COUPLING GLACIO-HYDROLOGICAL RESPONSE TO CLIMATE VARIABILITY IN MT EVEREST REGION IN CENTRAL HIMALAYA

Mt. Everest region in the central Himalaya is one of the most heavily glacierized parts of the Himalaya that is characterized by large debris-covered glaciers and many glacial lakes. The glaciers and ice are important sources of fresh water and play vital role in modulating the climate and the hydrological process. Previous studies from different parts of the Himalaya and around the world have revealed climate change at regional and global-scale and in general, shrinking of glaciers and ice caps. Climate change is thus, expected to impact in many ways to Cryosphere, hydrological process, and human livelihood. Temperature is often suggested to be increasing and considered as the main driver of change, however, in the higher elevations where the glaciers exist, climatic data are rarely available and limiting the climate related interpretation. This study is therefore conducted with the aim of linking variation of glaciers, glacial lakes, and river flow to local climatic trends in the higher elevations of Mt. Everest region. The study uses a comprehensive multi-temporal data from different sources: satellite observations, ground hydro-meteorological stations, and regular gridded and reanalysis climate data from the regional and global products (1960s to 2013). First, using the weather data from ground stations, gridded, and reanalysis products, the climatic trends and climate variability are evaluated. From 1979 to 2013, temperature has increased by 0.052 °C a-1, while the precipitation has shown an increasing tendency in 1960s to early 1990s and significantly decreasing afterward. During 1994–2013 period, at an elevation of ~ 5000 m, minimum temperature (0.072 ± 0.011 °C a-1) has increased more than maximum temperature (0.009 ± 0.012 °C a-1), with an average temperature increase of 0.044 ± 0.008 °C a-1 in the last two decades. The increases in the temperature are observed during the pre- and post-monsoon months, favouring melting ice close to the glacier terminus. At the same elevation, precipitation has significantly decreased (-9.3 ± 1.8 mm a-1) for all months, corresponding to a loss of 47 % during the monsoon. Second, the glacier changes are studied within the Sagarmatha (Mt. Everest) National Park (SNP; glacier area: ~ 400 km2) between 1962 and 2011, using multi-temporal optical satellite imagery, assisted by topographic maps. During the period, glaciers have experienced a surface area loss of 13.0 ± 3.1 %, an average terminus retreat of 403 ± 9 m, a Snow-Line Altitude (SLA) upward shifting of 182 ± 22 m, and an increasing of debris- covered area by 17.6 ± 3.1 %. An accelerated rate of glacier shrinkage is observed after the 1990s, which is caused not only due to increased temperature, but also as a result of a significant decreasing precipitation over the last decades. Moreover, selected glaciers have indicated a significant decreasing glacier flow velocities from the 1990s to recent year and a significant loss of glacier thickness (0.73 ± 0.63 m a-1) in the last decade. Third, a complete mapping and characterization of a total of 624 glacial lakes with surface area of 7.43 km2 (±18 %) are conducted in the SNP, with particular focus on conditions related to the formation of lakes using 2008 satellite imagery. Further, evolutions of glacial lakes are examined using the satellite imagery and topographic maps between 1963 and 2013. Three types of glacial lakes (supra, pro, and unconnected) present in the SNP have their distinctive potential to explain the glaciological and climatic conditions. Results show that the slope of the glacier where lakes are located influence the supraglacial lake formation. Furthermore, the slope to glacier upstream favours the formation of the supraglacial lakes, as a boundary condition. The formation of proglacial lakes is related to the growing and coalescing of the supraglacial lakes. The unconnected lakes are evaluated as a useful indicator of precipitation trend. During the study period (1960s–2011), both number and surface area of supraglacial lakes has continuously increased (number +109.7 %; area +13.3 %) with an accelerated rate in the last decade due to increase in the glacier melting. Proglacial lakes are more or less constant in both numbers and size, except Imja Lake that have exceptionally increased, while the surface area of unconnected lakes has increased from 1960s–1990s (+4.3 %) and decreased from early 1990s afterward (-10.9 %). The thesis has shown that the accelerated rate of glacier shrinkage and the decreasing of the unconnected lakes in the last decades are associated to decreasing precipitation. Supraglacial lakes behaviour confirms the acceleration of the negative mass balance of glaciers due to the reduced ice velocities caused by decreased precipitation. Finally, the hydrological dynamics of the Dudh Koshi river examined by stochastic frequency analysis, physically-based hydrological models, and multilinear regression using river discharge data and climate data. The analysis suggests that the Dudh Koshi river discharge is mainly dependent on precipitation from 1960s to 2000s, however a non-stationarity in the river discharge is observed since the early 2000s, indicating increased discharge, not justifiable by the observed weakening monsoon. The study concludes by underlining that an accelerated glacier melting as observed through the glacier change analysis affects an increasing of the discharge.