Water relation characteristics ofAlhagi sparsifolia and consequences for a sustainable management

Water relation characteristics of the desert legumeAlhagi sparsifolia were investigated during the vegetation period from April to September 1999 in the foreland of Qira oasis at the southern fringe of the Taklamakan Desert, Xinjiang Uygur Autonomous Region of China. The seasonal variation of predawn water potentials and of diurnal water potential indicated thatAlhagi plants were well water supplied over the entire vegetation period. Decreasing values in the summer months were probably attributed to increasing temperatures and irradiation and therefore a higher evapotranspirative demand. Data from pressure-volume analysis confirmed thatAlhagi plants were not drought stressed and xylem sap flow measurements indicated thatAlhagi plants used large amounts of water during the summer months. Flood irrigation had no influence on water relations inAlhagi probably becauseAlhagi plants produced only few fine roots in the upper soil layers. The data indicate thatAlhagi sparsifolia is a drought-avoiding species that utilizes ground water by a deep roots system, which is the key characteristic to adjust the hyper-arid environment. Because growth and survival ofAlhagi depends on ground water supply, it is important that variations of ground water depth are kept to a minimum. The study will provide a theoretical basis for the restoration and management of natural vegetation around oasis in arid regions.

     

Water relation characteristics of<Emphasis Type="Italic">Alhagi sparsifolia</Emphasis> and consequences for a sustainable management

Water relation characteristics of the desert legumeAlhagi sparsifolia were investigated during the vegetation period from April to September 1999 in the foreland of Qira oasis at the southern fringe of the Taklamakan Desert, Xinjiang Uygur Autonomous Region of China. The seasonal variation of predawn water potentials and of diurnal water potential indicated thatAlhagi plants were well water supplied over the entire vegetation period. Decreasing values in the summer months were probably attributed to increasing temperatures and irradiation and therefore a higher evapotranspirative demand. Data from pressure-volume analysis confirmed thatAlhagi plants were not drought stressed and xylem sap flow measurements indicated thatAlhagi plants used large amounts of water during the summer months. Flood irrigation had no influence on water relations inAlhagi probably becauseAlhagi plants produced only few fine roots in the upper soil layers. The data indicate thatAlhagi sparsifolia is a drought-avoiding species that utilizes ground water by a deep roots system, which is the key characteristic to adjust the hyper-arid environment. Because growth and survival ofAlhagi depends on ground water supply, it is important that variations of ground water depth are kept to a minimum. The study will provide a theoretical basis for the restoration and management of natural vegetation around oasis in arid regions.     

Evaporation from seasonally frozen bare and vegetated ground at various groundwater table depths in the Ordos Basin,Northwest China

In cold climates, the process of freezing–thawing significantly affects the ground surface heat balance and water balance. To better understand the mechanism of evaporation from seasonally frozen soils, we performed field experiments at different water table depths on vegetated and bare ground in a semiarid region in China. Soil moisture and temperature, air temperature, precipitation, and water table depths were measured over a 5‐month period (November 1, 2016, to March 14, 2017). The evaporation, which was calculated by a mass balance method, was high in the periods of thawing and low in the periods of freezing. Increased water table depth in the freezing period led to high soil moisture in the upper soil layer, whereas lower initial groundwater levels during freezing–thawing decreased the cumulative evaporation. The extent of evaporation from the bare ground was the same in summer as in winter. These results indicate that a noteworthy amount of evaporation from the bare ground is present during freezing–thawing. Finally, the roots of Salix psammophila could increase the soil temperature. This study presents an insight into the joint effects of soil moisture, temperature, ground vegetation, and water table depths on the evaporation from seasonally frozen soils. Furthermore, it also has important implications for water management in seasonally frozen areas.     

Responses of distribution pattern of desert riparian forests to hydrologic process in Ejina oasis

By using the theories and methods of landscape ecology and the technology of GIS and RS, a study has been carried out on the responses of distribution pattern of desert riparian forest to hydrologic process on the basis of the hydrologic data from 1990 to 2000 and the TM image of 2001 year. The results showed that: (1) there appears an even distribution pattern for the relative forest area in oasis, however, the degenerated forest diaplays an increasing tendency from west to east; (2) the desert riparian forest in Ejina is in completely degenerated process at the patch scale; (3) the number of patch is influenced not only by hydrologic process,but also by agricultural activity such as cultivation. The severe deterioration of the degraded vegetation in whole oasis initiates from lower reaches, and gradually impels to upstream; the fragmentation of landscape in the terminal site is more obvious, which is influenced by river shape and decreasing flux of water. It is found that the influence of surface hydrologic process to the ground hydrologic process of desert riparian forest in Ejina oasis is little for the recent ten years. The relative area of the degenerated forest increased with increasing ground water depth in the direction of parallel to river channel. On the contrary, in the direction perpendicular to river channel, there is a decreasing tendency for the average patch area of the forest and the degenerated forest with increasing ground water depth.     

Responses of distribution pattern of desert riparian forests to hydrologic process in Ejina oasis

By using the theories and methods of landscape ecology and the technology of GIS and RS, a study has been carried out on the responses of distribution pattern of desert riparian forest to hydrologic process on the basis of the hydrologic data from 1990 to 2000 and the TM image of 2001 year. The results showed that: (1) there appears an even distribution pattern for the relative forest area in oasis, however, the degenerated forest diaplays an increasing tendency from west to east; (2) the desert riparian forest in Ejina is in completely degenerated process at the patch scale; (3) the number of patch is influenced not only by hydrologic process, but also by agricultural activity such as cultivation. The severe deterioration of the degraded vegetation in whole oasis initiates from lower reaches, and gradually impels to upstream; the fragmentation of landscape in the terminal site is more obvious, which is influenced by river shape and decreasing flux of water. It is found that the influence of surface hydrologic process to the ground hydrologic process of desert riparian forest in Ejina oasis is little for the recent ten years. The relative area of the degenerated forest increased with increasing ground water depth in the direction of parallel to river channel. On the contrary, in the direction perpendicular to river channel, there is a decreasing tendency for the average patch area of the forest and the degenerated forest with increasing ground water depth.     

The effects of partial rootzone drying on root, trunk sap flow and water balance in an irrigated pear (Pyrus communis L.) orchard

Partial rootzone drying (PRD) means that part of the root system is watered as in full irrigation while the rest is exposed to soil drying. This practice is predicted to influence field hydrological circle. We studied the effect of this practice on soil water distribution, root and trunk sap flow, water consumption of pear trees, and capillary contribution from ground water table and water balance for three months in an irrigated orchard with a shallow ground water table. The irrigation treatments included: (a) conventional flooded irrigation (CFI), (b) fixed partial rootzone drying (FPRD), and (c) alternate partial rootzone drying (APRD). Root and trunk sap flows were monitored using a heat-pulse sap flow meter. The results showed that there were significant differences of soil water content in both sides of rootzone under partial drying. The capillary contribution from ground water table was significantly increased in APRD and FPRD when compared with CFI. More significantly, the total irrigation amount was greatly reduced, by 43.64 and 45.84%, respectively, for APRD and FPRD. The two PRD treatments used more soil-stored water while CFI had more drainage. The root sap flow on the wet side was substantially enhanced as a result of PRD, and was greater than that from same side in CFI. The trunk sap flow in FPRD and APRD was smaller than that in CFI. On average, both APRD and FPRD reduced plant daily water consumption by about 9.96 and 17.97%, respectively, when compared to CFI during the PRD period. Daily root water flow was a significant function of the reference evapotranspiration. The daily trunk water flow was also related to the reference evapotranspiration but the CFI carried more water than APRD and FPRD under the same evaporation demand, suggesting a restriction of transpirational water loss in the PRD trees. CFI needed a higher soil water content to carry the same amount of trunk flow than the PRD trees, suggesting the hydraulic conductance of roots in PRD trees enhanced, and the roots had a greater water uptake capacity than in CFI when the average soil water content in the rootzone was the same.     

Modelling the effects of soil type and root distribution on shallow groundwater resources

Vegetated, shallow groundwater environments typically have high environmental and economic value. A sound understanding of the complex interactions and feedbacks between surface vegetation and groundwater resources is crucial to managing and maintaining healthy ecosystems while responding to human needs. A vegetated shallow groundwater environment was modelled using the software HYDRUS 2D to investigate the effects of several combinations of soil type and root distributions on shallow groundwater resources. Three rainfall regimes coupled to both natural and anthropogenically affected groundwater conditions were used to investigate the effect that combinations of four soil types and five root distributions can have on (a) groundwater level drops, (b) groundwater depletion, (c) groundwater recharge and (d) water stress conditions. Vegetation with roots distributed across the whole unsaturated zone and vegetation with dimorphic root systems (i.e. roots having larger concentrations both near the surface and the capillary fringe) behaved differently from vegetation growing roots mainly near the saturated zone. Specifically, vegetation with roots in the unsaturated zone caused water‐table drops and groundwater depletions that were half the amount due to deep‐rooted vegetation. Vegetation with a large portion of roots near the soil surface benefited from rainfall and was less vulnerable to water‐table lowering; as such, the fraction of the total area of roots affected by water stress conditions could be 40% smaller than in the case with deep‐rooted vegetation. However, roots uniformly distributed in the unsaturated zone could halve groundwater recharge rates observed in bare soils. Our analysis provided insights that can enable the formulation of site‐ and purpose‐specific management plans to respond to both human and ecosystem water requirements. Copyright © 2015 John Wiley & Sons, Ltd.     

USING A COMBINED SLOPE HYDROLOGY/STABILITY MODEL TO IDENTIFY SUITABLE CONDITIONS FOR LANDSLIDE PREVENTION BY VEGETATION IN THE HUMID TROPICS

The susceptibility of cut slopes to landsliding can be reduced in certain circumstances by the establishment of a vegetation cover. However, the hydrological implications of allowing a cover to develop may offset the mechanical benefits of soil reinforcement by roots. The balance between hydrological and mechanical effects is critical on slopes which are susceptible to the development of an infiltration-induced transitory perched water table, a common cause of landslides in deep, tropical residual soils. This balance is likely to change both between slopes of different types as well as temporally on any given slope. The net effect of a vegetation cover must be predicted either before natural vegetation covers are allowed to encroach on bare slopes, or if engineers are considering the use of trees as a protective measure. This paper presents a method of calculating the impact of a vegetation cover on slope stability. Simulations carried out on a wide range of slope types suggest that where failure is most likely to be triggered by infiltration rather than ground water rise, large-scale vegetation covers may contribute to instability. Whether vegetation had a positive or negative impact on slope stability was controlled by the permeability of the soil matrix, whilst the magnitude of impact was controlled by the soil strength and the slope height.     

Neural networks to simulate regional ground water levels affected by human activities

In arid regions, human activities like agriculture and industry often require large ground water extractions. Under these circumstances, appropriate ground water management policies are essential for preventing aquifer overdraft, and thereby protecting critical ecologic and economic objectives. Identification of such policies requires accurate simulation capability of the ground water system in response to hydrological, meteorological, and human factors. In this research, artificial neural networks (ANNs) were developed and applied to investigate the effects of these factors on ground water levels in the Minqin oasis, located in the lower reach of Shiyang River Basin, in Northwest China. Using data spanning 1980 through 1997, two ANNs were developed to model and simulate dynamic ground water levels for the two subregions of Xinhe and Xiqu. The ANN models achieved high predictive accuracy, validating to 0.37 m or less mean absolute error. Sensitivity analyses were conducted with the models demonstrating that agricultural ground water extraction for irrigation is the predominant factor responsible for declining ground water levels exacerbated by a reduction in regional surface water inflows. ANN simulations indicate that it is necessary to reduce the size of the irrigation area to mitigate ground water level declines in the oasis. Unlike previous research, this study demonstrates that ANN modeling can capture important temporally and spatially distributed human factors like agricultural practices and water extraction patterns on a regional basin (or subbasin) scale, providing both high-accuracy prediction capability and enhanced understanding of the critical factors influencing regional ground water conditions.     

Prescribed‐fire effects on rill and interrill runoff and erosion in a mountainous sagebrush landscape

Changing fire regimes and prescribed‐fire use in invasive species management on rangelands require improved understanding of fire effects on runoff and erosion from steeply sloping sagebrush‐steppe. Small (0·5 m²) and large (32·5 m²) plot rainfall simulations (85 mm h^–1, 1 h) and concentrated flow methodologies were employed immediately following burning and 1 and 2 years post‐fire to investigate infiltration, runoff and erosion from interrill (rainsplash, sheetwash) and rill (concentrated flow) processes on unburned and burned areas of a steeply sloped sagebrush site on coarse‐textured soils. Soil water repellency and vegetation were assessed to infer relationships in soil and vegetation factors that influence runoff and erosion. Runoff and erosion from rainfall simulations and concentrated flow experiments increased immediately following burning. Runoff returned to near pre‐burn levels and sediment yield was greatly reduced with ground cover recovery to 40 per cent 1 year post‐fire. Erosion remained above pre‐burn levels on large rainfall simulation and concentrated flow plots until ground cover reached 60 per cent two growing seasons post‐fire. The greatest impact of the fire was the threefold reduction of ground cover. Removal of vegetation and ground cover and the influence of pre‐existing strong soil‐water repellency increased the spatial continuity of overland flow, reduced runoff and sediment filtering effects of vegetation and ground cover, and facilitated increased velocity and transport capacity of overland flow. Small plot rainfall simulations suggest ground cover recovery to 40 per cent probably protected the site from low‐return‐interval storms, large plot rainfall and concentrated flow experiments indicate the site remained susceptible to elevated erosion rates during high‐intensity or long duration events until ground cover levels reached 60 per cent. The data demonstrate that the persistence of fire effects on steeply‐sloped, sandy sagebrush sites depends on the time period required for ground cover to recover to near 60 per cent and on the strength and persistence of ‘background’ or fire‐induced soil water repellency. Published in 2009 by John Wiley & Sons, Ltd.     

Assessing Ground Water Development Potential Using Landsat Imagery

Seven villages in southeastern Kenya surround Mt. Kasigau and depend on the mountain's cloud forest for their water supply. Five of these villages have regularly experienced water shortages, and all village water supplies were contaminated with Escherichia coli bacteria. There is a need to economically find new sources of fresh ground water. Remote sensing offers a relatively quick and cost-effective way of identifying areas with high potential for ground water development. This study used spectral properties of features on Landsat remote sensing imagery to map linear features, soil types, surface moisture, and vegetation. Linear features represented geologic or geomorphologic features indicating either shallow ground water or areas of increased subsurface hydraulic conductivity. Regarding soil type, black soils were identified as potential indicators of shallow aquifers based on their relatively lower elevation and association with river valleys. A vegetation map was created using unsupervised classification, and three of the resulting vegetation classes were observed to be commonly associated with wet areas and/or ground water discharge. A wetness map, created using tasseled cap analysis, was used to identify all areas of high ground moisture, including those that corresponded to vegetated areas. The linear features, soil type, vegetation, and wetness maps were overlaid to produce a composite that highlighted areas with the highest potential for ground water development. Electrical resistivity surveys confirmed that areas highlighted by the composite image had relatively shallow depths to the water table. Some figures in this paper are available in color in the online version of the paper.     

Soil water dynamics and water balance on a tropical coral island

Understanding soil water dynamics and the water balance of tropical coral islands is important for the utilization and management of their limited freshwater resources, which is only from rainfall. However, there is a significant knowledge gap in the influence of soil water on the water cycle of coral islands. Soil water dynamics and the water balance of Zhaoshu Island, Xisha Archipelago were thus investigated using soil moisture measurements and the Hydrus-1D model from October 2018 to September 2019. Over the study period, vegetation transpiration, soil evaporation, groundwater recharge and storage in the vadose zone were approximately 196, 330, 365 and 20 mm, occupying 22%, 36%, 40% and 2% of annual rainfall total (911 mm), respectively. For the wet season (from May to October) these values became 75, 202, 455 and 40 mm, occupying 10%, 26% and 59% and 5% of the seasonal rainfall total (772 mm), respectively. During the dry season (from November to April), a dry soil layer between 40 and 120 cm depth of the soil profile was identified that prevented water exchange between the upper soil layers and the groundwater resulting in the development of deep roots so that vegetation could extract groundwater to supplement their water requirements. Vegetation not only consumes all dry season rainfall (140 mm) but extracts water deeply from groundwater (90 mm) as well as from the vadose layer (20 mm). As such, the vegetation appears to be groundwater-dependent ecosystems. The research results aid us to better understand the process of water dynamics on coral islands and to protect coral island ecosystems.     

The use of natural tracers as indicators of soil-water movement in a temperate semi-arid region

In a semi-arid area of southern Australia a change in land use from Eucalyptus scrub to cropping with wheat is shown to have caused considerable change in the mechanism of the movement of soil water and the amount of deep drainage. Chloride concentrations of soil water have been used to show the mean annual amount of deep drainage increases from less than 0.1 to 3 mm yr.⁻¹ following clearing of the native vegetation.

The concentration of environmental tritium in soil water beneath the native vegetation is consistent with the hypothesis that some relatively recent water (post 1960) has penetrated to depths of at least 12m along channels occupied by living roots. Where the native vegetation has been cleared, no water which fell as rain since 1960 was found at depths greater than 2.5 m.

¹⁸O and deuterium concentrations suggest that some water movement to the roots of the native vegetation is in the vapour phase.     

Varying water utilization of Haloxylon ammodendron plantations in a desert‐oasis ecotone

Haloxylon ammodendron is a desert shrub used extensively in China for restoring degraded dry lands. An understanding of the water source used by H. ammodendron plantations is critical achieving sustainable vegetation restoration. We measured mortality, shoot size, and rooting depth in 5‐, 10‐, 20‐, and 40‐year‐old H. ammodendron plantations. We examined stable isotopic ratios of oxygen (δ¹⁸O) in precipitation, groundwater, and soil water in different soil layers and seasons, and in plant stem water to determine water sources at different shrub ages. We found that water acquisition patterns in H. ammodendron plantations differed with plantation age and season. Thus, the main water source for 5‐year‐old shrubs was shallow soil water. Water sources of 10‐year‐old shrubs shifted depending on the soil water conditions during the season. Although their tap roots could absorb deep soil water, the plantation main water sources were from soil water, and about 50% of water originated from shallow and mid soil. This pattern might occur because main water sources in these plantations were changeable over time. The 20‐ and 40‐year‐old shrubs acquired water mainly from permanent groundwater. We conclude that the main water source of a young H. ammodendron plantation was soil water recharged by precipitation. However, when roots reached sufficient depth, water originated mainly from the deep soil water, especially in the dry season. The deeply rooted 20‐ and 40‐year‐old shrubs have the ability to exploit a deep and reliable water source. To achieve sustainability in these plantations, we recommend a reduction in the initial density of H. ammodendron in the desert‐oasis ecotone to decelerate the consumption of shallow soil water during plantation establishment.     

鄱阳湖国家级自然保护区湿地植被的干旱响应及影响因素

近年来鄱阳湖干旱事件频发,干旱导致的气象水文要素变化直接影响植被生长状况,尤其是对于地上植被生物量的影响极为显著.研究鄱阳湖干旱事件对于湿地植被的影响,对于保护鸟类栖息地,认识湿地生态功能和结构的变化具有重要的现实意义.利用长期卫星遥感数据,结合植被生物量野外调查,以2003和2006年极端干旱年份为出发点,从湿地植被面积、生物量密度和总生物量的角度分析了鄱阳湖湿地植被生物量对于极端干旱的响应.研究表明:湿地植被面积、生物量密度以及总生物量均呈现双峰分布特征,在4和11月分别达到上、下半年的峰值.2003年植被生物量与多年均值一致,2006年下半年植被面积、生物量密度以及总生物量均明显超出多年均值.影响湿地植被面积的主要因素为鄱阳湖水位变化;而影响植被生物量密度的主要因素为气温和水位,退水时间提前对于生物量密度影响最大;总生物量同时受到植被面积与植被生物量密度的综合影响,其中植被面积的影响更大,植被面积对于总生物量的影响在2006年表现得比2003年更加显著.总之,2006年湿地植被对水文干旱的响应要比气象干旱强烈得多.     

Mapping groundwater‐dependent ecosystems using remote sensing measures of vegetation and moisture dynamics

Protection of groundwater‐dependent ecosystems (GDEs) is an important criterion in sustainable groundwater management, particularly when human water consumption is in competition with environmental water demands; however, the delineation of GDEs is commonly a challenging task. The Groundwater‐dependent Ecosystem Mapping (GEM) method proposed here is based on interpretation of the land surface response to the drying process derived from combined changes in two multispectral indices, the Normalised Difference Vegetation Index and the Normalised Difference Wetness Index, both derived from Landsat imagery. The GEM method predicts three land cover classes used for delineation of potential GDEs: vegetation with permanent access to groundwater; vegetation with diminishing access to groundwater; and water bodies that can persist through a prolonged dry period. The method was applied to a study site in the Ellen Brook region of Western Australia, where a number of GDEs associated with localised groundwater, diffuse discharge zones, and riparian vegetation were known. The estimated accuracy of the method indicated a good agreement between the predicted and known GDEs; Producer's accuracy was calculated as up to 91% for some areas. The method is most applicable for mapping GDEs in regions with a distinct drying period. Copyright © 2012 John Wiley & Sons, Ltd.     

Assessing herbicide concentrations in the saturated and unsaturated zone of a Chalk aquifer in southern England

The behavior of the herbicides isoproturon (IPU) and chlortoluron (CTU) in ground water and shallow unsaturated zone sediments were evaluated at a site situated on the Chalk in southern England. Concentrations of IPU in ground water samples varied from < 0.05 to 0.23 microgram/L over a five-year period of monitoring, and were found to correlate with application of the pesticide. Concentrations of pesticides in ground water samples collected during periods of rising water table were significantly higher than pumped samples and suggest that rapidly infiltrating recharge water contains higher herbicide concentrations than the native ground water. Significant variations in herbicide concentrations were observed over a three-month period in ground water samples collected by an automated system, with concentrations of IPU ranging from 0.1 to 0.5 microgram/L, and concentrations of a recent application of CTU ranging from 0.2 to 0.8 microgram/L. Different extraction methods were used to assess pore water concentrations of herbicides in the unsaturated zone, and samples were analyzed by standard HPLC analysis and immunoassay (ELISA) methods. These data indicated highly variable concentrations of herbicide ranging from 4 to 200 g/ha for HPLC and 0.01 to 0.04 g/ha for ELISA, but indicate a general pattern of decreasing concentrations with depth. The results of this study indicate that transport of IPU and CTU through the unsaturated zone to shallow ground water occurs and that this transport increases immediately following herbicide application. Measured concentrations of herbicides are generally lower than specified by the European Union Drinking Water Directive, but are observed to spike above this limit. These results imply that, while delivery of pesticides to ground water can occur as a result of normal agricultural practices, the impact on potable supplies is likely to be negligible due to the potential for degradation during the relatively long travel time through the unsaturated zone and high degree of dilution that occurs within the aquifer. As a result of the wide variation in concentrations detected by different techniques, it is suggested that for future site investigations more than one sampling strategy be employed to characterize the occurrence of pesticide residues and elucidate the transport mechanisms.     

Vegetation responses to integrated water management in the Ejina basin,northwest China

The Ejina basin, which is located in arid and semi‐arid areas of northwest China, has experienced severe environmental deterioration in the past several decades, and an exploratory project was launched by the Chinese Government in 2001 to restore this degraded ecosystem. In this study, multi‐scale remotely sensed data and field investigations were used to quantify the responses of vegetation to the implementation of integrated water management under this project. In terms of the seasonal accumulated Normalized Difference Vegetation Index (SAN) variation, (1) the vegetation in 80·4% of the oasis regions showed an increasing or recovering trend, and increasing SAN trends with a magnitude greater than 0·14 a^?1 mainly resulted from cultivated land reclamation; (2) the vegetation in 91·5% of the desert regions presented an increasing trend, and the statistically significant trends mainly appeared in the middle and lower Ejina basin; (3) the vegetation in 19·6% of oasis and 5·1% of desert regions showed a decreasing or degrading trend, mainly where rivers diminished and along artificial concrete canals; and (4) opposite signs of vegetation trends occurred simultaneously along some natural rivers experiencing water reduction, with a decreasing trend generally appearing in the high SAN regions, whereas an increasing trend was seen in the low SAN regions. The broad vegetation recovery observed was due to the comprehensive improvement of the water environment, which was attributed to both the increase in runoff entering the Ejina basin and the adoption of engineering measures. Vegetation degradation in the area mainly resulted from deterioration of the local water environment, which was closely related to the problems of water management. The results of this study can be used as a reference for adjusting the current water resource management strategy to effectively restore this ecosystem. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of remote sensing‐based evapotranspiration estimates using a water transfer numerical simulation under different vegetation conditions in an arid area

Daily actual evapotranspiration (AET) and seasonal AET values are of great practical importance in the management of regional water resources and hydrological modelling. Remotely sensed AET models and Landsat satellite images have been used widely in producing AET estimates at the field scale. However, the lack of validation at a high spatial frequency under different soil water conditions and vegetation coverages limits their operational applications. To assess the accuracies of remote sensing‐based AET in an oasis‐desert region, a total of 59 local‐scale daily AET time series, simulated using HYDRUS‐1D calibrated with soil moisture profiles, were used as ground truth values. Of 59 sampling sites, 31 sites were located in the oasis subarea and 28 sites were located in the desert subarea. Additionally, the locally validated mapping evapotranspiration at high resolution with internalized calibration surface energy balance model was employed to estimate instantaneous AET values in the area containing all 59 of the sampling sites using seven Landsat subimages acquired from June 5 to August 24 in 2011. Daily AET was obtained using extrapolation and interpolation methods with the instantaneous AET maps. Compared against HYDRUS‐1D, the remote sensing‐based method produced reasonably similar daily AET values for the oasis sites, while no correlation was observed for daily AET estimated using these two methods for the desert sites. Nevertheless, a reasonable monthly AET could be estimated. The correlation analysis between HYDRUS‐1D‐simulated and remote sensing‐estimated monthly AET values showed relative root‐mean‐square error values of 15.1%, 12.1%, and 12.3% for June, July, and August, respectively. The root mean square error of the summer AET was 10.0%. Overall, remotely sensed models can provide reasonable monthly and seasonal AET estimates based on periodic snapshots from Landsat images in this arid oasis‐desert region.     

Study on vegetation ecological water requirement in Ejina Oasis

The Ecological Water Requirement (EWR) of desert oasis is the amount of water required to maintain a normal growth of vegetation in the special ecosystems. In this study EWR of the Ejina desert oasis is estimated through the relational equation between normalized difference vegetation index (NDVI), productivity and transpiration coefficient, which was established by a combination of the RS, GIS, GPS techniques with the field measurements of productivity. The results show that about 1.53×108 m3 water would be needed to maintain the present state of the Ejina Oasis, and the ecological water requirement would amount to 3.49×108 m3 if the existing vegetation was restored to the highest productivity level at present. Considering the domestic water requirement, river delivery loss, oasis vegetation water con-sumption, farmland water demand, precipitation recharge, etc., the draw-off discharge of the Heihe River (at Longxin Mount) should be 1.93×108―2.23 ×108 m3 to maintain the present state of the Ejina Oasis, and 4.28×108―5.17×108 m3 to make the existing vegetation be restored to the highest productiv-ity level at present.