Effects of climate variation and vegetation restoration on runoff in two adjacent forested watersheds in southwestern China

The Upper Minjiang River basin (UMR) is a water resource area for over 20 million inhabitants in the downstream Chengdu Plain and ecological barrier for the Yangtze River Basin in China. The recently decreasing runoff in the UMR poses serious challenges to regional water resource security, but thus to partition the contributions of climate variation and vegetation restoration on runoff variation is of great significance for regional water resource management and ecological restoration. This study utilized two Budyko-based models (Choudhury-Yang equation and Fu equation) to quantify the influences of climate variation and vegetation restoration on runoff changes in two adjacent 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 the both watersheds, with an abrupt change point identified in 1995. Compared with the period I (1982–1994), mean annual runoff in the period II (1995–2010) decreased by 11.7% (-72.7 mm) and 17.9% (-72.1 mm) in the Heishui (HS) watershed and Zhenjiangguan (ZJG) watershed, respectively. Both 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 changes in Ep had negligible influences on runoff declines. The two watersheds experienced vegetation restoration and exhibited greening trends from 1982 to 2010. Compared with the period I, the forest areas in the HS and ZJG during the period II increased by 10.5% and 18.9%, respectively. The changes in the underlying surface and the reduction of precipitation contributed to 60.8%–69.9% and 29.0%–38.7% of the runoff decline, respectively. This study contributes to an in-depth understanding of the mechanisms of runoff alteration in forested watershed under climate variation and vegetation restoration, and supports regional water resource management.