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Microclimatic Factors Affecting Evaporation and Transpiration
... While at the time it may have seemed impossible to accurately quantify water use to aid in irrigation scheduling, only a short time later Penman (1948) developed formulae to reliably estimate and predict ET from turf. Later, Monteith integrated a surface resistance term resulting in the development of the Penman-Monteith equation (Monteith et al., 1965;Penman et al., 1967). The Penman-Monteith equation was updated and simplified by FAO (Allen et al. 1998) and ASCE (Allen et al. 2005) by utilizing some assumed constant parameters for mown turfgrass resulting in the FAO-56 Penman-Monteith equation and ASCE Penman-Monteith equation, respectively. ...
Knowledge of water use rates and responses to deficit irrigation practices in cool‐season turfgrasses is important, particularly in climates where irrigation is required to maintain turf quality. This is Part I of two companion papers reviewing cool‐season turfgrass water use and requirements. Part I presents the history of early water conservation and quantification methods in turfgrass and summarizes research on cool‐season turfgrass evapotranspiration (ET) and deficit irrigation. Part II summarizes research on cool‐season turfgrass drought resistance completed since 1980. Summaries of research from > 60 experiments related to ET include: measured ET rates in various climates; comparisons of actual ET to estimated ET; crop coefficients; deficit irrigation; and how plant characteristics and management practices influence ET. Summarized daily evapotranspiration (ETc) rates for Agrostis L. spp., fine fescues (Festuca L. spp.), tall fescue (Festuca arundinacea Schreb.), Lolium spp., and Poa spp. were a weighted average ETc and coefficient of variation (CV) of 6.12 (19.0%), 5.52 (16.8%), 7.79 (14.5%), 5.90 (19.9%), and 5.35 (30.2%) mm d–1, respectively, with an overall ETc of 6.25 mm d–1 (26.9%). Average crop coefficients have ranged from 0.85 to 0.95 (0.91 weighted average across species) and vary slightly during growing months. Minimum deficit irrigation replacement levels for acceptable turf quality have ranged from 59 to 74% of ET under different irrigation intervals. Further study is warranted to investigate differences in crop coefficients among cultivars within species, cultural influences, deficit irrigation replacement levels, and to standardize measurement protocol to improve consistency in crop coefficients across future turfgrass experiments.
... The fourth method is the energy balance equation, which is used for ET estimation (equivalent to the latent heat flux LE) using various surface properties such as albedo, leaf area index, vegetation indices and T S [18] . The most issue hindering the measurement of ET at ground stations is its dependency on some surface parameters that are so difficult to measure over large and heterogeneous areas [2,[19][20][21][22] . Thus, the effectiveness of such methods will not exceed the areas where parameters are measured. ...
A study was carried out to estimate the actual evapotranspiration (ET) over a 1074 km 2 of the humid area of Perak State (Malaysia), where water and evaporation cycle deeply influences the climate, natural resources and human living aspects. Images from both Terra and Aqua platforms of the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor were used for ET estimation by employing the Surface Energy Balance Algorithm for Land (SEBAL) model. As a part of the accuracy assessment process, in-situ measurements on soil temperature and reference ET (ET 0) were recorded at the time of satellite overpass. In order to enhance the accuracy of the generated ET maps, MODIS images were subjected to sub-pixel analysis by assigning weights for different surface cover (urban, agriculture and multi-surface areas) reflections. The weightage process was achieved by integrating ET from pure pixels with the respective site-specific ET 0 of each land cover. The enhanced SEBAL model estimated ET exhibited a good correlation with the in-situ measured Penman-Montieth ET 0 , with R 2 values for the Aqua and the Terra platforms of 0.67 and 0.73, respectively. However, the correlation of the non-enhanced ET maps resulted in R 2 values of 0.61 and 0.68 for the Aqua and the Terra platforms, respectively. Hence, the results of this study revealed the feasibility of employing the sub-pixel analysis method for an accurate estimation of ET over large areas.
... Some authors suggest that LE/Rn > 1 can occur due to the advection of sensible heat over the irrigated area, which comprises an input of energy from outside the system that would contribute to plant evapotranspiration (Payero et al., 2003;Gavilán and Berengena, 2007). Moreover, Penman et al. (1967) indicated that in situations in which a crop that covers the ground is under suitable and intermittent irrigation, the air temperature near the canopy can be lower than the temperature at the level slightly above the canopy. In this case, sensible heat flux is negative (vertically downward), as opposed to the water vapor flux, generating negative β values. ...
Melon plants (Cucumis melo L.) are grown in the state of Rio Grande do Norte (RN), the largest producer of melons in Brazil, with plastic mulch and agrotextiles. Studies of crop evapotranspiration (ET) under these conditions are required to ensure adequate irrigation. This study aimed to determine the crop coefficients (Kc) of irrigated melon plants grown with mulch and agrotextiles in the region of Mossoró, RN, based on the Bowen Ratio Energy Balance (BREB) method. Two experiments were conducted at different times during the 2009/2010 season in a melon producing area (4°59'52" S, 37°23'09" W, and 54 m elevation) to define ET and Kc. Due to the plastic cover and reduced precipitation during the experiments, the Kc obtained by the BREB method was considered the basal Kc - KcbBREB. The results were compared with the Kcb from the FAO 56 Bulletin. There was close agreement between BREB and FAO ET measurements (12 % underestimation by the FAO method for the entire crop season), with sizeable differences only during the initial phenological stage. The mean KcbBREB values of the two field campaigns were 0.26, 0.96 and 0.63 for the initial, midseason and late stages, respectively. The high KcbBREB value in the initial growth phase may be related to the effect of the plastic mulch and agrotextiles on the energy balance at the surface. The relationship between KcbBREB and fc had high correlation, making possible an estimation of the melon Kcb based on the level of crop ground cover.
... Evapotranspiration rates were 2.6, 3.8, and 2.5 mm·day -1 during early, middle, and late growth periods, respectively. These differences in evapotranspiration were not totally attributable to weather conditions, because potential evapotranspiration rates as calculated by Penman's equation (Penman et al., 1967) were 4.5, 4.0, and 3.4 mm·day -1 during early, middle, and late Received for publication 8 Nov. 1989. Supported by state and Hatch Act funds allocated to the Georgia Agricultural Experiment Stations and grant funds from the Richard King Mellon Foundation. ...
Sweetpotatoes [Ipomoea batatas (L.) Lam cv . Georgia Jet] were grown on two soil types in drainage lysimeters under controlled soil water regimes during 1982 and 1983. Water regimes consisted of irrigating the sweetpotatoes throughout growth when soil water tension at 23 cm exceeded 25, 50, or 100 kPa or by allowing a 100-kPa water stress before root enlargement, during early root enlargement, or throughout root enlargement. Water use and marketable yields were greater when sweetpotatoes were grown on a Tifton loamy sand (fine loamy, siliceous, thermic, Plinthitic Paleudult) than when grown on a Bonifay sand (loamy, siliceous, thermic, Grossarenic, Plinthitic Paleudult). Water use, marketable yield, and yield of U.S. #1 grade roots generally decreased when soil water tensions exceeded 25 kPa before irrigation, although soil water stress of 100 kPa during storage root development did not significantly affect yield. Regression equations are provided to describe the relationships of water use to plant age and to compute daily evapotranspiration: pan evaporation ratios (crop factors) for sweetpotatoes irrigated at 25, 50, and 100 kPa of soil water tension.
... Thus, it appears that a values reported in the literature for maize, grass, and alfalfa are very similar and vary in a narrow range. Penman (1956) and Penman et al. (1967) suggested that the changes in plant color have very little influence on a and that the differences in ET caused by differences in a for agronomical vegetation are quite modest. Most agronomical vegetation surfaces have similar color. ...
We compared daily net radiation (R n) estimates from 19 methods with the ASCE-EWRI R n estimates in two climates: Clay Center, Nebraska (sub-humid) and Davis, California (semi-arid) for the calendar year. The performances of all 20 methods, including the ASCE-EWRI R n method, were then evaluated against R n data measured over a non-stressed maize canopy during two growing seasons in 2005 and 2006 at Clay Center. Methods differ in terms of inputs, structure, and equation intricacy. Most methods differ in estimating the cloudiness factor, emissivity (e), and calculating net longwave radiation (R nl). All methods use albedo (a) of 0.23 for a reference grass/alfalfa surface. When comparing the performance of all 20 R n methods with measured R n, we hypothesized that the a values for grass/alfalfa and non-stressed maize canopy were similar enough to only cause minor differences in R n and grass-and alfalfa-reference evapotranspiration (ET n and ET n) estimates. The measured seasonal average a for the maize canopy was 0.19 in both years. Using α = 0.19 instead of α = 0.23 resulted in 6% overestimation of R n. Using α = 0.19 instead of α = 0.23 for ET o and ET r estimations, the 6% difference in R n translated to only 4% and 3% differences in ET o and ET n respectively, supporting the validity of our hypothesis. Most methods had good correlations with the ASCE-EWRI R n (r 2 > 0.95). The root mean square difference (RMSD) was less than 2 MJ m -2 d -1 between 12 methods and the ASCE-EWRI R n at Clay Center and between 14 methods and the ASCE-EWRI R n at Davis. The performance of some methods showed variations between the two climates. In general, r 2 values were higher for the semi-arid climate than for the sub-humid climate. Methods that use dynamic ε as a function of mean air temperature performed better in both climates than those that calculate e using actual vapor pressure. The ASCE-EWRI-estimated R n values had one of the best agreements with the measured R n (r 2 = 0.93, RMSD = 1.44 MJ m -2 d-1), and estimates were within 7% of the measured R n. The R n estimates from six methods, including the ASCE-EWRI, were not significantly different from measured R n. Most methods underestimated measured R n by 6% to 23%. Some of the differences between measured and estimated R n were attributed to the poor estimation of R n1. We conducted sensitivity analyses to evaluate the effect of R n1 on R n, ET o, and ETr. The Rnl effect on R n was linear and strong, but its effect on ET o and ET r was subsidiary. Results suggest that the Rn data measured over green vegetation (e.g., irrigated maize canopy) can be an alternative R n data source for ET estimations when measured R n data over the reference surface are not available. In the absence of measured R n, another alternative would be using one of the R n models that we analyzed when all the input variables are not available to solve the ASCE-EWRI R n equation. Our results can be used to provide practical information on which method to select based on data availability for reliable estimates of daily R n in climates similar to Clay Center and Davis. © 2010 American Society of Agricultural and Biological Engineers ISSN 2151-0032.
... Devido à complexidade e ao alto custo dos dispositivos de medidas da ETo, inúmeros são os métodos de estimativa, bem como, farta é a disponibilidade de textos sobre o assunto entre os quais destacam-se Gangopadhyaya et al. (1966), Penman et al. (1967), Jensen (1973), Jensen et al. (1990), Doorenbos & Pruitt (1977), Berlato & Molion (1981), Brutsaert (1982), Villa Nova & Reichardt (1989), Camargo & Sentelhas (1997), Pereira et al. (1997), Medeiros (1998) e Allen et al. (1998). ...
... Presently either grass (Allen et al. 1998) or alfalfa (Wright 1982) is taken as reference crop surfaces depending on the agro-climatology of the area. (1967) observed that leaf area, stage of growth, canopy cover and ground shading, row spacing and orientation, and light reflection for potato and sugar beet crops influenced plant ET. Penman et al. (1967) studied the relation between the leaf area and canopy shading of the ground and conceptualized full cover stage -when the plants in the adjacent rows overlapped and masked the underlying soil from direct sunlight. Ritchie and Burnett (1971) measured the fractional ground cover, dry matter production and leaf area for dry land cotton and identified a threshold LAI (of 2.7) at which potential evaporation was reached from row crops; and beyond the threshold canopy LAI, evaporation was maximum, practically independent of plant factors until soil water was observed to limit evaporation. ...
This paper offers a historical retrospective on the remote sensing of crop coefficients for obtaining actual crop evapotranspiration.
We present the canopy reflectance-based approach of crop coefficients and show the usefulness of high-resolution airborne
imagery as a tool for monitoring the actual crop growth changes and characterizing in-field variability in an objective manner.
The plants growing on the soil are influenced by atmospheric or climatic factors which represent the boundary conditions at the soil-plant-atmosphere interface. Soil and plant conditions also have an impact on the atmosphere or microclimate. These interactions are quite complicated and are not fully understood. In this unit emphasis will be on the atmospheric interrelations of soils and plants.
Evapotranspiration (ET) from crops is a complex process involving many climatic, soil, and plant variables. Since evapotranspiration is the major water “output” in the hydrologic cycle for many regions of the world, a great amount of research has been conducted and many methods proposed to estimate evapotranspiration (Hagan
et al., 1967; Proc. Conf. Evapotransp., 1966).
Under variable climatic conditions the irrigation regime for particular crops changes from year to year, both in terms of irrigation rate and of distribution during the vegetation period, depending on precipitation deficiency. Since the effect and value of irrigation depend on its timeliness, special care must be taken to determine the optimum time for irrigation.
This chapter discusses the nuclear techniques for soil water. Research on soil water and irrigation depends on the determination of actual soil moisture at many different experiment sites, at different depths in the soil, and with different irrigation and other treatments. The operation of the neutron moisture meter depends on the fact that neutrons are readily scattered by water but not by soil. It has also been found that the amount of neutron scattering is proportional to the amount of water in the soil. The neutron moisture meter consists of a source of fast neutrons, a detector to identify the slow neutrons that result from scattering of the fast neutrons by water, and a counter to count them. There are many advantages to this technique of moisture measurement—the method is nondestructive, the measurement can be done in the field, and there are no samples to take or carry; after the initial placement of the access tube, each measurement is rapid; repeated measurements can be carried out over a long period without disturbing the soil; it is easy to measure at different depths at the same point, and; a large volume of soil is sampled. The main disadvantage is the expense of the equipment and its weight of 8–15 kg, while lack of uniformity, high chloride content, and stones can cause errors in some soils.
We are concerned with radiation and the microclimate above the soil because they have such large influences on:
1)
Evaporation of water from soil.
2)
Transpiration of water from plants.
3)
Soil and air temperature.
4)
Plant growth (plant growth is closely related to transpiration and to soil and air temperatures).
This new edition is a major revision of the popular introductory reference on hydrology and watershed management principles, methods, and applications. The book's content and scope have been improved and condensed, with updated chapters on the management of forest, woodland, rangeland, agricultural urban, and mixed land use watersheds. Case studies and examples throughout the book show practical ways to use web sites and the Internet to acquire data, update methods and models, and apply the latest technologies to issues of land and water use and climate variability and change.
Food security and environmental conservation are two of the greatest challenges facing the world today. It is predicted that food production must increase by at least 70% before 2050 to support continued population growth, though the size of the world's agricultural area will remain essentially unchanged. This updated and thoroughly revised second edition provides in-depth coverage of the impact of environmental conditions and management on crops, resource requirements for productivity and effects on soil resources. The approach is explanatory and integrative, with a firm basis in environmental physics, soils, physiology and morphology. System concepts are explored in detail throughout the book, giving emphasis to quantitative approaches, management strategies and tactics employed by farmers, and associated environmental issues. Drawing on key examples and highlighting the role of science, technology and economic conditions in determining management strategies, this book is suitable for agriculturalists, ecologists and environmental scientists.
This research was carried out to determine the effects of furrow, sprinkler, drip, mobile nozzle, mobile drip and LEPA (Low Energy Precision Application) irrigation methods on the yields and Water Use Efficiency of cotton in the South Eastern Anatolian Project (GAP) Area and in the Aegean Region. According to the results obtained, the highest yield was produced from the drip irrigation method. The drip, sprinkler and mobile nozzle forms of irrigation require 31, 28 and 26% less irrigation water respectively than furrow irrigation when related to the optimum yields and the feasible irrigation water quantities in the GAP Area. The drip irrigation method has utilised the irrigation water more efficiently than the other methods. LEPA and mobile drip did not show any significant difference in the effective use of irrigation water compared to furrow irrigation. Similar results were obtained in the Aegean Region of Turkey.
Theoretical and empirical models used to estimate the long-term water
balance of small catchments are inadequate as a check on the observed
water balance. Under rigorous research conditions, the observational
data are accurate to about ± 13-30% of yield, while the models
are accurate only to about ± 10-60% of yield. Analysis of
watershed data shows that the uncertainty in any comparison of
theoretical and observed values usually is much greater than the
difference between the values compared. In general, such comparisons are
not helpful either in confirming or challenging the accuracy of water
balance measurements, or in assessing the watertightness of catchment
areas.
surface, which is related to the surface saturation deficit. We show that this diffusion resistance model is not applicable either to vegetation canopies or to soil, and we have developed a combination formula relating the evaporation from a drying soil to the potential evaporation. This method uses the surface temperature as the only additional measurement to the standard combination formula .for potential evaporation, and it compares favorably with the energy balance-Bowen ratio method over a drying soil.
Data are presented on the microclimate above the crop canopy of an extended area of mature tea growing in southern Malawi. Stomatal opening was significantly greater following the onset of the rainy season although the difference did not appear to affect the partitioning of solar radiation between evaporation and sensible heat exchange as indicated by calculated values of the Bowen ratio.
Ten forms of the Penman combination evapotranspiration equation are reviewed and compared with lysimeter estimates at three locations. Aerodynamic and canopy resistance forms by Monteith and Thorn and Oliver and an empirical form by Wright best predict daily lysimeter measurements at Kimberly, Idaho, and Coshocton, Ohio. Canopy resistances of 40-80 s/m and momentum roughness heights of 15 mm for clipped grass and 45-70 mm for tall grass and alfalfa result in best estimates by the Monteith and Thorn-Oliver methods. The original Penman and Priestley-Taylor versions underestimated evapotranspiration in the arid Kimberly environment. Average standard errors of estimate of the best equations average about 0.8 mm/day over growing seasons.
A model is presented for calculating the daily evaporation rate from a
crop surface. It applies to a row crop canopy situation in which the
soil water supply to the plant roots is not limited and the crop has not
come into an advanced stage of maturation or senescence. The crop
evaporation rate is calculated by adding the soil surface and plant
surface components (each of these requiring daily numbers for the leaf
area index), the potential evaporation, the rainfall, and the net
radiation above the canopy. The evaporation from the soil surface
Es is calculated in two stages: (1) the constant rate stage
in which Es is limited only by the supply of energy to the
surface and (2) the falling rate stage in which water movement to the
evaporating sites near the surface is controlled by the hydraulic
properties of the soil. The evaporation from the plant surfaces
Ep is predicted by using an empirical relation based on local
data, which shows how Ep is related to Eo through
the leaf area index. The model was used to obtain the total evaporation
rate E = Es + Ep of a developing grain sorghum
(Sorghum bicolor L.) canopy in central Texas. The results agreed well
with values for E measured directly with a weighing lysimeter.
A dynamic programming model of irrigation scheduling is developed which accounts for stochastic weather conditions, results
in simple irrigation decision rules, and can be operated on current microcomputers. The model employs heat unit intervals
instead of chronological time to define the dynamic equations of the crop-soil system. Procedures are outlined for estimating
the transition probabilities of climate within the heat unit intervals. When compared to maximum yield irrigation scheduling,
the model increases net returns of corn, sorghum, and cotton by $10.00 to $30.00 per acre.
By using a simplification of the aerodynamical expression for bulk mass transfer over a rough evaporating surface, the quality of generalized empirical statistical wind functions in Penman's equations is assessed. It is shown that Penman's revised wind function for the open water case is good enough to be preferred over any attempt for more adaptations. It is also shown that for the case of potential evaporation (or reference crop evaporation) a recent approach by Doorenbos and Pruitt forms an appreciable improvement. Comparison with another recently derived generalization by Thom and Oliver shows that the generalized wind functions preferred hold also under conditions of atmospheric instability. Finally limitations are discussed of using reference crop evaporation formulas with only a generalized wind function in agricultural water management problems.
This paper describes a representation of the distribution of sensible and latent heat from the surface through the atmospheric boundary layer which has been formulated for use in a 10-level primitive equation model atmosphere. The transfer process is represented in two parts : (i) the transfer of energy across the Earth's surface into the lowermost 100 mb layer of the model atmosphere; and (ii) the subsequent redistribution of this energy through two or more such layers by small-scale convection. The fluxes of energy across the surface are calculated using empirical ‘bulk aerodynamic’ relationships. In land regions consideration of the energy balance at the surface is also necessary, and diurnal variations of radiation are taken into account. The redistribution of energy by small-scale convection is represented by convective adjustments which ensure that a certain neutral lapse rate of temperature is never exceeded. Some results of the incorporation of these effects into the 10-level model are described.
The energy balance of an upland heath dominated by heather (Calluna vulgaris) was measured in dry and wet weather. Median values of both transpiration and evaporation rates were ca. 2 mm hr-1. The median Bowen ratio for the dry canopy was 2.0 and for the wet canopy 0.6. On dry days the median value of the saturation deficit was only 3.8 mb and that of the climatological resistance was 30 s m-1. The bulk stomatal resistance increased from ca. 50 s m -2 in the morning to over 290 s m-1 in the afternoon with an overall median value of 110 s m-1. Transpiration from the dry canopy was controlled by a combination of small saturation deficits and large stomatal resistances. The median value of the boundary-layer resistance of the canopy was 22 s m-1 and was low partly because of a large low-level drag coefficient. Saturation deficits on wet days were close to zero and evaporation of intercepted water proceeded at close to the equilibrium rate, being largely limited by the low fluxes of available energy. The water loss from heather was compared with simulated losses from coniferous forest, herbaceous crops and grassland in the same conditions to evaluate the effects of vegetation on water loss from catchments.
An approach is presented to model soil moisture dynamics in irrigated fields. A generalised conceptual model is proposed for moisture transfer in response to hydraulic and thermal gradients. A model to stimulate moisture extraction by roots is presented, based on a philosophy related to fundamental physical principles and recent experimental evidence. The combined models are interpreted numerically by the finite element method and a number of numerical techniques are developed to treat time-dependent, nonlinear and moving boundary conditions. The approach is sufficiently general and is independent of the method of water application being considered. Its benefits are particularly magnified in studies of localised irrigation, where complex soil-water distribution patterns evolve.
After review of the purposes that have in the course of time been served by investigations on aerial water vapour pressure inside and just above plant stands, measured vapour pressure profiles within a maize crop are reported and discussed. Special attention is paid to measured horizontal homogeneity of water vapour pressure, of importance in relation to accuracy needed for mean vertical flux calculations and time averaged single measurements.Nach einer bersicht ber im Laufe der Zeit durchgefhrte Untersuchungen der Verteilung des Wasserdampfes innerhalb und knapp ber Pflanzenbestnden wird ber in einem Maisfeld gemessene Dampfdruckprofile berichtet. Besondere Beachtung wird der gemessenen horizontalen Homogenitt der Dampfdruckverteilung in bezug auf die fr Berechnungen des mittleren vertikalen Flusses erforderliche Genauigkeit und den zeitlich gemittelten Einzelmessungen gewidmet.
Several approaches to the evaluation of potential evapotranspiration (PE) are discussed from the point of view of the energy balance equation. Hourly and daily PE were evaluated for an irrigated ryegrass site near Simcoe in Southern Ontario during the summer of 1967 using the Bowen ratio and Penman solution of the energy balance equation, Thornthwaite's temperature equation and the water equivalent of the net radiation. The Bowen ratio values were used as the standard against which the other methods were compared. For the evaluations, measurements of temperature, humidity and wind profiles, net radiation, soil heat flux, screen temperature and humidity and wind run were made.
Excellent results were obtained from the Penman equation (using Businger's with function) for individual hourly periods on days when previous rainfall or irrigation ensured true PE conditions. On other drier days, Penman values were consistently too large. The water equivalent of the net radiation was highly correlated with Bowen ratio values on a hourly basis but overestimated consistently. A poor correlation between Thornthwaite values and the net radiation is demonstrated on a daily basis, which indicates its tenuous connection with the energy balance equation.
The relation between cumulative dry matter productivity of the ryegrass and cumulative PE is shown to be linear, but the productivity rate changed from 1 g/525 mm when the crop was irrigated to 1 g/1287 mm after irrigation had ceased.
An equation for Potential Evaporation (PE) proposed by Priestley and Taylor in 1972 has fewer data requirements than the well established Penman Potential Transpiration (Et) equation. From their definitions, PE and Et values should both provide acceptable estimates of Reference Crop Evapotranspiration (ETo), as defined by Doorenbos and Pruitt. Analysis of mean monthly climatic data from 30 tropical stations, widely spread within the latitude zone 25°N to 25°S, showed that PE and Et estimates agreed closely when monthly rainfall exceeded monthly Et. The minimum data requirements for the Priestley-Taylor equation are daily net radiation and mean air temperature. The Penman equation additionally requires daily data for humidity and run of wind. As reliable field net radiometers become more widely available, the Priestley-Taylor PE equation offers a satisfactory alternative to the Penman Et equation for estimating ETo in humid tropical climates.
The potential evapotranspiration (ETP), as defined by Thornthwaite, is considered as a physical process and it does not take into account the complexities of the interactions between crop, soil and climatological factors. A better definition, given by Penman, introduces some factors related to the crop characteristics. Bouchet and Robelin consider that the potential evapotranspiration is the envelope of the different crops' maximum evapotranspirations (ETM) with non-limiting leaf area: it gives an indication of the water requirements of the ecological environment. The value of the maximum evapotranspiration in an arid region depends on the kind of crop chosen. This study gives an analysis of some data obtained with different crops in the Mediterranean region of Tunis (Tunisia). The energy balance method has shown the importance of another source of energy in this semi-arid region - advection. Under conditions of large scale advection (oasis effect), the ratio between maximum evapotranspiration and net radiation exceeds generally unity. The water loss from the different crops receiving optimum irrigation treatment, varies widely. The factors which could influence the maximum crop evapotranspiration are described as follows: (a) the air characteristics (temperature, humidity, wind-speed) flowing from a dry area into the experimental irrigated area; (b) the solar energy interception and the crop architecture; and (c) the extent of the transpirating surface (leaf development, leaf area index) and its characteristics (geometry, stomatal density, tropism). It is concluded that, for a large area, cultivated with a defined crop, the measured evapotranspiration from this crop would be a more accurate method of irrigation control. It is difficult in those arid conditions to evaluate the water requirements for a cropped surface by climatological formulae because of the importance of the physiological factors.
Studies of the interception process in grassy species are reported for a series of laboratory tests. Procedures, equipment, and techniques were developed to determine the magnitudes of rainfall interception on grasses and to measure the resultant losses. It was found in vigorously growing grass plots that the evaporation of a given amount of intercepted moisture was accompanied by a like reduction in the amount of evapotranspiration from the plants. Total moisture use was approximately the same in plots with wet and dry leaf surfaces. The interception storage component and precipitation passing through the vegetative canopy as combined stemflow, throughfall and drip (STD) were measured to determine the relationships and magnitudes. Storage capacities were observed to agree essentially with those reported by other investigators although some variations can occur with different storm types and intensities. Values of STD were found to be quite significant even at the beginning of small storms.
The heat‐transfer processes which occur at the Earth's surface have direct bearing on many of the physical problems in agriculture that relate to the plant environment. The evapotranspiration from a crop can be estimated from the disposition of solar and sensible energy available at the earth‐air interface. A suitable measurement of the heat budget, including net radiation, heat transfer by soil conduction, air‐temperature gradient above the crop, and vapor pressure gradient above the crop, will account for the fraction of solar energy used hourly or daily for evapotranspiration. The simple theory, a brief description of the experimental equipment, and some illustrative measurements are discussed.
Evapotranspiration rates of 75- and 100-cm. crops of sudangrass were similar and considerably larger than that of the 140-cm. height. This difference appeared to be due to the presence of seedheads rather than physiological maturity. It was also shown, for days of similar energy
input, that evapotranspiration increased with wind speed. These facts emphasize the point that for well-watered sudangrass, physical rather than physiological factors regulated the evapotranspiration.