The hydrological impact of greening and climate change on the Mu Us Sandy land of China under the background of declining ecological efficiency

Extensive afforestation activities in the Mu Us Sandy Land (MUSL) of China have profoundly altered the underlying surface conditions, thereby impacting the carbon sequestration and water cycle. However, under the circumstances of the warming-humidifying trends in northwest China, it’s challenging to clarify direct and indirect effects of carbon sequestration increase and climate change on evapotranspiraton (ET) and water resource variation (represented by water yield, WY) in this typical fragile ecological zone. To address this, we calculated the Net Primary Productivity (NPP) by a modified CASA model, proposed an indicator (sensitivity of carbon sequestration, SC) to characterize the ecological efficiency of revegetation and found it decrease significantly (−0.77 a^-1; p < 0.05). Then we incorporated SC into a quantitative attribution analysis framework. The results indicated that the decoupled responses of ET and WY to six driving factors (NPP; precipitation, PRE; temperature, TEMP; vapor pressure deficit, VPD; solar radiation, RAD; wind speed, WS) presented significant spatio-temporal heterogeneity. Spatially, NPP dominated the positive and negative changes in ET and WY respectively, enhancing ET at a rate of 3.52 mm·a^-1 and suppressing WY at a rate of 4.12 mm·a^-1. Among climate factors, PRE and TEMP also explained 12 % and 8 % of ET change, respectively. Temporally, the interaction between NPP and ET steadily shrank at a rate of 0.003 a^-1, while that between most climate factors and ET increased to different degrees, indicating that ET may slightly shift from being driven by vegetation to being driven by climate. From the perspective of SC, we found that the lower the SC in an area, the greater the contribution of climate change to ET. While the unstable increase of PRE, decrease of SC, and the warming-humidifying trends were jointly intensifying the potential water resource crisis. The results help us balance the contradiction between ecological benefits and limited water resources.