Spatiotemporal water yield variations and influencing factors in the Lhasa River Basin, Tibetan Plateau

Climate and land use/cover changes are widely recognized as two main drivers of variations in ecosystem services including water yield. However, vegetation cover condition, which can also influence the hydrological cycle through evapotranspiration process, is seldom considered. In this study, we used the Seasonal Water Yield Model (SWYM) to assess the spatiotemporal water yield changes of Lhasa River Basin from 1990 to 2015, and analysed its influencing factors by focusing on precipitation change, land cover change, and vegetation cover change (indexed by Normalized Difference Vegetation Index, i.e. NDVI). We first examined the model through Morris Screening sensitivity analysis and validated it with observed flow data. Spatiotemporal variation of three indices of water yield, baseflow, quick flow and local recharge, were then assessed. To analyse the contribution of each factor to water yield change, three scenarios were built in which one factor was altered at a time. Results showed that, the precipitation and vegetation cover change were substantial during the study period, while land cover change was quite small. From 1990 to 2015, the baseflow, local recharge and quick flow decreased by 67.03%, 80.21% and 37.03% respectively, with the change mainly occurring during 2000-2010. The spatial pattern of water yield remained mostly unchanged. The upstream area had relatively high baseflow and local recharge, and was the main contributor of quick flow. The downstream area had relatively low or even zero baseflow, and most of its local recharge was negative due to high evapotranspiration. According to contribution analysis, precipitation and vegetation cover change were the main factors affecting water yield in the Lhasa River Basin. For baseflow, the influence of precipitation change was, on average, 7.98 times as big as vegetation cover change, and the influence of vegetation cover change was, on average, 115.45 times as big as land cover change. However, land cover change began to exert greater influence after 2010. We suggest that besides climate and land use/cover change, vegetation cover change should also be studied in greater depth to fully understand its effect on regional hydrological process and ecosystem service provision.