Stronger influences of grassland growth than grassland area on hydrological processes in the source region of the Yellow River

Concerns about regional hydrological responses to land use cover changes have arisen worldwide. However, the separation and quantification of the effects of changes in vegetation area and growth status on hydrological processes remains unclear. Here, we applied a distributed hydrology soil vegetation model (DHSVM) to analyze how the grassland growth state and surface area influence hydrological processes in the source region of the Yellow River (SRYR). The results showed that although the grassland surface area decreased slightly from 1981 to 2020, grassland growth (represented by the leaf area index) tended to increase from 2000. Grassland changes (including area and growth status) increased soil moisture storage and evapotranspiration to an overall extent of 2.03 % and 5.66 %, respectively; and most of the increase occurred after 2000 by 2.90 % and 9.40 %, respectively. These processes under grassland change led to an overall decrease in runoff of 2.41 %, with a much stronger decrease after 2000 of 4.3 %. Changes in grassland growth accounted for similar to 95 % of the runoff variations caused by grassland changes in the SRYR during the 40-year study period, compared to changes in grassland surface area. Simulations assuming different extents of grassland degradation suggest that such degradation can reduce evapotranspiration and soil moisture storage, thereby increasing runoff, whereas grassland restoration has the opposite effect. Our results highlighted the sensitivity of hydrological processes in the SRYR to grassland growth and the need for appropriate interventions to maintain and restore grasslands.