Identification of the dominant factors and altitudinal variation in water use efficiency in the Qinling-Daba Mountains

Water use efficiency (WUE) is commonly expressed as the ratio of gross primary productivity (GPP) to evapotranspiration (ET) and reflects the integrated impacts of water, energy, and carbon cycles on ecosystem processes. Understanding the spatiotemporal variations of WUE in the Qinling-Daba Mountains and its influencing factors will contribute to identifying the characteristics of the carbon and water cycles in Qinling-Daba Mountains within the context of climate change. This study analyzed spatiotemporal variations of WUE, GPP to transpiration ratio (GPP/T), transpiration to ET ratio (T/ET), soil moisture (SM), leaf area index (LAI), and vapor pressure deficit (VPD) in the Qinling-Daba Mountains and explored the dominant factors influencing changes in WUE. The study produced several interesting results: (1) The Distance between Indices of Simulation and Observation for the PML-GPP, PML-ET, SMCI, and MOD15A2H datasets are 0.89, 0.87, 1.22, and 1.02, respectively. Hence, they most accurately represent the conditions of GPP, ET, SM, and LAI in the Qinling-Daba Mountains, respectively. (2) GPP/T and WUE show similar spatiotemporal trends, unlike T/ET, which differs significantly. (3) At the regional scale, the contribution rate of the increase in GPP/T and the decrease in T/ET to the increase and decrease in WUE are 74.5 % and 106.83 %, respectively. At both regional and pixel scales, the increase in GPP/T and the decrease in T/ET respectively dominate the increase and decrease in WUE. (4) The increase in LAI, decrease in VPD, and increase in SM indirectly dominate the increase in WUE by controlling the increase in GPP/T at altitudes below 1000 m, between 1000 and 2700 m, and above 2700 m, respectively. The decrease in LAI indirectly dominates the decrease in WUE by controlling the decrease in T/ET at different altitude ranges. The interactions between meteorological and vegetation factors constrain WUE variations in the Qinling-Daba Mountains.