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During the night, plant water loss can occur either through the roots, as hydraulic redistribution (HR), or through the leaves via the stoma, as nocturnal transpiration (En), which was methodologically difficult to separate from stem refilling (Re). While HR and En have been reported across a range of species, ecosystem, and climate zone, there is little understanding on the interactions between En and/or Re and HR. As water movement at night occurs via gradients of water potential, it is expected that during periods of high atmospheric vapor pressure deficit (VPD), water loss via En will override water loss via HR. To test this hypothesis, sap flow in stems and roots of Populus euphratica Oliv. trees, growing in a riparian zone in a hyperarid climate, was measured once in a year. Nocturnal stem sap flow was separated into En and Re using the “forecasted refilling” method. Substantial nocturnal sap flow (38% of 24-hr flux on average) was observed and positively correlated with VPD; however, the strength of the correlation was lower (R2 = .55) than diurnal sap flow (Ed) (R2 = .72), suggesting that nocturnal stem sap flow was attributed to both water loss through the canopy and replenishment of water in stem tissues. Partitioning of nocturnal sap flow shows that Re constituted approximately 80%, and En ~20%, of nocturnal sap flow. The amount of root sap flow attributed to redistribution was negatively related to Ed (R2 = .69) and the amount of acropetally sap flow in stems, Re (R2 = .41) and En (R2 = .14). It was suggested that the magnitude of HR is more strongly depressed by Re that was recharge to the water loss via Ed than by En. It was consistent with whole-tree water balance theory, that the nighttime upward sap flow to xylem, stem refilling and transpiration, may depress hydraulic redistribution of roots. [ABSTRACT FROM AUTHOR]

Field experiments were conducted in the lower reaches of the Heihe River basin in the northwest of China to determine relationships between stream and groundwater and to quantify stream lateral seepage. The water table fluctuation, water balance and statistical analysis methods were used to analyze long-term experimental data. Results showed that the groundwater depth along the experimental section responded strongly to the streamflow of the East River in the basin. The streamflow of the East River in all periods significantly influenced the amplitude of groundwater depth within the range of 300 m from the riverbank; the stream lateral seepage was mainly controlled by the streamflow and its durations. The evapotranspiration of riparian forest had used larger proportions of groundwater storage in summer, close to approximating 60%. This study quantified stream lateral seepage to groundwater for different water conveyance and provides support for regional water resources management in an arid inland river basin. [ABSTRACT FROM AUTHOR]

In this study, the ability of a wavelet analysis-artificial neural network (WA-ANN) conjunction model for multi-scale monthly groundwater level forecasting was evaluated in an arid inland river basin, northwestern China. The WA-ANN models were obtained by combining discrete wavelet transformation with ANN. For WA-ANN model, three different input combinations were trialed in order to optimize the model performance: (1) ancient groundwater level only, (2) ancient climatic data, and (3) ancient groundwater level combined with climatic data to forecast the groundwater level for two wells in Zhangye basin. Based on the key statistical measures, the performance of the WA-ANN model was significantly better than ANN model. However, WA-ANN model with ancient groundwater level as its input yielded the best performance for 1-month groundwater forecasts. For 2- and 3-monthly forecasts, the performance of the WA-ANN model with integrated ancient groundwater level and climatic data as inputs was the most superior. Notwithstanding this, the WA-ANN model with only ancient climatic data as its inputs also exhibited accurate results for 1-, 2-, and 3-month groundwater forecasting. It is ascertained that the WA-ANN model is a useful tool for simulation of multi-scale groundwater forecasting in the current study region. [ABSTRACT FROM AUTHOR]

Analysis of the impacts of climate variability on evapotranspiration (ET) is of great importance in understanding climate variability and its effect on hydrological aspects. In this study, temporal and spatial variations in reference evapotranspiration (ET0) and actual evapotranspiration (AET) were comprehensively detected in the Taohe River Basin from 1981 to 2010. The spatial distributions of annual sensitivity of ET0 and AET to climatic variables were investigated. The quantitative contributions, the possible causes and dominant controlling factors were analyzed. The results suggested the following. (1) Considerable changes in ET0 occurred due to climate variability in the Gannan Plateau area (GPA) and Loess Plateau area (LPA) by 3.54 mm/yr and 3.39 mm/yr from 1981 to 2010, respectively. (2) Net solar radiation was the most sensitive factor on ET0. The dominant factor was air temperature leading to increase in ET0 due to high sensitive coefficient to ET0 and also exhibited significant increasing magnitude. (3) Precipitation (PPT) was the most sensitive factor for AET. AET in the GPA is controlled by air temperature and in LPA is controlled by PPT. This study provides a beneficial reference to agriculture, water resource and eco-environment management strategies in this region for associated policymakers and stakeholders. [ABSTRACT FROM AUTHOR]