Impacts of altitudinal ecohydrological dynamic changes on water balance under warming climate in a watershed of the Qilian Mountains, China

In alpine areas of northwest China, one of the major concerns is the rapid warming and stimulated vegetation growth consume more water and reduce available water for downstream oasis development. Investigating the response of these ecohydrological dynamics to climate change is thus crucial, but is also challenging because of tremendous variability of vegetation, hydrology, and climate in elevation and complex interactions between them. Here, we performed numerical simulations in a mountainous watershed covering a range of contrasting climatic conditions and vegetation characteristics representative of the Qilian Mountains, China. The simulations were run with a dynamic global vegetation model LPJ-WHyMe to quantify spatiotemporal changes of vegetation (e.g., species and net primary production (NPP)) and hydrological components (e.g., runoff and evapotranspiration (ET)) in recent decades (1982-2018). The simulated results were compared with those derived from MODIS and observations. Results show that the favorable climate condition for vegetation growth appears around the freezing altitude (3000-3250 m asl) where the NPP, ET, and water use efficiency (WUE = NPP/ET) exhibit a 'humped' peak value while runoff increases with precipitation towards higher altitudes. The warming and moistening climate and elevated CO2 since 1982 have favored vegetation growth, leading to uphill migration of the treeline and the 'humped' peak to higher elevation (3250-4000 m asl). The climate warming and stimulated vegetation growth consumed 83 % of the increased precipitation (34.6 mm) in the whole catchment. The greatly increased ET and thus decreased runoff were found at high elevation (above 4000 m asl) due to the reduced freezing days (up to 43 days) in the warming climate. Meanwhile, the substantially elevated WUE and moistening climate can balance the increased ET in the vegetation occupation areas below 4000 m asl, leading to increased runoff. The results indicate that despite a little increase of runoff in the recent decades, stimulated vegetation growth and climate warming could reduce water resources availability in the mountains if climate warming continues and/or climate wetting ceases in future.