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During the spring of 2014, 130 million m3 of water were
released from the United States' Morelos Dam on the lower Colorado River to
Mexico, allowing water to reach the Gulf of California for the first time in
13 years. Our study assessed the effects of water transfer or ecological
environmental flows from one nation to another, using remote sensing...
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... riparian corridor was divided into seven river Reaches from the Northern International Border (NIB) near Yuma, AZ to the estuary (Fig. 1). Landsat NDVI (n = 4 to 5 sum- mer season scenes/year) was compared for each of the seven river Reaches for the wetted, inundation zone and the overall riparian ...
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In this study, we used moderate resolution imaging spectroradiometer (MODIS) satellite images to quantify the sedimentation processes in a cascade of six hydropower dams along a 700-km transect in the Paranapanema River in Brazil. Turbidity field measurement acquired over 10 years were used to calibrate a turbidity retrieval algorithm based on MODI...
Citations
... Ultimately, such fine-scale understanding would ideally be linked-at a similar resolution, but larger extent-to remotely sensed vegetation greenness metrics so that conclusions are easily scalable across catchments. Metrics like normalized difference vegetation index (NDVI) reflect changes in water availability to vegetation (Aguilar et al., 2012), which has been observed to correlate with ET (Nagler et al., 2018). Because surface water and shallow groundwater sustain vegetation greenness (Werstak et al., 2010;Fu and Burgher, 2015), vegetation greenness has been hypothesized to reflect stream drying. ...
Geologic, geomorphic, and climatic factors have been hypothesized to influence where streams dry, but hydrologists struggle to explain the temporal drivers of drying. Few hydrologists have isolated the role that vegetation plays in controlling the timing and location of stream drying in headwater streams. We present a distributed, fine-scale water balance through the seasonal recession and onset of stream drying by combining spatiotemporal observations and modeling of flow presence/absence, evapotranspiration, and groundwater inputs. Surface flow presence/absence was collected at fine spatial (~80 m) and temporal (15-min) scales at 25 locations in a headwater stream in southwestern Idaho, USA. Evapotranspiration losses were modeled at the same locations using the Simultaneous Heat and Water (SHAW) model. Groundwater inputs were estimated at four of the locations using a mixing model approach. In addition, we compared high-frequency, fine-resolution riparian normalized vegetation difference index (NDVI) with stream flow status. We found that the stream wetted and dried on a daily basis before seasonally drying, and daily drying occurred when evapotranspiration outputs exceeded groundwater inputs, typically during the hours of peak evapotranspiration. Riparian NDVI decreased when the stream dried, with a ~2-week lag between stream drying and response. Stream diel drying cycles reflect the groundwater and evapotranspiration balance, and riparian NDVI may improve stream drying predictions for groundwater-supported headwater streams.
... However, long-term effects of environmental flows on floodplain vegetation have not been described because of a lack of longer-term datasets [1,19]. Therefore, long-term effects of environmental water on floodplain ecosystems need to be monitored and modeled to better manage environmental water [20]. ...
River floodplains are among the most dynamic and diverse ecosystems on the planet. They are at risk of degradation due to river regulation and climate change. Environmental water has been delivered to floodplains to maintain environmental health by mimicking natural floods. It is important to understand the long-term effects of environmental water to floodplain vegetation to support its management. This study used Normalized Differences Vegetation index (NDVI) from the 30-year Landsat datasets of the Hattah Lakes floodplain in Australia to investigate the drivers of vegetation dynamics. We developed generalized additive mixed models (GAMM) to model responses of vegetation to environmental water, natural floods, precipitation, temperature, and distance to water across multiple spatial and temporal scales. We found the effect of environmental water on floodplain vegetation to be quite different from that of natural floods in both space and time. Vegetation in most areas of Hattah Lakes will respond to natural floods within one month of flooding, while positive responses to environmental water occur 1 to 3 months after inundation and are more restricted spatially. For environmental water planning, managers need to be aware of these differences. The implementation of new infrastructure to transport or retain environmental water on floodplains needs to be planned carefully, with continuous monitoring of rainfall and natural floods. Whilst environmental floods do not mimic the effect of natural floods, they do provide some positive benefits that can partially offset effects of reduced natural floods.
... Among other things, this agreement included measures to promote the ecological health of the CRD. In keeping with Minute 319, in 2014, 105,392 acre-feet of water was released into the CRD in a one-time event, allowing water to reach the Gulf of California for the first time in 13 years [26]. The flow was designed to simulate historical pulse-flows into the CRD and to enhance wetlands in the area. ...
Due to increasing water use, diversion and salinization, along with subsidence and sea-level rise, deltas in arid regions are shrinking worldwide. Some of the most ecologically important arid deltas include the Colorado, Indus, Nile, and Tigris-Euphrates. The primary stressors vary globally, but these deltas are threatened by increased salinization, water storage and diversion, eutrophication, and wetland loss. In order to make these deltas sustainable over time, some water flow, including seasonal flooding, needs to be re-established. Positive impacts have been seen in the Colorado River delta after flows to the delta were increased. In addition to increasing freshwater flow, collaboration among stakeholders and active management are necessary. For the Nile River, cooperation among different nations in the Nile drainage basin is important. River flow into the Tigris-Euphrates River delta has been affected by politics and civil strife in the Middle East, but some flow has been re-allocated to the delta. Studies commissioned for the Indus River delta recommended re-establishment of some monthly water flow to maintain the river channel and to fight saltwater intrusion. However, accelerating climate impacts, socio-political conflicts, and growing populations suggest a dire future for arid deltas.
... Riparian vegetation water use is dependent on hydrological processes that include groundwater/surface (blue water) availability, timing, and duration of large streamflow events, and whose dynamics are derived from climatic controls on precipitation and mountain snowpack [77], changes in carbon balance [78,79], carbon uptake [80], and canopy temperature [81]. Future climate projections indicate that dryland regions face generalized temperature increases with largest warming in the summer months, and a likely decrease in precipitation, particularly in the cool season [82][83][84]. ...
Declines in riparian ecosystem greenness and water use have been observed in the delta of the Lower Colorado River (LCR) since 2000. The purpose of our case study was to measure these metrics on the U.S. side of the border between Hoover and Morelos Dams to see if declining greenness was unique to the portion of the river in Mexico. In this case study, five riparian reaches of the LCR from Hoover to Morelos Dam since 2000 were studied to evaluate trends in riparian ecosystem health. We measure these riparian woodlands using remotely sensed measurements of the two-band Enhanced Vegetation Index (EVI2; a proxy for greenness); daily evapotranspiration (ET; mmd−1) using EVI2 (ET(EVI2)); and an annualized ET based on EVI2, the Phenology Assessment Metric (PAM ET), an annualized ET using Landsat time-series. A key finding is that riparian health and its water use has been in decline since 2000 on the U.S. portion of the LCR, depicting a loss of green vegetation over the last two decades. EVI2 results show a decline of −13.83%, while average daily ET(EVI2) between the first and last decade had a decrease of over 1 mmd−1 (−27.30%) and the respective average PAM ET losses were 170.91 mmyr−1 (−17.95%). The difference between the first and last five-year periods, 2000–2005 and 2016–2020, showed the largest decrease in daily ET(EVI) of 1.24 mmd−1 (−32.61%). These declines come from a loss in healthy, green, riparian plant-cover, not a change in plant water use efficiency nor efficient use of managed water resources. Our results suggest further deterioration of biodiversity, wildlife habitat and other key ecosystem services on the U.S. portion of the LCR.
... Consequently, transpiration can contribute significantly to total ET if the water added is adequate to wet the root zone, indicated by the surface/sub-surface soil moistures [3,[11][12][13][14]. In drylands, ET is an effective indicator of vegetation resilience and resistance to drought conditions, since water is the major constraint on plant productivity [15,16]. In the hydrological cycle, ET also serves as a key process that links energy, carbon and water cycles [17]. ...
Drylands cover about 40% of the world's land area and support two billion people, most of them living in developing countries that are at risk due to land degradation. Over the last few decades, there has been warming, with an escalation of drought and rapid population growth. This will further intensify the risk of desertification, which will seriously affect the local ecological environment, food security and people's lives. The goal of this research is to analyze the hydrological and land cover characteristics and variability over global arid and semi-arid regions over the last decade (2010-2019) using an integrative approach of remotely sensed and physical process-based numerical modeling (e.g., Global Land Data Assimilation System (GLDAS) and Famine Early Warning Systems Network (FEWS NET) Land Data Assimilation System (FLDAS) models) data. Interaction between hydrological and ecological indicators including precipitation, evapotranspiration, surface soil moisture and vegetation indices are presented in the global four types of arid and semi-arid areas. The trends followed by precipitation, evapotranspiration and surface soil moisture over the decade are also mapped using harmonic analysis. This study also shows that some hotspots in these global drylands, which exhibit different processes of land cover change, demonstrate strong coherency with noted groundwater variations. Various types of statistical measures are computed using the satellite and model derived values over global arid and semi-arid regions. Comparisons between satellite-(NASA-USDA Surface Soil Moisture and MODIS Evapotranspiration data) and model (FLDAS and GLDAS)-derived values over arid regions (BSh, BSk, BWh and BWk) have shown the over and underestimation with low accuracy. Moreover, general consistency is apparent in most of the regions between GLDAS and FLDAS model, while a strong discrepancy is also observed in some regions, especially appearing in the Nile Basin downstream hyper-arid region. Data-driven modelling approaches are thus used to enhance the models' performance in this region, which shows improved results in multiple statistical measures ((RMSE), bias (ψ), the mean absolute percentage difference (ψ)) and the linear regression coefficients (i.e., slope, intercept, and coefficient of determination (R 2)).
... Evapotranspiration (ET) -consisting of both evaporation from plant and soil surfaces and transpiration through stomata -is a key process in the hydrological cycle which links water, carbon, and energy cycles . In drylands, water is the dominant constraint on plant productivity such that ET can be used as an indicator of vegetation resilience and resistance to drought (Nagler et al., 2018a(Nagler et al., , 2018b, and remote sensing currently provides widely-used estimates of ET at spatiotemporal resolutions relevant for on-the-ground management Fisher et al., 2017). There are three primary methods for estimating ET from remote sensing: (i) empirical/statistical methods which upscale field measurements of ET using remotelysensed VIs (e.g., Nagler et al., 2005aNagler et al., , 2005bNagler et al., , 2009Yang et al., 2006); (ii) semi-empirical methods that use remotely sensed land surface temperature (LST) and/or VIs to derive key parameters in physical models of latent heat flux (e.g., Cleugh et al., 2007;Mu et al., 2011Mu et al., , 2007aYuan et al., 2010); and (iii) surface energy balance (SEB) methods which estimate latent heat flux as the residual in the SEB equation using remotely-sensed surface reflectance and LST data (e.g., Anderson et al., 2008Anderson et al., , 1997Colaizzi et al., 2012). ...
... In fact, the present river bathymetry results from the interaction between natural processes and anthropic pressures affecting the reach since centuries, like damming, water exploitation for several uses, land use change for agriculture, river channel embanking and sediment mining, which drove to a significant degradation of the river bed (Surian and Rinaldi 2003;Lanzoni et al. 2015;Nones et al. 2018a) with consequent negative effects on local fauna and flora. On the one side, a reduction of the water flow is detrimental for riparian vegetation and biota (Nagler et al. 2018). On the other side, a water discharge decrease means a reduction in sediment transport (Guerrero et al. 2013), with a detrimental effect on the ecological status (Vesipa et al. 2017: Nones 2019. ...
The paper proposes a simplified methodology to track the evolution of vegetation patterns over a central sandbar of the Po River, Italy, by means of a fixed video camera installed on the top of a bridge pier. Looking downstream, the camera ac-quires images every twelve hours while hourly water levels are derived from a ra-dar hydrometer located 300 m upstream of the study area. The vegetation growth rate is computed analysing several images covering the period July-December 2017, characterized by a dry period during the summer/autumn and a flood at the end of the year. The tracking of the vegetation patterns bounds provides some general indications on the role of a transient hydrology on the plants’ dynamics.
Revitalization of the Colorado River delta riparian corridor and increasing riparian plant greenness and water use may be accomplished by added water and restoration efforts to offset declines measured since 2000 by Landsat. We use the two‐band Enhanced Vegetation Index (EVI2; a proxy for greenness) and evapotranspiration (ET, mm/day) using EVI2 and potential ET(ETo) from Yuma Valley. We assess if restoration with only 7.5% landcover had an impact on the unrestored reach‐level landcover by measuring these two metrics, EVI2 and ET(EVI2) by comparing restored and unrestored areas. A key finding is that over 21‐years EVI2 in the unrestored corridor decreased by 23.6% and ET(EVI2) decreased by 32% (0.87 mm/day). Since 2011, the unrestored reaches lost EVI2 (11%) and −0.73 mm/day ET(EVI2) (28%), but restored sites increased in EVI2 (36%) and 0.58 mm/day ET(EVI2) (20%). Water delivered to restored sites increased EVI2 by 33.6% and 1.29 mm/day ET(EVI2) (58%). EVI2 and ET(EVI2) as a function of the Standardized Precipitation Evapotranspiration Index for drought periods highlight that restored and unrestored sites respond differently. Unrestored reaches are in decline; restored sites show increases in EVI2 and ET(EVI2). Restored sites do not have a significant impact on unrestored adjacent area, but smaller surface flows, a greater reliance on directed agricultural return flows, and deliveries of water to active restoration sites have revitalized habitat and increased ecosystem services in the delta.
Hydrological and bioclimatic processes that lead to drought may stress plants and wildlife, restructure plant community type and architecture, increase monotypic stands and bare soils, facilitate the invasion of non‐native plant species and accelerate soil erosion. Our study focuses on the impact of a paucity of Colorado River surface flows from the United States (U.S.) to Mexico. We measured change in riparian plant greenness and water use over the past two decades using remotely sensed measurements of vegetation index (VI), evapotranspiration (ET) and a new annualized phenology assessment metric (PAM) for ET. We measure these long‐term (2000–2019) metrics and their short‐term (2014–2019) response to an environmental pulse flow in 2014, as prescribed under Minute 319 of the 1944 Water Treaty between the two nations. In subsequent years, small‐directed flows were provided to restoration areas under Minute 323. We use 250 m MODIS and 30 m Landsat imagery to evaluate three vegetation indices (NDVI, EVI, EVI2). We select EVI2 to parameterize an optical‐based ET algorithm and test the relationship between ET from Landsat and MODIS by regression approaches. Our analyses show significant decreases in VIs and ET for both the 20‐year and post‐pulse 5‐year periods. Over the last 20 years, EVI Landsat declined 34% (30% by EVIMODIS) and ETLandsat‐EVI declined 38% (27% by ETMODIS‐EVI), overall ca. 1.61 mm/day or 476 mm/year drop in ET; using PAM ETLandsat‐EVI the drop was from 1130 to 654 mm/year. Over the 5 years since the 2014 pulse flow, EVILandsat declined 20% (13% by EVIMODIS) and ETLandsat‐EVI declined 23% (4% by ETMODIS‐EVI) with a 0.77 mm/day or a 209 mm/year 5‐year drop in ET; using PAM ETLandsat‐EVI the drop was from 863 to 654 mm/year. Data and change maps show the pulse flow contributed enough water to slow the rate of loss, but only for the very short‐term (1–2 years). These findings are critically important as they suggest further deterioration of biodiversity, wildlife habitat and key ecosystem services due to anthropogenic diversions of water in the U.S. and Mexico and from land clearing, fires and plant‐related drought which affect hydrological processes.
