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

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 Byr & aring;ns 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.