Is the runoff coefficient increasing or decreasing after ecological restoration on China's Loess Plateau?

Abstract The “Grain for Green” project (GGP) was launched in 1999 on China's Loess Plateau to reduce soil erosion, which had far-reaching impacts on the local eco-hydrological processes. In this study, we use monthly runoff depth and precipitation datasets spanning 1961 to 2015 for 16 primary basins of the plateau to reveal changes in runoff generation capacity before and after the GGP. We use a Budyko-based elasticity method to calculate the runoff depth (R1) and runoff coefficient (C1) exclusively attributable to land use/cover change. Results indicate that the mean annual runoff coefficients (C0) decline by 26%–76% from the periods 1961–1999 to 2000–2015. The annual observed runoff depth (R0) and C0 for 75% of basins show significant downward trends during 1961–1999; after implementation of the GGP, both annual R0 and annual C0 for over 50% of basins show upward trends. The study further finds that the increase of erosive rainfall during the period 2000–2015, whose mean increasing rate reaches 4.6 mm/year, is the main reason for the upward trends of R0 and C0. After removing the effect of precipitation variation during this period, we find that 11 out of 16 basins show decreased trends for C1, with the downward rate between 2.4% and 6.0% per year. The reduction rate in semi-arid areas is about four times the rate in semi-humid areas. The results remind us to consider the carrying capacity of local water resources when implementing soil and water conservation measures across the Loess Plateau.