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Accurate estimates of evapotranspiration (ET) are essential for the conservation of ecosystems and sustainable management of water resources in arid and semiarid regions. Over the last two decades, several empirical remotely sensed ET models (ERSETMs) had been developed and extensively used for regional-scale ET estimation in arid and semiarid ecosystems. These ERSETMs were constructed by combining datasets from different sites and relating measured daily ET to corresponding meteorological data and vegetation indices at the site scale. Then, regional-scale ET on a pixel basis can be estimated, using the established ERSETMs. The estimation accuracy of these ERSETMs at the site scale plays a fundamental and crucial role in regional-scale ET estimation. Recent studies have revealed that ET estimates from some of these models have significant uncertainties at different spatiotemporal scales. However, little information is available on the performance of these ERSETMs at the site scale. In this study, we compared eight ERSETMs, using ET measurements from 2013 to 2018 for two typical eddy covariance sites (Tamarix site and Populus site) in an arid riparian ecosystem of Northwestern China, intending to provide a guide for the selection of these models. Results showed that the Nagler-2013 model and the Yuan-2016 model outperformed the other models. There were substantial differences in the ET estimation of the eight ERSETMs at daily, monthly, and seasonal scales. The mean ET of the growing season from 2013 to 2018 ranged from 465.93 to 519.65 mm for the Tamarix site and from 386.22 to 437.05 mm for the Populus site, respectively. The differences in model structures and characterization of both meteorological conditions and vegetation factors were the primary sources of different model performance. Our findings provide useful information for choosing models and obtaining accurate ET estimation in arid regions.

Widely distributed in arid and semiarid regions, phreatophytic roots extend into the saturated zone and extract water directly from groundwater. In this paper, we implemented a vegetation optimality model of root dynamics (VOM-ROOT) in the Noah land surface model with multi parameterization options (Noah-MP LSM) to model the extraction of groundwater through phreatophytic roots at a riparian site with a hyperarid climate (with precipitation of 35 mm/yr) in northwestern China. VOM-ROOT numerically describes the natural optimization of the root profile in response to changes in subsurface water conditions. The coupled Noah-MP/VOM-ROOT model substantially improves the simulation of surface energy and water fluxes, particularly during the growing season, compared to the prescribed static root profile in the default Noah-MP. In the coupled model, more roots are required to grow into the saturated zone to meet transpiration demand when the groundwater level declines over the growing season. The modeling results indicate that at the study site, the modeled annual transpiration is 472 mm, accounting for 92.3% of the total evapo- transpiration. Direct root water uptake from the capillary fringe and groundwater, which is supplied by lateral groundwater flow, accounts for approximately 84% of the total transpiration. This study demonstrates the importance of implementing a dynamic root scheme in a land surface model for adequately simulating phreatophytic root water uptake and the associated latent heat flux.

Accurate estimates of evapotranspiration (ET) in arid ecosystems are important for sustainable water resource management due to competing water demands between human and ecological environments. Several empirical remotely sensed ET models have been constructed and their potential for regional scale ET estimation in arid ecosystems has been demonstrated. Generally, these models were built using combined measured ET and corresponding remotely sensed and meteorological data from diverse sites. However, there are usually different vegetation types or mixed vegetation types in these sites, and little information is available on the estimation uncertainty of these models induced by combining different vegetation types from diverse sites. In this study, we employed the most popular one of these models and recalibrated it using datasets from two typical vegetation types (shrub Tamarix ramosissima and arbor Populus euphratica) in arid ecosystems of northwestern China. The recalibration was performed in the following two ways: using combined datasets from the two vegetation types, and using a single dataset from specific vegetation type. By comparing the performance of the two methods in ET estimation for Tamarix ramosissima and Populus euphratica, we investigated and compared the accuracy of ET estimation at the site scale and the difference in annual ET estimation at the regional scale. The results showed that the estimation accuracy of daily, monthly, and yearly ET was improved by distinguishing the vegetation types. The method based on the combined vegetation types had a great influence on the estimation accuracy of annual ET, which overestimated annual ET about 9.19% for Tamarix ramosissima and underestimated annual ET about 11.50% for Populus euphratica. Furthermore, substantial difference in annual ET estimation at regional scale was found between the two methods. The higher the vegetation coverage, the greater the difference in annual ET. Our results provide valuable information on evaluating the estimation accuracy of regional scale ET using empirical remotely sensed ET models for arid ecosystems.

Balancing human demands for water with environmental requirements to maintain functioning ecosystems requires the quantification of ecological water requirements. In arid regions, high spatial variability of vegetation cover and different water consumption of plant species make it different to estimate reasonable ecological water requirements. We developed a simple and practical approach that estimates the vegetation water requirements (VWRs) of desert riparian ecosystems. This model is species-specific and spatially-explicit; it considers the water consumption characteristics required by different species and highlights the impacts that high vegetation cover spatial variability has in arid regions on evapotranspiration. The model was parameterized based on the observation of the water consumption of two typical desert riparian species, Populus euphratica and Tamarix spp., in the lower basin of the Tarim River in northwestern China. Comparisons between the modeling results and measured data for two mature Populus and Tamarix stands indicate that the model is reasonable predictive. A case study in the lower basin of the Tarim River demonstrated the model’s practicality and transferability. This model could run based on near real-time or forest weather data and spatial vegetation patterns, and provides a continuous estimation of the temporal and spatial variations of the VWR. Particularly, this model forecasts VWRs under different vegetation spatial distribution and coverage scenarios, and evaluates the impacts and consequences of different management actions. This model can serve as a useful tool for management agencies interested in improving their decisions to allocate river water between human activities and natural ecosystems in arid regions.

The White method is a simple but the most frequently applied approach to estimate groundwater evapotranspiration (ETg) from groundwater level diurnal signals. Because of a lack of direct measurements of ETg, it is difficult to evaluate the performance of the White method, particularly in field environments with variable groundwater fluctuations. A 2-year field observation in a hyper-arid riparian tamarisk (Tamarix spp.) stand with deep groundwater depth in the lower Tarim River basin of China was conducted to measure the surface evapotranspiration (ETs) and groundwater table. The performance of the White method and the influences of the variable groundwater fluctuations on the determinations of the specific yield (S-y) and recharge rate of groundwater (r) in the White method were investigated. The results showed that the readily available S-y determined by Loheide's method was feasible but must be finely determined based on the soil textures in the layers in which the groundwater level fluctuated. A constant S-y value for a defined porous medium could be assumed regardless of the discharge or recharge processes of groundwater. The time span of 0000h to 0600h for r estimation for the White method worked best than other time spans. A 2-day moving average of r values further enhance ETg estimation. Slight effects of environmental or anthropogenic disturbances on the diurnal fluctuations of groundwater level did not influence the ETg estimations by the White method. Our results provide valuable references to the application of the White method for estimating daily ETg in desert riparian forests with deep groundwater depth. Copyright (c) 2015 John Wiley & Sons, Ltd.

Evapotranspiration (ET) and its controlling mechanism over the desert riparian forests in arid regions are the important scientific basis for the water resources managements of the lower reaches of the inland rivers of China. Nearly three years of continuous measurements of surface ET, soil water content at different depths and groundwater table over a typical Tamarix spp. stand and a typical Populus euphratica stand were conducted in the lower reach of the Tarim River. The ET seasonal trends in the growing season were controlled by plant phenology, and ET in non-growing season was weak. The diurnal variations of ET resulting from the comprehensive effects of all atmospheric factors were significantly related with reference ET. The spatial pattern of ET was determined by vegetation LAI, more vegetation coverage, more ET amount. Groundwater is the water source of surface ET, and the soil water in shallow layers hardly took part in the water exchange in the groundwater-soil-plant-air system. The temporal processes of ET over the Tamarix stand and the Populus stand were similar, but the water consumption of the well-grown Populus euphratica was higher than that of the well-grown Tamarix spp. Further analysis indicates that plant transpiration accounts for most of the surface ET, with soil evaporation weak and negligible; groundwater table is a crucial factor influencing ET over the desert riparian forests, groundwater influences the processes and amounts of ET by controlling the growth and spatial distribution of desert riparian forests; quantifying the water stress of desert riparian forests using groundwater table is more appropriate, rather than soil water content. Based on the understanding of ET and water movements in the groundwater-soil-plant-air system, a generalized framework expressing the water cycling and its key controlling mechanism in the lower reaches of the inland rivers of China is described, and a simple model to estimate water requirements of the desert riparian forests is presented.

Determining the maintenance and control mechanisms of the energy and water exchanges over the desert riparian forest zone in the lower Tarim River of China could improve our understanding of land surface processes of the groundwater-dependent ecosystems in the hinterland area of the Eurasian continent under hyper-arid climate. Nearly three years of continuous measurements of meteorological factors, surface water and energy fluxes, soil water content, and groundwater table were carried out over a riparian Tamarix spp. stand in the lower Tarim River basin. Because of scarce precipitation and low air humidity, the diurnal and seasonal variations of most meteorological factors exhibited simple unimodal dynamics. However, the seasonal variation of latent heat exchange (LE) was distinctly related to plant phenology. The sensible heat exchange (H) exhibited reverse course with the LE seasonal course, which suggested seasonal variation of the H was controlled by the LE course. In the daytime, a good linear relationship existed between evapotranspiration (ET) and reference evapotranspiration. The yearly ET over the tamarisk stand was approximately 500 mmyear(-1), and the mean daily ET is about 3.85 mm d(-1) in a vibrant growing season. The groundwater table and the soil water content (SWC) near the groundwater table exhibited obvious seasonal and diurnal variations in the growing seasons but SWC in the shallow soil layer did not, suggesting the groundwater was the water source of the tamarisk. Our analysis indicated that under a hyper-arid climate, the energy and water exchanges over riparian tamarisk stands in the lower Tarim River basin showed certain features: (1) plant transpiration accounted for most of the surface ET, with soil evaporation weak and negligible; (2) the seasonal processes of surface energy and water exchanges were highly related with plant phenology; (3) the diurnal processes of ET resulted from the comprehensive effects of all atmospheric factors; and (4) ET depends on groundwater, rather than precipitation. (C) 2014 The Authors. Published by Elsevier B.V.

Global climate change has significant impacts on agricultural production. Future climate change will bring important influences to the food security. The CERES-Wheat model was used to simulate the winter wheat growing process and production in Panzhuang Irrigation District (PID) during 2011–2040 under B2 climate scenario based on the Special Report on Emissions Scenarios (SRES) assumptions with the result of RCMs (Regional Climate Models) projections by PRECIS (Providing Regional Climates for Impacts Studies) system introduced to China from the Hadley Centre for Climate Prediction and Research. The CERES-Wheat model was calibrated and validated with independent field-measured growth data in 2002–2003 and 2007–2008 growing season under current climatic conditions at Yucheng Comprehensive Experimental Station (YCES), Chinese Academy of Sciences (CAS). The results show that a significant impact of climate change on crop growth and yield was noted in the PID study area. Average temperature at Yucheng Station rose by 0.86°C for 1961–2008 in general. Under the B2 climate scenario, average temperature rose by 0.55°C for 2011–2040 compared with the baseline period (1998–2008), which drastically shortened the growth period of winter-wheat. However, as the temperature keep increasing after 2030, the top-weight and yield of the winter wheat will turn to decrease. The simulated evapotranspiration shows an increasing trend, although it is not very significant, during 2011–2040. Water use efficiency will increase during 2011–2031, but decrease during 2031–2040. The results indicate that climate change enhances agricultural production in the short-term, whereas continuous increase in temperature limits crop production in the long-term.

The eddy covariance flux measurements from 2003 to 2006 at a tropical rain forest site in Xishuangbanna, southwest China, were used to study the ecosystem evapotranspiration (ET). During the four study years, the mean annual precipitation and air temperature were slightly lower than their long-term averages (1322 vs 1487 mm and 20.1 vs 21.7 degrees C, respectively). Mean annual ET in the four study years was estimated at 1029 mm, accounted for 78% of the corresponding annual precipitation. Mean daily ecosystem ET was 2.6 mm day(-1) during the dry season from November to April and was 3.1 mm day(-1) during the rainy season from May to October. The ET diurnal variations in the cool-dry season (from November to February), the hot-dry season (March and April) and the rainy season had similar maximum values, although the soil water content, leaf area index (LAI) and climate conditions differed greatly. ET was mainly controlled by soil water availability in the hot-dry season, by LAI in the early rainy season (May and June) and by atmospheric conditions in the mid-to-late rainy season (from July to October) and the cool-dry season. The total ET was substantially higher than the corresponding precipitation in the dry season. The extra amount of water evapotranspirated in the dry season was mainly due to the depletion of soil water stored in the previous rainy season. Fog deposition during the dry season also played a role in providing water for ET. Our results indicated the importance of interannual interactions of water balance in the seasonal distribution of ecosystem ET at this site. Copyright (C) 2010 John Wiley & Sons, Ltd.

Three macroscopic root water uptake models (Molz-Remson, Feddes, and Selim-Iskandar) have been widely used in modeling water flow in soil-plant systems. In this article, we use evapotranspiration data and soil water content data obtained from lysimeter measurements and root distribution data obtained in a nearby wheat field to evaluate the accuracy of these root water uptake models in predicting the soil water content profiles in the winter wheat field. To account for the role of root distribution, we modified the Molz-Remson model and the Selim-Iskandar model with the Feddes reduction function, and the Feddes model with the root length density. These models, with and without modifications, were then individually incorporated into the soil water flow equations as a sink term. The flow equations were solved numerically with the measured evapotranspiration data as input, and the predicted soil water content profiles were compared with the measured profiles to evaluate the validity of the root water uptake models. The comparison showed that: (1) the average deviation of the Molz-Remson (linear version) model, the Feddes model, and the Selim-Iskandar model could reach as high as 25,70, and the non-linear version of the Molz-Remson model performed slightly better, with an average deviation of 13%; (2) modifications made to the Molz-Remson models and the Selim-Iskandar model did not achieve any improvement in the model predictions, but the modified Feddes model significantly improved the prediction accuracy, reducing the average deviation to 5.6%.