Attribution of decadal runoff changes by considering remotely sensed snow/ice melt and actual evapotranspiration in two contrasting watersheds in the Tienshan Mountains

Water processes in the Tienshan Mountains have undergone great changes under the impacts of climate warming and human activities. Here, based on the Budyko framework, we investigated the respective contributions of climate change and human activities to decadal runoff changes (1982–2014) in two contrary watersheds, i.e., the humid Kashi watershed in the Yili River valley and the arid Boertala watershed in the Ebinur Lake basin, in the Tienshan Mountains, where closing the water balance is difficult due to the scarcity of precipitation gauges. To achieve closure of the decadal water balance of the study watersheds, we estimated evapotranspiration from a complementary relationship model and snow/ice melt using the degree-day method with the remote sensing snow cover product. We found that the increase of rainfall (+61.6 mm) and snow/ice melt (+57.3 mm) dominated the increase of runoff (+87.2 mm, from 398.2 mm in 1982–1992 to 485.4 mm in 1993–2014) in the Kashi watershed, contributing 47.1 mm (54.8%) and 43.8 mm (51.0%), respectively, to the increase of runoff. Unlike the Kashi watershed, human activities contributed −13.8 mm (−103.7%) to the runoff (+13.9 mm, from 61.4 mm in 1982–1997 to 75.3 mm in 1998–2014) in the Boertala watershed. Most of the increase in rainfall (+62.5 mm) and snow/ice melt (+13.6 mm) was consumed by the increase of ET (+64.0 mm) instead of runoff. Further analysis showed that most of the increases in snow/ice melt and ET were contributed from the high (>2500 m) and low (<1500 m) elevation regions, respectively. Snowmelt showed a decreasing trend in low elevation regions. Our study highlighted the importance of snow/ice melt and ET in understanding decadal changes in runoff in the Tienshan Mountains.