Estimation of water budget components and its driving factors analysis in arid grassland

Clarifying the dependence of the grassland water budget change and its components on environmental factors is significant for the sustainable development of dryland ecosystems. Here, the Hydrus-1D model was used to simulate the water budget of natural grassland for 42 years (1980-2021). The standardized precipitation evapotranspiration index (SPEI) and soil moisture deficit index (SMDI) were used to analyze the soil drought evolution characteristics and the water use dynamic of the grassland in dry and wet years. Here, the calibrated Hydrus-1D model accurately identified the dynamic of grassland soil moisture in 2020 and 2021. The simulated data showed that evaporation (E) and transpiration (T_c) were the main pathways of soil water consumption, accounting for 96.5 % and 86.1 % of rainfall in dry and wet years, respectively. The soil water storage did not present a difference in precipitation proportion in dry (2.4 %) and wet (1.2 %) years, and the deep percolation accounted for a maximum of 12.8 % in wet years. Rainfall from 380 to 400 mm was the threshold, as it not only corresponds to the maximum water use efficiency (T_c/ET = 0.52, ET = E + T_c) but also serves as an important turning point for drought and deep percolation (below 150 cm) within the soil. The structural equation model further indicated that the dependence of E and T_c on meteorological factors was the main reason for the change of their proportions in dry and wet years. T_c was more sensitive to meteorological factors (R² = 0.63), while E was not (R² = 0.27). The SMDI had a greater impact on determining the threshold for water budget components than the SPEI. These results deepen the understanding of the hydrological process of grasslands in sandy areas, including the interaction between water budget components and environmental factors in wet and dry scenarios.