The change of annual stream flow in the Shiyang river basin, a typical arid-inland basin in north-west China, was investigated using hydrological, meteorological and water-related human activities' data of the past 50 years. The long-term trends of the hydrological time series were examined by non-parametric techniques, including the Pettitt and Mann-Kendall tests. Double cumulative curves and multi-regression methods were used to separate and quantify the effects of climate changes and human activities on the stream flows. The results show that the study area has been experiencing a significant upward warming trend since 1986 and precipitation shows a decreasing trend in the mountainous region but an increasing trend in the plains region. All stream flows in the upper reach and lower reaches of the Shiyang river exhibit decreasing tendencies. Since 1970, human activities, such as irrigation, have had a significant effect on the upstream flow, and account for 60% of total flow decreases in the 1970s. However, climate changes are the main reason for the observed flow decreases in the 1980s and 1990s, with contributions to total flow decrease of 68% and 63%, respectively. Before 1975, flow decreases in the upper reaches were the main factor causing reduced flows in the lower reaches of the Shiyang river. After 1975, the effect of human activities became more pronounced, with contributions of 63%, 68% and 56% to total flow decreases in the lower reaches of the Shiyang river in the periods 1975 to 1980, 1980s and 1990s, respectively. As a result, climate change is responsible for a large proportion of the flow decreases in the upstream section of the catchment during the 1980s and 1990s, while human activities have caused flow decreases downstream during the same period.

Abstract:Tamarix elongata Ledeb is a desert shrub found in the desert region of Northwest China and is commonly cultivated as a sand-holding plant in this region. To understand its water requirement and the effects of climate conditions on its growth, trunk xylem sap flows of irrigated 8-year-old Tamarix elongata Ledeb plants were monitored continuously with heat-pulse sap flow meters for the entire season. Soil moisture contents at 0-300 cm layer depth were also measured with a tube type time domain reflectometry (Tube-TDR). Meteorological factors, i.e. solar radiation, air temperature, relative humidity and wind speed were simultaneously monitored by an automatic weather station at the site. Daily and seasonal variations of the trunk sap fluxes and their correlations with the meteorological factors, reference evapotranspiration and soil moisture contents in the root-zone were analysed. The results indicated that frost influenced the trunk sap flux greatly under irrigated conditions, although the flux generally fluctuated with the variation of environmental factors and showed a mean trunk sap flux of 4Ð18 l d1 . There was a significantly exponential relationship between sap flux and the reference value of crop evapotranspiration, with a correlation coefficient of R 2 D 0Ð7172. The sap flux also had a significant correlation with the soil water contents at a depth of 150-300 cm from soil surface (R 2 D 0Ð5014). The order of the main meteorological factors affecting the sap flux of Tamarix elongata Ledeb trees was solar radiation > air temperature > vapour pressure deficit > relative humidity > wind speed.

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