Studies of glacier mass-balance processes and the climate response of glacier-fed rivers in the Himalaya

Abstract

The climate change impact on the water security of the glacierised catchments of High Mountain Asia can potentially be amplified by the shrinkage of snow and ice reserve. However, uncertainties in quantitative descriptions of cryospheric processes, those of input data, and a serious lack of field observations compromise the predictability of Himalayan glaciers and glacier-fed rivers. In order to address some of the aspects of the broader issues, we choose specific problems related to the mass balance processes of glaciers and the climate response of glacier-fed rivers in the Himalaya. In the thesis, we develop modelling and field-based novel methods to quantify important processes related to the accumulation and ablation of ice on debris covered Himalayan glaciers. As changing glacier runoff is the key to understanding the future changes in Himalayan rivers, we analyse the climate response of the runoff of glacier-fed rivers. The major outcomes of the thesis are the following. 1) Avalanching is a significant contributor to the accumulation of snow and ice on a large class of Himalayan glaciers. Field-based glaciological mass balance measurement ignores the avalanche contribution. No attempts have been made to quantify the net avalanche contribution to mass balance in the Himalaya. We first discuss diagnostic criteria to identify strongly avalanche-fed glaciers, and then develop an approximate method to quantify the magnitude of the avalanche contribution to the accumulation. Our simulations show that, in three well studied Himalayan glaciers, ~95% of the total accumulation is controlled by avalanches. This study led to the first quantification of avalanches and the associated biases in the glaciological mass balance of these glaciers. Also, point out the strong control of avalanches on the dynamics of these glaciers. 2) Sub-seasonal ablation measurement by glaciological method involves tremendous human effort and often poses serious logistical challenges on debris-covered Himalayan glaciers. Measurement of vertical temperature profiles within debris allows estimation of point scale sub-debris ablation and is less labour-intensive than the glaciological method. Here, we developed novel techniques for estimating sub-seasonal ablation from vertical temperature profiles within the debris. Our analysis suggests that the accuracy of the ablation estimates from vertical temperature profiles is comparable to that of the glaciological method. Therefore, this measurement could be a convenient way of making accurate field measurements of local ablation rate over debris covered glaciers. 3) The future changes in runoff from a catchment under a given climate forcing is determined by the corresponding climate sensitivity. Understanding the runoff sensitivity from the glaciersied Himalaya is important in the backdrop of potential climate-change impacts. Here, we analyse the sensitivities of river summer runoff to precipitation and temperature changes in two Himalayan catchments from the different climatic regimes. Our analysis suggests that summer runoff from the glacierised parts of the catchments is sensitive to temperature changes but is insensitive to precipitation changes. The behaviour of the summer runoff from the non-glacierised parts is exactly the opposite. Such precipitation-independent runoff from the glacierised parts stabilises the catchment runoff against precipitation variability to some degree. We also show that the impact of the future glacier loss on the interannual variability of summer runoff may be significant in these two catchments. The outcomes from the thesis will contribute to improved predictability of surface mass balance processes of Himalayan glaciers and that of the changing runoff of glacier-fed Himalayan rivers.