Altitudinal variation in impacts of snow cover, reservoirs and precipitation seasonality on monthly runoff in Tibetan Plateau catchments

Abstract. Although of great importance for long-term, effective water resource allocation, current knowledge of monthly runoff variability, its spatio-temporal characteristics, and underlying key drivers, including their sensitivity to climate change and other human impacts, is limited. With a particular focus on 10 sub-basins along an elevation gradient (1000 to 5900 m.a.s.l.) in the hydrologically complex, seasonally cold Yalong River basin, China, this study developed an extended Budyko framework based on monthly water balances (2002–2016) to consider snow storage dynamics (∆S_snow) separately from other terrestrial water storage changes (∆S’), including those related to hydropower reservoir construction. Results showed that snow accumulation and snowmelt are main drivers of runoff seasonality in the upper sub-catchments of the Yalong River basin, with propagating impacts also on lower-elevation snow-free sub-catchments, which are increasingly under the additional influence of hydropower reservoirs. This creates a relatively strong altitudinal heterogeneity in drivers of monthly runoff, which has been hypothesized to occur also in other world regions including e.g. major European rivers of Alpine origin, although not yet quantified at similarly high spatio-temporal resolution. Furthermore, an observed decrease in runoff seasonality in the Yalong River at its Yangtze River outlet (that receives water from all 10 investigated sub-basins) was shown to be unrelated to snow storage changes and hence likely caused by trends in unfrozen precipitation seasonality and/or flow-modulating impacts of constructed reservoirs, natural lakes and groundwater, implying that further snow thinning may exacerbate such trends in the future. Implementing the variance decomposition method based on the extended Budyko framework, the intra-annual runoff variability (σ²_R) was captured by calculating the variance and covariance of influencing factors (R² values above 0.9 in most sub-basins) with the main contributors being variances of rainfall (P_r) and ∆S’. Methodologically, we have verified the substantial contribution of hydropower reservoir storage changes on total storage changes by independent analysis of reservoir storage data, supporting the applicability of the extended monthly Budyko framework for identifying dominant processes in the context of runoff generation and the rapid environmental changes that the Yalong River basin and other cold regions (not least of the Tibetan plateau) are currently experiencing.