Different responses of soil water to warming controlled by evapotranspiration during grassland degradation: A case study in the typical steppe of China

• Grassland degradation affects evapotranspiration mainly through influencing transpiration. • The effect of warming on evapotranspiration gradually increases with the degree of grassland degradation. • Grassland degradation alters soil water by modifying evapotranspiration. • Soil water exhibits varying responses to warming influenced by evapotranspiration components. Worldwide grasslands are experiencing degradation due to overgrazing and the effects of climate change, and warming-induced changes in degraded grasslands could accelerate degradation. As one of the direct manipulating factors of grassland degradation, soil water is governed by climatological and anthropogenic processes. Evapotranspiration (ET) plays a key role in regulating soil water content in these processes, but the underlying mechanisms remain unclear. In this study, a two-year field experiment was conducted in different degraded grasslands. The calibrated Hydrus-1D model was applied to identified the dynamic of ET and soil water in 2022 and 2023 under normal condition and warming (+1.5 °C) scenarios. ET decreased slightly with grassland degradation, mainly due to the reduction of transpiration (T). T/ET decreased as well, 57.87 %∼69.06 %, 51.71 %∼65.60 %, 31.76 %∼49.44 % and 15.54 %∼22.61 % in undegraded control (CK), light degraded (LD), moderately degraded (MD) and heavy degraded (HD) grasslands, respectively, during growing seasons. With the aggravation of grassland degradation, the influence of warming on ET was gradually enhanced. Soil water content (SWC) responded differently to degraded degrees. Additionally, warming showed more significant effects on SWC in LD and MD grasslands than in HD grassland. Furthermore, ET contribution to regulating SWC response to degradation and warming in LD and MD grasslands at 30–100 cm soil depth. Compared to other meteorological factors, vapor pressure deficit (VPD) exhibited significantly stronger correlation with T, and this correlation weakened gradually with grassland degradation, indicating a diminished regulatory role of vegetation in response to environmental moisture changes. Grassland T/ET during the growing season could serve as a potential parameter for predicting vegetation productivity. In dry conditions, irrigation at critical growth stages in the field can be implemented to achieve optimal results in enhancing water use efficiency and degraded grassland restoration. These findings not only can improve our understanding of the hydrological processes and water budget during grassland degradation, but also provide valuable guidelines for the management of water resources and the restoration of ecological environment.