Changes in the mountain river discharge in the northern Tien Shan since the mid-20th Century: Results from the analysis of a homogeneous daily streamflow data set from seven catchments

This study is an assessment of the changes in seasonal and monthly flow in seven catchments draining the northern Tien Shan Mountains in Central Asia over a period from the 1950s to the present day. The purpose is to provide a first assessment of the flow response to climate change in regionally important catchments given their contribution to the water resource. All the catchments have a natural flow regime, and are therefore sensitive to climate change, but differ in area, elevation and glacial extent. Trends in flow were characterised using the Mann-Kendall test for standard meteorological seasons and individual months for mean flow, five flow quantiles and peak-over-threshold series for the period 1974-2013 at all sites and from the 1950s where data were available. The results were related to trends in seasonal temperature and precipitation from the regional high-elevation meteorological stations and glacier mass balance, equilibrium line altitude (ELA) and accumulation area ratio (AAR) records from the Tuyuksu glacier. The results show no reduction in streamflow in any catchment or season in the northern Tien Shan since the 1950s. Positive trends in all flow indicators, including peakover-threshold frequency, were observed in catchments with higher glacierization of over 10% and extensive presence of rock glaciers and permafrost indicating increased melt over the period which is characterised by a long-term increase in temperature. These trends were most evident in autumn and winter. In catchments with low glacierization, variability in summer flow was controlled primarily by precipitation of the preceding cold season. Correlation with glacier mass balance was weak but changes in ELA and AAR indicate that production of liquid runoff at higher elevations contributes to increased streamflow partly compensating for the declining glacier area. The observed changes in streamflow do not suggest any immediate problems with water availability in the northern Tien Shan. On the contrary, increased autumn and winter flows point at a more prolonged recharge of reservoirs and aquifers though eventually this water source will be exhausted.