Effects of Climate Variation and Vegetation Restoration on Runoff in Two Adjacent Forested Watersheds in Southwestern China

The Upper Minjiang River (UMR) basin is a water resource area for over 20 million inhabitants in the downstream Chengdu Plain and an ecological barrier for the Yangtze River Basin in China. The recently decreasing runoff trend in the UMR poses serious challenges to regional water resource security. However, the causes of runoff variations in such large high‐altitude forested basins remain poorly understood. In this study, we utilised two Budyko‐based models (the Choudhury–Yang equation and the Fu equation) to quantify and compare the driving factors of runoff changes in two adjacent high‐altitude forested watersheds in the UMR from 1982 to 2010. Our results showed that annual runoff had significant decreasing trends ( p < 0.05) from 1982 to 2010 in both watersheds, with an abrupt change point identified in 1995. Compared with Period I (1982–1994), mean annual runoff in Period II (1995–2010) decreased by 11.7% (−72.7 mm) in the Heishui (HS) watershed and 17.9% (−72.1 mm) in the Zhenjiangguan (ZJG) watershed, respectively. Both Budyko‐based models indicated that the changes in the underlying surface were the dominant factor contributing to runoff reduction in the two watersheds, followed by decreased precipitation. The two watersheds experienced vegetation restoration and exhibited greening trends from 1982 to 2010, and the forest areas increased by 10.5% in HS and 18.9% in ZJG from Period I to Period II. The changes in the underlying surface and the reduction of precipitation contributed to 61.4%–71.5% and 29.8%–39.1% of the runoff decline, respectively. This study contributes to an in‐depth understanding of the mechanisms of runoff alteration in high‐altitude forested basins under climate variation and vegetation restoration, and supports regional water resource management.