orchard daily evapotranspiration as compared to the energy balance method.
Evaporation measurements using two Bowen ratio energy balance (BREB) systems in a remote high altitude montane grassland catchment of the Drakensberg Mountains, Cathedral Peak, South Africa are reported on. Various methods of data verification and rejection of inaccurate measured air temperature and water vapour pressure gradients are examined. A theoretical analysis, based on the equivalent temperature, results in data rejection procedures using the measurement of the air temperature profile difference. Data rejection is necessary whenever the Bowen ratio approaches −1, resulting in extremely inaccurate and impossibly large positive or negative sensible heat and latent energy fluxes. Using the simplified energy balance, it is shown that when the Bowen ratio approaches the limit of −1, for which the available energy flux density approaches 0 W m−2, conditions are pseudoadiabatic and isobaric and that such conditions can be depicted by the wet-bulb temperature isolines of the psychrometric chart. Disregarding evaporation estimates for which the Bowen ratio values are between arbitrarily chosen values remedies the problem to some extent. With this method, daily total evaporation may be reasonable but 20-min values unreasonable during mainly early morning and late afternoon periods. A more sensitive and dynamic approach is used to prevent BREB data from being excluded unnecessarily and to prevent rogue values escaping detection. Once the rejection procedures were applied, the 20-min BREB latent energy flux estimates compared well with measurements from a weighing lysimeter adjacent the site. Three methods were used to estimate the exchange coefficient K which allowed flux estimation for when BREB data are invalid or lacking. One method involved calculating K from wind speed only and the second method was based on the MOST-dependent temperature-variance method for which the 20-min standard deviation of 1-Hz air temperature data were used. From independent measurements of sensible heat H and latent energy LE, a time-invariant exchange coefficient K was also determined from measurements of the air temperature profile difference. These methods were used when there were invalid water vapour pressure data due to condensation in the hoses or problems with the cooled dew point mirror or when the fine-wire thermocouples were damaged or when there were unreliable estimates of the air temperature gradient. The time-invariant value for K used in one of the methods, 0.239 m2 s−1, was confirmed for a mixed grassland catchment using independent eddy covariance and surface-layer scintillometer measurements of H and Bowen ratio measurements of the air temperature profile difference.
The behaviour of 4-year-old kiwifruit (Actinidia deliciosa (A. Chev.) C. F. Liang & A. R. Ferguson) vines growing in a Springbank soil within the Kerikeri Irrigation Scheme was monitored over the 1982–83 season. Water stress was induced in two vines by withholding irrigation and preventing rainfall recharge. Fruit volume expansion appeared strongly linked to the hydraulic status of the vines and could be described by a simple model whereby fruit either expand at the maximum rate shown on well-watered vines, or not at all if water is limiting. Volume losses resulting from fruit softening after extreme water stress were quickly recovered upon the reapplication of irrigation and could be ignored for modelling the influence of water stress on harvest yields. In the absence of rainfall or irrigation, the readily available moisture in this soil is capable of maintaining unrestricted fruit volume expansion for 10 days in mid summer. This result is extrapolated to other soils within the Irrigation Scheme on the basis of their known physical properties — principally the depth to the first layer likely to impede root development. Practical implications for kiwifruit growers are discussed.
A rapid response drag anemometer for measuring streamwise and lateral components of horizontal windspeed is described. Theory of operation, design and calibration are discussed with emphasis on the electronic preconditioning of signals and problems associated with using a mechanically resonant system as a sensor. Field comparisons showed half-hourly means and standard deviations of the streamwise component to be within 8% and 5% of respective values obtained from a 3-dimensional sonic anemometer. The lateral component from the drag anemometer was significantly more noisy than that from the 3-D sonic due to induced oscillations arising from vortex shedding. After mechanical and electronic filtering, half-hourly standard deviation comparisons agreed to within 6% for this component. Friction velocities obtained from the drag anemometer in combination with a 1-D sonic, agreed with measurements from the 3-D sonic anemometer to within 4% over a measured range of 0.05 to 1.2 m s-1
Energy fluxes over an area of "homogeneous" suburban residential land-use in Vancouver, B.C., Canada are shown to vary by up to 25-40% within horizontal scales on the order of 102-103 m. Previously, variability of this magnitude has been expected to occur only at larger scales, between land-use zones or as urban-rural differences. In view of these findings, it is recognized that microadvective interaction between surface types at small scales may be important and can affect the energy balance even at larger scales. The present study discusses the small-scale spatial variability of energy fluxes and shows that it varies greatly for each term in the surface energy balance. Net radiation shows a relatively conservative behaviour (via albedo-surface temperature feedback) with little spatial variability. The turbulent fluxes (measured by eddy correlation at 28 m height), on the other hand, show a link between their temporal and spatial variability as the result of a temporally shifting source area which contains varying combinations of surface cover (using the dynamical source area concept of Schmid and Oke, 1990). As a result, part of the measured temporal variation is attributable to spatial differences in surface cover. Anthropogenic heat flux and storage heat flux (both modelled using a high resolution surface data-base) exhibit temporally varying spatial distributions. Their spatial pattern, however, is governed by nested scales of urban morphology (blocks, streets, properties, etc.). These differences in the source of variability between each component flux suggest a difficulty in the interpretation of the energy balance over urban areas, unless each term is spatially-averaged over the principal morphological units occurring in the area.
Accurate measurements of surface fluxes of carbon dioxide (CO2) and water (H2O) are important for several reasons and can be made using several types of instrumentation. For three C4 grasses—bermudagrass (Cynodon dactylon (L.) Pers.), a mixed species native tallgrass prairie, and sorghum (Sorghum bicolor (L.) Moench.)—we measured evapotranspiration (ET) using a canopy chamber (CC) and Bowen ratio/energy balance (BREB) instrumentation and we measured leaf CO2 uptake using a leaf chamber (LC), and, after accounting for soil CO2 fluxes, we calculated leaf uptake using a CC and BREB instrumentation. In addition, soil CO2 fluxes from bare soil were measured using a CC and soil chamber (SC). Measurements were made on 4 and 5 May 1994 at the Blackland Research Center, Temple, TX. Flux of CO2 into the leaf was considered positive and was expressed per unit ground area. Half-hour CC ET measurements were consistently and substantially greater than BREB measurements for all grasses, perhaps because of increased soil evaporation due to greater turbulence inside the CC. Leaf CO2 uptake measured using the three methods showed similar diurnal trends for all grasses (responding, primarily, to changes in photosynthetic photon flux density), but consistently tended to be greatest for BREB measurements. The regression equation for LC CO2 uptake as a function of BREB uptake had a slope not statistically different from 1.0, with large scatter likely because of limited leaf area sampled. CC CO2 uptake was consistently the least, partly because we may have underestimated soil CO2 flux in the CC. Half-hour soil CO2 fluxes from the CC were significantly greater (P < 0.05) than those from the SC for about two-thirds of the day on bare soil, perhaps because of large chamber ventilation rates. Differences of daytime soil CO2 fluxes averaged 0.07 mg m−2 s−1 (1.0 mg m−2 s−1 ≈ 22.7 μ mol m−2 s−1). These results show the consistency, repeatability and, we believe, accuracy of leaf CO2 uptake and soil CO2 flux measurements made using all methods.
Boundary layer meteorology is the study of the physical processes that take place in the layer of air that is most influenced by the earth's underlying surface. This text/reference gives an uncomplicated view of the structure of the boundary layer, the instruments available for measuring its mean and turbulent properties, how best to make the measurements, and ways to process and analyze the data. The main applications of the book are in atmospheric modelling, wind engineering, air pollution, and agricultural meteorology. The authors have pioneered research on atmospheric turbulence and flow, and are noted for their contributions to the study of the boundary layer. This important work will interest atmospheric scientists, meteorologists, and students and faculty in these fields.
The magnitude of the of the Webb et al. (1980) density corrections on water vapor fluxes due to the flux of sensible heat is examined in terms of the Bowen ratio. The correction due to latent heat is proportional only to the mole fraction of water vapor and is 5 times smaller than that for the sensible heat. The temperature regime within the path of the krypton hygrometer was measured to determine its effect in the Webb et al. (1980) corrections and the krypton O2 absorption corrections. The results of this experiment are inconclusive, but confirm the need for additional measurements. Two methods for determining water vapor from krypton hygrometer measurements are reexamined. An approximate method endorsed by the manufacturer and having errors of less than 3 percent for fluctuation smaller than 2 gm-3 is shown to underestimate the vapor flux by 14% under specific conditions of light winds and irrigated surfaces in arid regions (K. Kunkel, 1992, personal communication).
The theory of heat-flux meters describing the perturbation of a uniform, steady heat flux through a porous medium (soil) by a meter of different thermal conductivity was tested and confirmed under laboratory conditions. From the theory, an equation was derived for determining the heat flux density through the medium which is more accurate than direct calibration equations. The medium heat flux density is found by first calibrating a heat meter independently in two other media differing in thermal conductivity. Then, once the meter is inserted into the medium of interest, its signal and the temperature gradient in the direction of heat flow at 3–6 cm from the meter are measured. The method should serve to provide for more accurate assessment of energy budget of the earth's surface.
Eddy correlation measurements of sensible and latent heat flux are used with measurements of net radiation, soil heat flux, and other micrometeorological variables to develop the Penman-Monteith, Shuttleworth-Wallace, and modified Priestley-Taylor evapotranspiration models for use in a sparsely vegetated, semiarid rangeland. The Penman-Monteith model, a one-component model designed for use with dense crops, is not sufficiently accurate (r2 = 0.56 for hourly data and r2 = 0.60 for daily data). The Shuttleworth-Wallace model, a two-component logical extension of the Penman-Monteith model for use with sparse crops, performs significantly better (r2 = 0.78 for hourly data and r2 = 0.85 for daily data). The modified Priestley-Taylor model, a one-component simplified form of the Penman potential evapotranspiration model, surprisingly performs as well as the Shuttle worth-Wallace model. The rigorous Shuttleworth-Wallace model predicts that about one quarter of the vapor flux to the atmosphere is from bare-soil evaporation. Further, during daylight hours, the small leaves are sinks for sensible heat produced at the hot soil surface.
The amount of radiation incident upon a radiometer depends on the shape of the radiometer surface, its orientation with respect to the sources of radiation, the distance between sources and radiometer, and other factors. The geometric factor describing the ratio of radiation received by the radiometer emanating from a particular source, to the total received from all sources (assumed to be radiating at the same rate) is the view factor.An intuitive approach is made to the development of the view factor, based on a presentation by Nusselt (1928). The view factor for a radiometer viewing the ground surface is shown graphically as an aid to determining appropriate heights for exposing instruments. Finally, a comparison of the short-wave radiation balance on a spherical and flatplate radiometer is made.
A micro-Bowen ratio system, with β determined from temperature and humidity measurements very close to the soil surface (1 and 6 cm), was tested above bare soil by comparison with evaporation losses from a highly sensitive 6.1-m diameter lysimeter. The results indicated good accuracy in estimation of evaporation (E) during 8 days with moist soil-surface conditions, as well as 1 day of second-stage drying conditions when E averaged only 30–40% of a calculated reference crop evapotranspiration. Good agreement between estimated and measured E was also found, using a Bowen ratio system with a chilled mirror hygrometer sensing air sampled at 5 and 85 cm above the soil surface. On one of the study days a second Bowen ratio system (psychrometric with sensors at 5 and 85 cm) was employed as well as eddy-correlation instrumentation for measuring sensible heat flux (H), with H used in the energy balance equation to solve for LE. Close agreement with lysimeter-determined E and calculated E from these two systems was noted although in later afternoon hours some divergence occurred, probably produced as upwind soil surfaces developed inhomogeneous surface-moisture conditions. The results suggest that the microsystem approach should be applicable to under-canopy and small-plot studies where traditional methods are impractical.
When the sensible heat flux is uniform and directed upwards across a field then the air near the ground is warmer in the quiet zone and cooler in the wake zone when compared with conditions in the open. Other scalars show similar behaviour. In conditions of dry air advection, when evaporation in the open exceeds the equilibrium rate, evaporations is reduced in the quiet zone and is expected to be enhanced in the wake zone.Recent work on the aerodynamics of shelter has shown that there exists a quiet zone of reduced turbulence and smaller eddy size immediately behind windbreaks of all porosites. Beyond that, further downwind, lies an extended wake region of increased turbulence with eddy sizes returning to upwind scale. There is evidence to show that turbulent transport of heat, vapour and carbon dioxide is reduced in the quiet zone and enhanced in the wake.
A previous study of evaporation from an area of patterned woodland (tiger-bush) in Niger by Culf et al. (1993) has demonstrated the need to determine the contribution from the bare soil strips which occupy 67% of the surface area. They measured total evaporation from the entire land surface using eddy correlation, but not the individual contributions from the soil and vegetation components. This distinction is necessary to create accurate models of evaporation from the tiger-bush as different processes operate in the two components. The previous study in the same area relied upon untested modelling of the bare soil evaporation as no direct measurements were available. In the present study, carried out during HAPEX-Sahel, hourly fluxes of evaporation from a large patch of bare soil within the tiger-bush were measured using a Bowen ratio system. The data obtained show in detail how soil evaporation varies after rainfall as the surface dried out. Comparison is made on an hourly and daily basis of actual and potential evaporation. A two-phase model based on the Ritchie (1972) approach is calibrated using these data and the model is used to calculate the soil evaporation component of the water balance over a number of seasons with different rainfall. This analysis shows that over the entire tiger-bush area, annual soil evaporation is normally ∼28% of annual rainfall, but this percentage increases markedly in dry years. The implications are that in dry years runoff from these bare soil areas will decrease by a greater percentage than rainfall because a greater proportion of rainfall is lost as soil evaporation.
When crops are grown in a row configuration, heat and mass transfer within the soil-canopy system influence the energy and water balance of the crop. Field experiments were conducted near Lubbock, TX, to examine the energy balance of the soil and canopy separately, in cotton (Gossypium hirsutum L.) under a variety of aerial to obtain the field energy balance, including total latent heat flux (LE). Latent heat flux from the crop canopy (LE c ) was determined from sap flow measurements of transpiration. Latent heat flux from the soil (LE s ) was computed as the difference between LE and LE c (...)
The response requirements of the sensors, in particular the vertical
wind sensor, used for eddy correlation measurements are discussed. Data
are presented to allow estimation of the errors due to inadequate cosine
and frequency response of the vertical wind sensor. A Gill propeller
anemometer is examined as an example to show how large the errors might
be.
This paper reports the results of the analysis of measured turbulence regimes near the forest floor in an old-growth Douglas-fir stand on a south-facing slope in northern Vancouver Island, Canada. Primary instrumentation included one eddy correlation unit, which consisted of a three-dimensional sonic anemometer, a krypton hygrometer and a fine wire thermocouple, and four home-made hot wire anemometers.
The general features of the turbulence regimes near the forest floor within this stand were similar to those observed previously in other stands with an open trunk space. The high value of the ratio of the wind speed inside the stand to that outside (0.42) suggested the existence of a secondary maximum in the stand wind profile. The wind speed near the forest floor was approximately a logarithmic function of height with an effective roughness length of 0.012 m. The average turbulence intensity was 0.86. Power spectra for the streamwise and lateral velocity components exhibited a bimodal distribution in contrast with a unimodal distribution for the spectrum of the vertical component. Near the forest floor, latent heat and sensible heat generally flowed down humidity and temperature gradients, respectively. Some unique features were also observed, namely the suppression of the vertical velocity variance by the moderate to strong temperature inversion in the daytime and the occurrence of a very small eddy diffusivity for sensible heat flux. FOR. SCI. 39(2):211-230.
Simplified expressions describing the frequency response of eddy correlation systems due to sensor response, path-length averaging, sensor separation and signal processing are presented. A routine procedure for estimating and correcting for the frequency response loss in flux and variance measurements is discussed and illustrated by application to the Institute of Hydrology's Hydra eddy correlation system.The results show that flux loss from such a system is typically 5 to 10% for sensible and latent heat flux, but can be much larger for momentum flux and variance measurements in certain conditions.A microcomputer program is included which, with little modification, can be used for estimating flux loss from other eddy correlation systems with different or additional sensors.
This paper describes wind-tunnel experiments on the flow around single and multiple porous windbreaks (height H), sheltering a model plant canopy (height H/3). The mean wind is normal to the windbreaks, which span the width of the wind tunnel. The incident turbulent flow simulates the adiabatic atmospheric surface layer. Five configurations are examined: single breaks of three solidities (low, medium, high; solidity = 1 - porosity), and medium-solidity multiple breaks of streamwise spacing 12H and 6H. The experimental emphases are on the interactions of the windbreak flow with the underlying plant canopy; the effects of solidity; the differences in shelter between single and multiple windbreaks; and the scaling properties of the flow. Principal results are: (1) the "quiet zones" behind each windbreak are smaller in multiple than single arrays, because of the higher turbulence level in the very rough-wall internal boundary layer which develops over the multiple arrays. Nevertheless, the overall shelter effectiveness is higher for multiple arrays than single windbreaks because of the "nonlocal shelter" induced by the array as a whole. (2) The flow approaching the windbreak decelerates above the canopy but accelerates within the canopy, particularly when the windbreak solidity is high. (3) A strong mixing layer forms just downwind of the top of each windbreak, showing some of the turbulence and scaling properties of the classical mixing layer formed between uniform, coflowing streams. (4) No dramatic increase in turbulence levels in the canopy is evident at the point where the deepening mixing layer contacts the canopy (around x/H = 3) but the characteristic inflection in the canopy wind profile is eliminated at this point.
An open path infrared absorption based instrument for fast response measurements of H2O and CO2 fluctuations is described. This instrument performed reliably in several field experiments in both terrestrial and marine environments, on both fixed (tower) and mobile (boat, plane) flux platforms. Noise levels for H2O and CO2 concentrations were less than 10 mg/m3 and 300 g/m3, respectively for frequencies between 0.005 and 10 Hz. Drifts in instrument output, associated with changes in instrument temperature, are compensated for electronically.
The spatial resolution of meteorological observations of scalars (such as concentrations or temperature) and scalar fluxes (e.g., water-vapour flux, sensible heat flux) above inhomogeneous surfaces is in general not known. It is determined by the surface area of influence orsource area of the sensor, which for sensors of quantities that are subject to turbulent diffusion, depends on the flow and turbulence conditions.
Functions describing the relationship between the spatial distribution of surface sources (or sinks) and a measured signal at height in the surface layer have been termed thefootprint function or thesource weight function. In this paper, the source area of levelP is defined as the integral of the source weight function over the smallest possible domain comprising the fractionP of the total surface influence reflected in the measured signal. Source area models for scalar concentration and for passive scalar fluxes are presented. The results of the models are presented as characteristic dimensions of theP=50% source areas (i.e., the area responsible for 50% of the surface influence): the maximum source location (i.e., the upwind distance of the surface element with the maximum-weight influence), the near and the far end of the source area, and its maximal lateral extension. These numerical model results are related directly to non-dimensional surface-layer scaling variables by a non-linear least squares method in a parameterized model which provides a user-friendly estimate of the surface area responsible for measured concentrations or fluxes. The source area models presented here allow conclusions to be made about the spatial representativeness and the localness (these terms are defined in the text) of flux and concentration measurements.
Eddy correlation measurements of sensible heat and evaporation fluxes from a kiwifruit orchard confirm earlier, less comprehensive measurements that the spatially-averaged evaporation rate from well-watered and well-sheltered orchard blocks in the northern part of New Zealand occurs at the equilibrium rate (Eq. In this humid coastal climate the rapid onset of dewfall after sunset prevents night-time water use, which contrasts with other work showing that nocturnal transpiration may represent 25% of the daily water economy of a kiwifruit vine.Sensible heat fluxes measured at both shelter height (h) and 2.25h agreed within 5%, implying the existence of a constant flux region above shelter height. Under the low windspeed conditions common in mid-summer, the aerodynamic resistance to scalar transport between the canopy and the height of the windbreaks is shown to be of comparable magnitude (about 65 s m−1) to both the canopy ‘stomatal’ and boundary-layer resistances. One consequence of this is that transpiration will be less sensitive to stomatal regulation. Independent of imposed wind and radiative conditions, the effect of the close-spaced windbreaks and crop geometry is to create an orchard environment that is an efficient absorber of radiation but which is relatively poorly coupled with overhead conditions through turbulent exchange.
This study tests the evaporation energy combination model developed by Shuttleworth and Wallace (1985) with data collected in a subarctic wetland. The modelled evaporation was compared with evaporation calculated from the Bowen ratio energy balance technique over a range of leaf area indices (LAI) from non-vegetated to fully vegetated conditions. The Shuttleworth-Wallace (SW) model was in excellent agreement with the measured evaporation for hourly and day-time totals for all values of LAI. This gives a particular advantage to the SW model compared to the simple Penman-Monteith combination equation. A comparison of measured and modelled total evaporation for all days yielded a root mean square error and mean bias error of 0.98 and −0.13 MJ m−2 day−1, respectively. The model also shows good agreement with the measured evaporation on an hourly basis. Although the results of this study are encouraging, we are cautious because this test is not truly independent. The need for additional investigation and testing of certain model parameters is recognized. In this study, we assume that eddy diffusivity within the canopy decreased exponentially and was controlled by a decay constant which varied with LAI. However, there is little information available to validate this treatment. Net radiation at the soil surface was computed from net radiation over the canopy and an exponential function of LAI, which was held constant over the course of the growing season.
Accurate measurements of carbon dioxide (CO2) flux from soil are important because this flux is an important component of the surface carbon budget, and a good indicator of the level of soil microbial and root activity. Half-hour CO2 fluxes from bare soil were measured using soil chamber and Bowen ratio/energy balance (BREB) methods for 4 days in December 1992, at the Blackland Research Center, Temple, TX. Soil chamber CO2 measurements were made sequentially at nine positions in the field. Three CO2 BREB systems were used. The CO2 flux was ≈ 0 in the early morning and after sunset and was maximum (slightly less than 0.1 mg m−2 s−1 (2.3 μmol m−2 s−1)) near midday. The coefficient of variation (CV) of chamber CO2 fluxes across the nine positions averaged 40% throughout the day, indicating the need for a large number of chamber measurements to obtain a representative CO2 flux measurement. The CV of the three daily BREB CO2 fluxes was less than 2%, indicating the BREB CO2 fluxes from the three systems were equal. There was good agreement between fluxes from the two methods. The average chamber and BREB CO2 fluxes for the entire period of measurements were 0.039 and 0.042 mg m−2 s−1, respectively, while the root mean square difference between half-hour fluxes from the two methods was 0.017 mg m−2 s−1. The methods are complementary and can both be used for soil CO2 flux measurements. The chamber method is low cost and easy to use, and offers the possibility of replicated measurements over space. The BREB method integrates over a large spatial area and is thus less affected by the high spatial variability of soil CO2 flux.
The stomatal conductances of the leaves and panicles of a sparse dryland millet crop grown at ICRISAT Sahelian Center, Sadoré, in southwest Niger were measured using a diffusion porometer and an infrared gas analyser, respectively. Leaf conductances were found to be high, up to 12 mm s−1 or 480 mmol m−2 s−1, and varied according to the leaf surface, age and position in the canopy. These data were combined with measurements of leaf area index to calculate canopy conductance. Because of the low leaf area (maximum 1.3), canopy conductances were low and varied both diurnally and seasonally. Transpiration was calculated using the above canopy conductance values using the Penman-Monteith and Shuttleworth-Wallace models. In comparison to the Shuttleworth-Wallace model, it was found that the Penman-Monteith equation underestimated transpiration when the soil was dry and overestimated it when the soil was wet. These differences in transpiration arise because of the modification of the in-canopy vapour pressure deficit caused by heat and water vapour fluxes from the soil, a mechanism which is only present in the Shuttleworth-Wallace model.
When the atmospheric turbulent flux of a minor constituent such as CO2 (or of water vapour as a special case) is measured by either the eddy covariance or the mean gradient technique, account may need to be taken of variations of the constituent's density due to the presence of a flux of heat and/or water vapour. In this paper the basic relationships are discussed in the context of vertical transfer in the lower atmosphere, and the required corrections to the measured flux are derived.
If the measurement involves sensing of the fluctuations or mean gradient of the constituent's mixing ratio relative to the dry air component, then no correction is required; while with sensing of the constituent's specific mass content relative to the total moist air, a correction arising from the water vapour flux only is required. Correspondingly, if in mean gradient measurements the constituent's density is measured in air from different heights which has been pre‐dried and brought to a common temperature, then again no correction is required; while if the original (moist) air itself is brought to a common temperature, then only a correction arising from the water vapour flux is required.
If the constituent's density fluctuations or mean gradients are measured directly in the air in situ , then corrections arising from both heat and water vapour fluxes are required.
These corrections will often be very important. That due to the heat flux is about five times as great as that due to an equal latent heat (water vapour) flux. In CO2 flux measurements the magnitude of the correction will commonly exceed that of the flux itself. The correction to measurements of water vapour flux will often be only a few per cent but will sometimes exceed 10 per cent.
This paper completes the construction of a two-layer model for the energy balance of sparse canopies begun by Van den Hurk and McNaughton (1994). The model is based on the Lagrangian theory of Raupach (1989). In the earlier work, we showed that the average effect of the near-field component of the scalar concentration profile can be represented by a ‘near-field’ resistor. Here we calculate values for the upper and lower ‘far-field’ resistors using Raupach's expression for the far-field diffusivity and his empirical descriptions of the profiles of vertical velocity variance and Lagrangian integral time scale (Raupach, 1988). The boundary-layer resistance for the foliage of the overstorey canopy is also reassessed. Calculations are carried out for a range of possible canopies. Representative values are chosen for use where a detailed description of the vegetation is unavailable.
The resistors of the new model are compared with those of earlier two-layer models. The new ‘far-field’ resistors are smaller than the corresponding ‘aerodynamic’ resistors of the earlier models of Shuttleworth and Wallace (1985) and Choudhury and Monteith (1988) while the new boundary-layer resistor can be somewhat larger, depending on the choices made in the calculation. We could not find experimental data on canopy energy balances suitable for deciding which model is best. Instead, we have attempted to verify the model using measurements of surface temperature and calculated values of the ‘excess’ resistance, but the results are not definitive. A major conclusion is that turbulent transport near the ground beneath an overstorey canopy is poorly known and therefore is poorly represented in all models. Another conclusion is that current models for the excess resistance are inadequate.
Comparison of Penman±Monteith, Shuttle-worth±Wallace and modified Priestley±Taylor evapotranspira-tion models for wildland vegetation in semiarid rangeland Density fluctuations and use of the Krypton hygrometer in surface flux measure-ments
- D I Stannard
- B D Tanner
- E Swiatek
- J P Greene
Stannard, D.I., 1993. Comparison of Penman±Monteith, Shuttle-worth±Wallace and modified Priestley±Taylor evapotranspira-tion models for wildland vegetation in semiarid rangeland. Water Resources Res. 29, 1379±1392. Tanner, B.D., Swiatek, E., Greene, J.P., 1994. Density fluctuations and use of the Krypton hygrometer in surface flux measure-ments. Proc. Management of Irrigation and Drainage Systems Conf., July 1993, Utah, Irrigation and Drainage Div., ASCE, pp. 945±952.
A rapid-response 2-D drag anemometer for atmo-spheric turbulence measurements Soil and canopy energy balances of a row crop at partial cover
- A E Green
- M J Judd
- J K Mcaneney
- M S Astill
- P T Prendergast
- ±
- J M Ham
- J L Heilman
- R J Lascano
Green, A.E., Judd, M.J., McAneney, J.K., Astill, M.S., Prendergast, P.T., 1991. A rapid-response 2-D drag anemometer for atmo-spheric turbulence measurements. Boundary Layer Meteorol. 57, 1±15. Ham, J.M., Heilman, J.L., Lascano, R.J., 1991. Soil and canopy energy balances of a row crop at partial cover. Agron. J. 83, 744±753.
Energy balance determinations close to the soil surface using a micro-Bowen ratio system An open path, fast response infrared absorption gas analyzer for H 2 O and CO 2 Micrometeorological and chamber measure-ments of CO 2 flux from bare soil
- H Ashktorab
- W O Pruitt
- U K T Paw
- W V D L George
- T P Meyers
Ashktorab, H., Pruitt, W.O., Paw, U.K.T., George, W.V., 1989. Energy balance determinations close to the soil surface using a micro-Bowen ratio system. Agric. For. Meteorol. 46, 259±274. Auble, D.L., Meyers, T.P., 1992. An open path, fast response infrared absorption gas analyzer for H 2 O and CO 2. Boundary Layer Meteorol. 59, 243±256. Dugas, W.A., 1993. Micrometeorological and chamber measure-ments of CO 2 flux from bare soil. Agric. For. Meteorol. 67, 115±128. Dugas, W.A., Reicosky, D.C., Kiniry, J.R., 1997. Chamber and micrometeorological measurements of CO 2 and H 2 O fluxes for three C 4 grasses. Agric. For. Meteorol. 83, 113±133.
A wind tunnel study of turbulent flow around single and multiple windbreaks, part 1: velocity fields
- Judd