Effects of inter-basin transfers on watershed hydrology and vegetation greening in a large inland river basin

The effects of inter-basin transfers (IBTs) on watershed hydrological balances and associated ecosystem processes remain poorly understood in arid regions because of data scarcity and the complexity of ecosystem responses to water management in many parts of the world. To fill this gap, the objective of this study was to quantify the effect of IBTs on watershed hydrological regimes and the associated vegetation dynamics. We conducted a case study on the Shiyang River Basin, a typical mountain-oasis-desert inland river basin in northwestern China, using water balance and wavelet analysis methods. Long-term (1980-2020) monitoring data from river discharge and groundwater tables were used to construct the water balances and quantify the periodicity of the surface water and groundwater. We observed that over 2.90 billion m3 of transferred water during 2003-2020 mitigated the declining trend in groundwater, resulting in a change in water storage trend from -91.9 mm/y to -53.7 mm/y. IBTs contributed approximately 230 % of the increase in observed river runoff, 21 %-60 % (42 % on average) of the increase in total water storage, and 1 %-32 % (12 % on average) of the evapotranspiration deficit (i.e., the portion of actual evapotranspiration beyond climate-driven evapotranspiration). IBTs also significantly altered the seasonal fluctuation and periodicity of river flows and groundwater. Increased water availability promoted vegetation recovery and resulted in multiscale resonance periodicities with vegetation, ranging from monthly to interannual scales. Our results indicate that IBTs markedly modified the water cycle, hydrological regimes, and ecosystem processes in the study basin. Thus, we call for long-term monitoring of the timing, frequency, and magnitude of ecohydrological modifications from IBTs to provide critical information for effective watershed management policies to achieve long-term sustainable development in water-stressed regions.