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 (Rrain), snowmelt runoff (Rsnow), and glacier meltwater (Rglacier), 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 Rrain dominates mean annual runoff with a proportion of 42%, followed by Rsnow (37%) and Rglacier (21%). In terms of inter-annual variation, Rrain and Rsnow show increasing trends (0.93 (p < 0.05) and 0.31 (p > 0.05) mm per year), while Rglacier 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 Rrain (5.03 mm) and Rsnow (3.19 mm) but has a limited effect on Rglacier (−0.06 mm), while warming increases Rrain (10.69 mm) and Rglacier (5.79 mm) but decreases Rsnow (−6.12 mm). The negative effect of glacier shrinkage on Rglacier has outweighed the positive effect of warming on Rglacier, resulting in the tipping point (peak water) for Rglacier having passed. Runoff projections indicate that future decreases in Rglacier and Rsnow could be offset by increases in Rrain 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 Rglacier in the future, considering the large inter-annual fluctuations and high uncertainty in precipitation projection.