Response of runoff and its components to climate change in the Manas River of the Tian Shan Mountains

Abstract

A warming–wetting climate trend has led to increased runoff in most watersheds in the Tian Shan Mountains over the past few decades. However, it remains unclear how runoff components, that is, rainfall runoff (R_rain), snowmelt runoff (R_snow), and glacier meltwater (R_glacier), responded to historical climate change and how they will evolve under future climate change scenarios. Here, we used a modified Hydrologiska Byråns Vattenbalansavdelning (HBV) model and a detrending method to quantify the impact of precipitation and temperature changes on runoff components in the largest river (Manas River) on the northern slope of the Tian Shan Mountains from 1982 to 2015. A multivariate calibration strategy, including snow cover, glacier area, and runoff was implemented to constrain model parameters associated with runoff components. The downscaled outputs of 12 general circulation models (GCMs) from the Sixth Coupled Model Intercomparison Project (CMIP6) were also used to force the modified HBV model to project the response of runoff and its components to future (2016–2100) climate change under three common socio-economic pathways (SSP126, SSP245, and SSP585). The results indicate that R_rain dominates mean annual runoff with a proportion of 42%, followed by R_snow (37%) and R_glacier (21%). In terms of inter-annual variation, R_rain and R_snow show increasing trends (0.93 (p < 0.05) and 0.31 (p > 0.05) mm per year), while R_glacier exhibits an insignificant (p > 0.05) decreasing trend (−0.12 mm per year), leading to an increasing trend in total runoff (1.12 mm per year, p > 0.05). The attribution analysis indicates that changes in precipitation and temperature contribute 8.16 and 10.37 mm, respectively, to the increase in runoff at the mean annual scale. Climate wetting (increased precipitation) increases R_rain (5.03 mm) and R_snow (3.19 mm) but has a limited effect on R_glacier (−0.06 mm), while warming increases R_rain (10.69 mm) and R_glacier (5.79 mm) but decreases R_snow (−6.12 mm). The negative effect of glacier shrinkage on R_glacier has outweighed the positive effect of warming on R_glacier, resulting in the tipping point (peak water) for R_glacier having passed. Runoff projections indicate that future decreases in R_glacier and R_snow could be offset by increases in R_rain due to increased precipitation projections, reducing the risk of shortages of available water resources. However, management authorities still need to develop adequate adaptation strategies to cope with the continuing decline in R_glacier in the future, considering the large inter-annual fluctuations and high uncertainty in precipitation projection.