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Climate change, population growth and extreme events can trigger social crises and instability. The processes that dominate a society’s emergence, resilience and collapse, and the complex interactions among such processes, operating within a small region, at a multicentury or even larger time scale, remain to be identified. The causes or driving forces responsible for societal changes must be identified for a plausible explanation. Historical records provide unique examples of societies that have failed to develop buffers and strategic resilience against climate change and natural variability. Using a wide range of observations from China’s Hexi Corridor, the complex interactive processes linking climate change with human society over the past two millennia were investigated. This paper proposes a domino effect resulting from a society’s failure to respond to climate change in which individual small problems create a greater challenge over long time spans. Building resilience against the impacts of climate change requires a deep understanding of social and environmental feedbacks to create a reliable buffer against future changes. This study offers lessons learned from the past 2,000 years that remain relevant today, given the projected changes in climate and extreme events. Social responses to changes can trigger further crises. This study uses historical data and finds society’s failures to respond to climate change in China’s Hexi Corridor amplified problems.

Populus euphratica Oliv. is a relatively ‘conservative’ water user even when growing in favourable water conditions, but the mechanistic understanding of this has received little attention. We undertook an experiment to determine trees water use by measuring stem and root sap flow (Fs), variation in stem diameter (Ds), leaf stomatal gas-exchange (e.g. conductance, gs and transpiration, Tr) and water potential $$\left( {{\psi _{\text{L}}}} \right)$$ during the growing season for P. euphratica. There was a hysteretic ‘apparent feedforward’ of stomatal response to increasing VPD. Mean of gs was not significantly different among months. $${\psi _{\text{L}}}$$ was negatively related to gs and Tr, but in contrast, stem Fs was positively associated to Tr but not to gs. There was no lag in the daily onset and cessation of Fs between the bottom and top of the trunk, possibly due to the short distance between measurement points (about 2 m), however, the lag time in Fs between the bottom of the trunk and roots, approximately 30 min, suggested that stored water was withdrawn first from the trunk and subsequently the roots. Daily contraction of Ds (− Ds) increased with increasing Fs during both day and night, and expansion of Ds (+ Ds) showed a logarithmic rise to a maximum with increasing Fs during the day. Day Fs and − Ds were also logarithmic with respect to VPD, with a correlation coefficient equal to 0.51 and 0.50, respectively (P

Accurate projection of future precipitation is a major challenge due to the uncertainties arising from the atmospheric predictors and the inherent biases that exist in the global circulation models. In this study, we employed multivariate recursive nesting bias correction (MRNBC) and multiscale wavelet entropy (MWE) to reduce the bias and improve the projection of future (i.e., 2006–2100) precipitation with artificial intelligence (AI)-based data-driven models. Application of the developed method and the subsequent analyses are performed based on representative concentration pathway (RCP) scenarios: RCP4.5 and RCP8.5 of eight Coupled Model Intercomparison Project Phase-5 (CMIP5) Earth system models for the upstream of the Heihe River. The results confirmed the MRNBC and MWE were important statistical approaches prudent in simulation performance improvement and projection uncertainty reduction. The AI-based methods were superior to linear regression method in precipitation projection. The selected CMIP5 outputs showed agreement in the projection of future precipitation under two scenarios. The future precipitation under RCP8.5 exhibited a significantly increasing trend in relative to RCP4.5. In the future, the precipitation will experience an increase by 15–19% from 2020 to 2050 and by 21–33% from 2060 to 2090.

Accurate and reliable river flow forecasts attained with data-intelligent models can provide significant information about future water resources management. In this study we employed a 50-model ensemble of three data-driven predictive models, namely the support vector regression (SVR), multivariate adaptive regression spline (MARS) and M5 model tree (M5Tree) to forecast river flow data in a semiarid and ecologically significant mountainous region of Pailugou catchment in northwestern China. To attain stable and accurate forecast results, 50 different models were trained by randomly sampling the entire river flow data into 80% for training and 20% for testing subsets. To attain a complete evaluation of the ensemble-model based results, the global mean of six quantitative statistical performance evaluation measures: the coefficient of correlation (R), mean absolute relative error (MAE), root mean squared error (RMSE), Nash–Sutcliffe efficiency coefficient (NS), relative RMSE, and the Willmott’s Index (WI), and Taylor diagrams, including skill scores relative to a persistence model, were selected to assess the performances of the developed predictive models. The results indicated that all of the averaged R value attained was higher than 0.900 and all of the averaged NS values were higher than 0.800, representing good performance of the SVR, MARS and M5Tree models applied in the 1-, 2- and 3-day ahead modeling horizon, and this also accorded with the deductions made through an assessment of the Willmott’s Index. However, the M5Tree model outperformed both the SVR and MARS models (with NS = 0.917 vs. 0.904 and 0.901 for 1-day, 0.893 vs. 0.854 and 0.845 for 2-day, and 0.850 vs. 0.828 and 0.810 for 3-day forecasting horizons, respectively), which was in concurrence with the high value of WI. Therefore, based on the ensemble of 50 models, the performance of the M5Tree can be considered as superior to the SVR and MARS models when applied in a problem of river flow forecasting at multiple forecast horizon. A detailed comparison of the overall performance of all three models evaluated through Taylor diagrams and boxplots indicated that the 1-day ahead forecasting results were more accurate for all of the predictive models compared to the 2- and 3-day ahead forecasting horizons. Data-intelligent models designed in this study indicate that the M5Tree method could successfully be explored for short-term river flow forecasting in semiarid mountainous regions, which may have useful implications in water resources management, ecological sustainability and assessment of river systems.

Evidence exists of nighttime transpiration and its potential impact on plant/water relations for species in a diversity of ecosystems. However, relevant data related to typical desert riparian forest species remains limited. Accordingly, we measured sap flow velocity of Populus euphratica using the heat ratio method between 2012 and 2014. Nocturnal stem sap flow was separated into nighttime and stem refilling using the “forecasted refilling” method. Nighttime transpiration was observed for each phenophase. The highest value was during the full foliation period but lowest during leaf expansion and defoliation periods. The contribution of nighttime transpiration to daytime transpiration was an average of 15% but this was comparatively higher during the defoliation period. Relationships between nighttime transpiration, vapor pressure deficits, and air temperatures were more closely associated than with wind speed in all phenophases. Moreover, we found that nighttime transpiration linearly correlated to vapour pressure deficit during the first and the full foliation periods, but nighttime transpiration showed exponential correlations to air temperatures during the same phenophases. Additionally, environmental drivers of transpiration were significantly different between nighttime and daytime (P

Understanding forest ecosystem evapotranspiration (ET) is crucial for water-limited environments, particularly those that lack adequate quantified data such as the lower Heihe River basin of northwest China which is primarily dominated by Tamarix ramosissima Ledeb. and Populus euphratica Oliv. forests. Accordingly, we selected the growing season for 2 years (2012 and 2014) of two such forests under similar meteorological conditions to compare ET using the eddy covariance (EC) technique. During the growing seasons, daily ET of T. ramosissima ranged from 0.3 to 8.0 mm day−1 with a mean of 3.6 mm day−1, and daily ET of P. euphratica ranged from 0.9 to 7.9 mm day−1 with a mean of 4.6 mm day−1 for a total of 548 and 707 mm, respectively. The significantly higher ET of the P. euphratica stand was directly linked to high soil evaporation rates under sufficient water availability from irrigation. When the soil evaporation was disregarded, water use was comparable to two contrasting riparian forests, a P. euphratica forest with a total transpiration of 465 mm and a T. ramosissima forest with 473 mm. Regression analysis demonstrated that climate factors accounted for at least 80% of ET variation in both forest types. In conclusion, water use of the riparian forests was low and comparable in this arid region, that suggest the long-term plant adaptation to the local climate and conditions of water availability.

Prediction of groundwater depth (GWD) is a critical task in water resources management. In this study, the practicability of predicting GWD for lead times of 1, 2 and 3 months for 3 observation wells in the Ejina Basin using the wavelet-artificial neural network (WA-ANN) and wavelet-support vector regression (WA-SVR) is demonstrated. Discrete wavelet transform was applied to decompose groundwater depth and meteorological inputs into approximations and detail with predictive features embedded in high frequency and low frequency. WA-ANN and WA-SVR relative of ANN and SVR were evaluated with coefficient of correlation (R), Nash-Sutcliffe efficiency (NS), mean absolute error (MAE), and root mean squared error (RMSE). Results showed that WA-ANN and WA-SVR have better performance than ANN and SVR models. WA-SVR yielded better results than WA-ANN model for 1, 2 and 3-month lead times. The study indicates that WA-SVR could be applied for groundwater forecasting under ecological water conveyance conditions.

The study deals with the distribution of nutrients in wetland sediments, which provide the basis for revealing the wetland eutrophication processes and mechanisms of internal pollution sources. The total organic carbon (TOC), total nitrogen (TN), and total phosphorus (TP) contents and distribution characteristics of sediment samples were examined. The results showed that the TOC concentration ranged from 3.81 to 15.6 g/kg, the TN concentration ranged from 0.21 to 1.18 g/kg with a mean concentration of 0.66 g/kg, and the TP concentration ranged from 0.16 to 0.35 g/kg with a mean of 0.23 g/kg. Statistical analysis showed close correlations between TOC and TN (R2 = 0.96), and TN and TP (R2 = 0.97), which indicated that the TN and TP in the sediments were from similar sources. The concentrations of TOC, TN, and TP in Longbao Lake wetland sediments were too low for eutrophication to occur. Our investigation indicated that Longbao Lake undergoes natural evolution rather than anthropogenic activities.

Extensive research has found that nighttime transpiration (E n) is positively correlated to the vapour pressure deficit (VPD), that suggested E n was highest during the night under high temperatures and low humidity along with high soil water availability, typically for the riparian forest in the extreme arid region of China. This study used the heat ratio method to measure sap velocity (V s) for mature and saplings Populus euphratica Oliv., and then E n was conservatively calculated as total nocturnal sap flow (F s, the product of V s and sapwood area A s) between 01:00 to 06:00. A gas exchange system was used to measure the leaf transpiration rate (T r) and stomatal conductance (g s) of saplings. For mature trees, nighttime V s was extensive and logarithmic correlated to VPD (similar to daytime). For saplings, g s and T r was extensive in different months, and also a strong logarithmic relationship was found between V s and VPD for both daytime and nighttime periods. Both of stem sap flow and leaf gas exchange suggusted the occurrence of E n, whether mature or sapling trees. E n contribution to daily transpiration (E d) was high just as expected for P. euphratica, which was confirmed by proportional E n to E d (E n/E d) means taken in 2012 (24.99%) and 2013 (34.08%). Compared to mature trees, E n/E d of saplings in 2013 was lower with means of 12.06%, that supported further by the shorter duration times and less T r,n (16.64%) and g s,n (26.45%) of leaf, suggesting that E n magnitude is associated to individual the tree size, that effect to stored water of individual trees, although this hypothesis requires further research.

Being an important desert riparian forest in the lower reaches of the Heihe River Basin, Populus euphratica Oliv. forest functions as a natural barrier in maintaining and preserving the stability of local oases. Accordingly, accurately estimating the water use of P. euphratica is important for vegetation protection and water resource allocation. To date, little data are available for evaluating the hysteretic effects between sap flow and environmental variables, and for estimating the water use of desert riparian forest. In this study, we measured the sap flow velocity (V s ) of P. euphratica using the heat ratio method during the growing season of 2012. Based on the response of V s to solar radiation (R s ) and vapor pressure deficit (VPD), we estimated the hourly Vs and daily Vs using the multivariable linear regression and a modified Jarvis-Stewart (JS) model, respectively. Hysteretic response of R s to environmental variables was then evaluated using a sap flow model. We found the thresholds of R s responses to R s and VPD at both hourly and daily scales during the growing season, and successfully estimated the seasonal variations of hourly R s and daily R s using the JS model. At an hourly scale, the maximum R s occurred earlier than the maximum VPD by approximately 0.5 h but later than the maximum R s by approximate 1.0 h. At a daily scale, the maximum R s lagged behind the maximum VPD by approximately 2.5 h while occurred earlier than the maximum R s by approximately 2 h. However, hysteretic response of V s was weakened when R s and VPD were measured together using the JS model at both hourly and daily scales. Consequently, short-term and intensive field campaigns, where V s and environmental variables can be measured, may be used to estimate short-run sap flow and stand transpiration using only two environmental variables.

Spatial variations of soil microbial activities and its relationship with environmental factors are very important for estimating regional soil ecosystem function. Based on field samplings in a typical saline groundwater-irrigated region, spatial variations of soil microbial metabolic activities were investigated. Combined with groundwater quality analysis, the relationship between microbial activities and water salinity was also studied. The results demonstrated that moderate spatial heterogeneity of soil microbial activities presented under the total dissolved solids (TDS) of groundwater ranging from 0.23 to 12.24 g L(-1). Groundwater salinity and microbial activities had almost opposite distribution characteristics: slight saline water was mainly distributed in west Baqu and south Quanshan, while severe saline and briny water were dominant in east Baqu and west Huqu; however, total AWCD was higher in the east-center and southwest of Baqu and east Huqu, while it was lower in east Baqu and northwest Huqu. The results of correlation analyses demonstrated that high-salinity groundwater irrigation had significantly adverse effects on soil microbial activities. Major ions Ca(2+), Mg(2+), Cl(-), and SO4(2-) in groundwater decisively influenced the results. Three carbon sources, carbohydrates, amines, and phenols, which had minor utilization rates in all irrigation districts, were extremely significantly affected by high-salinity groundwater irrigation. The results presented here offer an approach for diagnosing regional soil ecosystem function changes under saline water irrigation.

Evapotranspiration is a major factor that controls hydrological process and its accurate estimation provides valuable information for water resources planning and management, particularly in extremely arid regions. The objective of this research was to evaluate the use of a support vector machine (SVM) to model daily reference evapotranspiration (ET0) using limited climatic data. For the SVM, four combinations of maximum air temperature (T max ), minimum air temperature (T min ), wind speed (U 2 ) and daily solar radiation (R s ) in the extremely arid region of Ejina basin, China, were used as inputs with T max and T min as the base data set. The results of SVM models were evaluated by comparing the output with the ET0 calculated using Penman–Monteith FAO 56 equation (PMF-56). We found that the ET0 estimated using SVM with limited climatic data was in good agreement with those obtained using the conventional PMF-56 equation employing the full complement of meteorological data. In particular, three climatic parameters, T max , T min , and R s were enough to predict the daily ET0 satisfactorily. Moreover, the performance of SVM method was also compared with that of artificial neural network (ANN) and three empirical models including Priestley-Taylor, Hargreaves, and Ritchie. The results showed that the performance of SVM method was the best among these models. This offers significant potential for more accurate estimation of the ET0 with scarce data in extreme arid regions.

Accurately predicting groundwater level (GWL) fluctuations is one of the most important issues for managing groundwater resources. In this study, the feasibility of predicting weekly GWL fluctuations in a coastal aquifer using the wavelet-adaptive neuro-fuzzy inference system (WANFIS) was investigated. WANFIS was a conjunction model that combined discrete wavelet transform and adaptive neuro-fuzzy inference system (ANFIS). GWL data of two wells located in the coastal aquifer of eastern Laizhou bay, China, were used to establish WANFIS model. The performances of WANFIS model, along with ANFIS model, were assessed in terms of the following statistical indices, such as coefficient of correlation (R), root mean square error, and mean absolute relative error. Compared with the best ANFIS models, the best WANFIS model gave a better prediction. Moreover, it was found that wavelet transform positively affected the ANFIS's predicting ability. In addition, the WANFIS model was also found to be superior to the best ANN model. This study indicated that WANFIS model was preferable and could be applied successfully due to its high accuracy and reliability for predicting GWL.

Inland lakes are a major water resource for arid regions. Determining basin scale is therefore an important indicator of variability for water resources in such regions. The goal of this study was to ascertain the appropriate Juyan Lake basin scale and its minimum ecological water requirement for the lake’s continued preservation. This was accomplished in three parts. First, an analysis was carried out on the inflow of the lower reach of Heihe River and any subsequent water surface area change to Juyan Lake. Second, an optimum Juyan Lake basin scale was determined by establishing existing relationships between reservoir capacity and lake area, water level and lake area, and variation in lake area during different times of the year. Third, the ecological water requirement of the lake was determined. Results showed that the decrease in surface runoff and ceased flow of the channel resulted in the drying up of the terminal lake of Heihe River. It was determined that 35.6 km2 is the most stable Juyan Lake surface area, and 0.55 × 108 m3 is the minimum replenishment quantity in which to maintain Juyan Lake throughout the year without it drying up.

Desert riparian forests are the main body of natural oases in the lower reaches of inland rivers; its growth and distribution are closely related to water use sources. However, how does the desert riparian forest obtains a stable water source and which water sources it uses to effectively avoid or overcome water stress to survive? This paper describes an analysis of the water sources, using the stable oxygen isotope technique and the linear mixed model of the isotopic values and of desert riparian Populus euphratica forests growing at sites with different groundwater depths and conditions. The results showed that the main water source of Populus euphratica changes from water in a single soil layer or groundwater to deep subsoil water and groundwater as the depth of groundwater increases. This appears to be an adaptive selection to arid and water-deficient conditions and is a primary reason for the long-term survival of P. euphratica in the desert riparian forest of an extremely arid region. Water contributions from the various soil layers and from groundwater differed and the desert riparian P. euphratica forests in different habitats had dissimilar water use strategies.

Most soil respiration measurements are conducted during the growing season. In tundra and boreal forest ecosystems, cumulative, non-growing season soil CO2 fluxes are reported to be a significant component of these systems’ annual carbon budgets. However, little information exists on soil CO2 efflux during the non-growing season from alpine ecosystems. Therefore, comparing measurements of soil respiration taken annually versus during the growing season will improve the accuracy of estimating ecosystem carbon budgets, as well as predicting the response of soil CO2 efflux to climate changes. In this study, we measured soil CO2 efflux and its spatial and temporal changes for different altitudes during the non-growing season in an alpine meadow located in the Qilian Mountains, Northwest China. Field experiments on the soil CO2 efflux of alpine meadow from the Qilian Mountains were conducted along an elevation gradient from October 2010 to April 2011. We measured the soil CO2 efflux, and analyzed the effects of soil water content and soil temperature on this measure. The results show that soil CO2 efflux gradually decreased along the elevation gradient during the non-growing season. The daily variation of soil CO2 efflux appeared as a single-peak curve. The soil CO2 efflux was low at night, with the lowest value occurring between 02:00–06:00. Then, values started to rise rapidly between 07:00–08:30, and then descend again between 16:00–18:30. The peak soil CO2 efflux appeared from 11:00 to 16:00. The soil CO2 efflux values gradually decreased from October to February of the next year and started to increase in March. Non-growing season Q10 (the multiplier to the respiration rate for a 10°C increase in temperature) was increased with raising altitude and average Q10 of the Qilian Mountains was generally higher than the average growing season Q10 of the Heihe River Basin. Seasonally, non-growing season soil CO2 efflux was relatively high in October and early spring and low in the winter. The soil CO2 efflux was positively correlated with soil temperature and soil water content. Our results indicate that in alpine ecosystems, soil CO2 efflux continues throughout the non-growing season, and soil respiration is an important component of annual soil CO2 efflux.

Aims As deep water uptake is limited by the low densities of fine roots at the subsoil layer at high evaporative demand, hydraulic redistribution (HR) is another possible mechanism that could account for the lack of water stress experienced by desert riparian phreatophytes during dry periods. The objectives of this study were to search for evidence that the roots of two desert riparian phreatophytes, Populus euphratica Oliv. and Tamarix ramosissima Ledeb., carry out HR and to investigate the pattern of this phenomenon in the broader scheme of plant water uptake by roots. Methods To demonstrate HR, we present data on patterns of sap flow in the stems or branches and lateral roots of those two phreatophyte species and soil volumetric moisture content where these species grow. Results During the dry season, we observed reverse or acropetal flow in the lateral roots of P. euphratica ,a pattern consistent with hydraulic lift. With the onset of heavy rains, this pattern reversed, indicating water movement from moist topsoil to dry subsoil, i.e. hydraulic descent. After lateral irrigation by creek, water moved downward to dry subsoil and outward to opposite sides, suggesting lateral hydraulic redistribution via roots at night and during the day, which may be mediated by stem tissues and, by inference, the radial sectoring in the xylem. Although no direct evidence indicated reverse sap flow of lateral roots and associated HR in T. ramosissima, several factors indicate that HR is occurring: (1) diel fluctuations of volumetric moisture content in the upper soil layer and (2) the identification of primary water sources as groundwater and vadose zone water through stable isotope studies. As a result, we inferred that HR occurs in T. ramosissima via adventitious roots with diameters of 2– 5m m and length of 60–100 cm in the upper soil layer, rather than via lateral roots; further investigation is needed to substantiate this. Conclusions We confirm and extend previous knowledge on HR patterns of P. euphratica and add a new species, T. ramosissima, to the wide list of existing species involved in HR. However, these two desert riparian phreatophytes exhibit remarkable differences

Tamarix spp. (Saltcedar) is a facultative phreatophyte that can tolerate drought when groundwater is not accessed. In addition to deep water uptake, hydraulic redistribution (HR) is another factor contributing to the drought tolerance of Tamarix spp. In this study, data on soil volumetric moisture content (θ), lateral root sap flow, and relevant climate variables were used to investigate the patterns, magnitude, and controlling factors of HR of soil water by roots of Tamarix ramosissima Ledeb. in an extremely arid land in Northwest China. Results showed evident diurnal fluctuations in θ at the depths of 30 and 50 cm, indicating “hydraulic lift” (HL). θ increased remarkably at 10 and 140 cm but decreased at 30 and 50 cm and slightly changed at 80 cm after rainfall, suggesting a possible “hydraulic descent” (HD). However, no direct evidence was observed in the negative flow of lateral roots, supporting HR (including HL and HD) of T. ramosissima. The HR pathway unlikely occurred via lateral roots; instead, HR possibly occurred through adventitious roots with a diameter of 2–5 mm and a length of 60–100 cm. HR at depths of 20–60 cm ranged from 0.01–1.77 mm/d with an average of 0.43 mm/d, which accounted for an average of 22% of the estimated seasonal total water depletion at 0–160 cm during the growing season. The climate factors, particularly vapor pressure deficit and soil water potential gradient, accounted for at least 33% and 45% of HR variations with depths and years, respectively. In summary, T. ramosissima can be added to the wide list of existing species involved in HR. High levels of HR may represent a considerable fraction of daily soil water depletion and substantially improve plant water status. HR could vary tremendously in terms of years and depths, and this variation could be attributed to climate factors and soil water potential gradient.

Ecologic patterns and community succession are generally controlled by hydrologic mechanisms, especially for plant distributions which are sensitive to habitat conditions. The hydrology characteristics of ecosystems mainly influenced plant ecological processes in water and salinity changes. In this paper, we analyzed the composition and characteristic of natural plant community, divided the plant community classes and discussed the effect of water and salinity gradients on plant species and community classes in Ejina Desert Oasis. The results demonstrated that Populus euphratica, Tamarix chinensis and Phragmites communis were the most important plant species that had the highest important values among forest, shrubs and herbaceous. Six plant community patterns were classified by cluster analysis in Ejina Desert Oasis. Species richness and species diversity were the highest near West River and East River channels of the core oasis area. The distributions of plant community were mainly influenced by the following factors: distance from river channel, groundwater level, soil water content, soil salinity and groundwater salinity. The water and salinity factors, which controlled the distributions of plant, were the main driving forces for ecosystem succession. The plant community succession is becoming toward the type of shrub + herb or low shrub with very drought-tolerant from the type of forest + shrub + herb with tall and high water consumption, when habitat conditions change from good to poor. The water gradients had more significant and more directed effect than salinity gradients on plant species and communities.

Water resources have been overexploited for agricultural irrigation and industrial production in the upper and middle reaches of the Heihe River, northwestern China. Due to inadequate water resources management, the runoff entering into the lower reaches has been continuously reduced in recent years. The Heihe River is the primary recharge source for the groundwater of the lower reaches, so the decrease in runoff has caused the groundwater level to decline. As a result, a series of ecological and environmental problems has now appeared in the lower reaches, including river-flow interruptions, drying up of associated lakes, degeneration of vegetative cover and so on. In view of these issues, the National Water Diversion Project was put into practice in July 2000. It has significantly increased the quantity and frequency of flows entering into the lower reaches of the Heihe River, and has recharged the groundwater and improved the water quality to some degree along the length of the river. The water deliveries have had obvious influences on the groundwater in the lower reaches. The groundwater level increase and groundwater quality improvement have been of great benefit in restoring the ecological environment that was destroyed in past years.

Water resources is a primary controlling factor for economical development and ecological environmental protection in the inland river basins of arid western China. Groundwater, as the important component of total water resources, plays a dominant role in the development of western China. In recent years, with the utilization ratio of surface water raised, the groundwater recharge rate has been reduced by surface water, and groundwater was exploited on a large-scale. This has led to the decline of groundwater levels and the degradation of eco-environments in the lower reaches of Heihe watershed, especially. Therefore, the study on the groundwater-level change in recent years, as well as simulating and predicting groundwater levels changes in the future is very significant to improve the ecological environment of the Heihe River Basin, coordinate the water contradiction, and allocate the water resources. The purpose of this study is to analyze the groundwater-level variations of the Ejina region basin on a large-scale, to develop and evaluate a conceptual groundwater model in Ejina Basin; according to the experimental observation data, to establish the groundwater flow model combining MODFLOW and GIS Software; simulated the regional hydrologic regime in recent 10 years and compared with various delivery scenarios from midstream; determined which one would be the best plan for maintaining and recovering the groundwater levels and increasing the area of Ejina Oasis. Finally, this paper discusses the possible vegetation changes of Ejina Basin in the future.

Field experiments on the CO2 flux of alpine meadow soil in the Qilian Mountain were conducted along the elevation gradient during the growing season of 2004 and 2005. The soil CO2 flux was measured using the Li-6400-09 soil respiration chamber attached to the Li-6400 portable photosynthesis system. The effects of water and heat and roots on the soil CO2 flux were statistically analyzed. The results show that soil CO2 flux along the elevation gradient gradually decreases. The soil CO2 flux was low at night, with lowest value occurring between 0200 and 0600 hours, started to rise rapidly during 0700–0830 hours, and then descend during 1600–1830 hours. The peak CO2 efflux appears during 1100–1600 hours. The diurnal average of soil CO2 efflux was between 0.56 ± 0.32 and 2.53 ± 0.76 μmol m−2 s−1. Seasonally, soil CO2 fluxes are relatively high in summer and autumn and low in spring and winter. The soil CO2 efflux, from the highest to the lowest in the ranking order, occurred in July and August (4.736 μmol m−2 s−1), June and September, and May and October, respectively. The soil CO2 efflux during the growing season is positively correlated with soil temperature, root biomass and soil water content.

The soil-plant system is a very important subsystem of the soil-plant-atmosphere continuum (SPAC). The water uptake by plant roots is an important subject in the research on water transport in this SPAC and is also the most active study direction in the fields of ecology, hydrology and environment. The study of the spatial distribution pattern of fine roots of plants is the basis of constructing a water absorption model of plant roots. Our study on the spatial distribution pattern of fine roots of Populus euphratica in a desert riparian forest shows that the density distribution of its root lengths can be expressed horizontally as a parabola. The fine roots are concentrated within the range of 0–350 cm from the tree trunk and their amount accounts for 91.9% of the total root mass within the space of 0–500 cm. In the vertical direction, the density distribution of the fine root lengths shows a negative exponential relation with soil depth. The fine roots are concentrated in the 0–80 cm soil layer, accounting for 96.8% of the total root mass in the 0–140 cm soil layer.

Groundwater vulnerability is a cornerstone in evaluating the risk of groundwater contamination and developing management options to preserve the quality of groundwater. Based on the professional model (DRASTIC model) and geographical information system (GIS) techniques, this paper carries out the shallow groundwater vulnerability assessment in the Zhangye Basin. The DRASTIC model uses seven environmental parameters (depth to water, net recharge, aquifer media, soil media, topography, impact of vadose zone, and hydraulic conductivity) to characterize the hydrogeological setting and evaluate aquifer vulnerability. According to the results of the shallow groundwater vulnerability assessment, the Zhangye Basin can be divided into three zones: low groundwater vulnerability risk zone (risk index 140). Under the natural conditions, the middle and high groundwater vulnerability risk zones of the Zhangye Basin are mainly located in the groundwater recharge zones and the important cities. The high, middle and low groundwater vulnerability risk zones of the Zhangye Basin cover around 17, 21 and 62% of study area, respectively.

The microclimate of a desert oasis in hyperarid zone of China was monitored using micrometeorological methods and compared with those of areas adjacent to forested land. Differences in ground-level photosynthetically active radiation (PAR) on clear, cloudy and dust storm days and their subtending causes are analysed and discussed. Desert oases serve the ecological functions of altering solar radiation, adjusting near-ground and land surface temperatures, reducing soil temperature differences, lowering wind velocity, and increasing soil and atmospheric humidity. The total solar radiation in the interior of the oasis was roughly half of that outside a forest canopy. During the growing season, air temperatures in Populus euphratica Oliv. (poplar) and Tamarix ramosissima Ledeb. (tamarisk) forests were 1.62 °C and 0.83 °C lower respectively than those in the areas around the forests. Furthermore, the taller the forest cover, the greater the temperature drops; air temperatures in the upper storey were greater than those in the lower storey, i.e., air temperature rose with increasing height. Over the growing season, the relative humidities of the air in the poplar and tamarisk forests were 8.5% and 4.2% higher respectively than those in areas around the forests. Mean wind velocity in poplar-forested lands was 0.33 m s−1, 2.31 m s−1 lower than that in the surrounding area. During dust storm days the PAR was significantly lower than that on cloudy or clear days, when it was high and varied in an irregular manner.

Given the increasing threat of water shortages in the extremely arid regions (rainfall

Nighttime sap flow is a potentially important factor that affects whole-plant water balance and water-use efficiency (WUE). Its functions include predawn disequilibrium between plant and soil water potentials as well as between the increments of oxygen supply and nutrient uptake. However, main factors that drive nighttime sap flow remain unclear, and researches related to the relationship between nighttime sap flow velocity and environmental factors are limited. Accordingly, we investigated the variations in the nighttime sap flow of Populus euphratica in a desert riparian forest of an extremely arid region, Northwest China. Results indicated that P. euphratica sap flow occurred throughout the night during the growing season because of the partial stomata opening. Nighttime sap flow for the P. euphratica forest accounted for 31%–47% of its daily sap flow during the growing season. The high value of nighttime sap flow could be the result of high stomatal conductance and could have significant implications for water budgets. Throughout the whole growing season, nighttime sap flow velocity of P. euphratica was positively correlated with the vapor pressure deficit (VPD), air temperature and soil water content. We found that VPD and soil water content were the main driving factors for nighttime sap flow of P. euphratica.