UNDERSTANDING HYDROLOGICAL PROCESSES OF RIVERS DRAINING THE TIBETAN PLATEAU

Rivers draining the Tibetan Plateau provide fresh water for approximately 40% of the world’s population. Understanding the dynamics of these river systems, including climatic controls, contributions from glacial and snow melt, groundwater storage, and the potential effects of climate change, is important for keeping this part of the world supplied with fresh water. We focused on the Mekong River as an example of a large river draining the eastern Tibetan Plateau. The Mekong, one of the world’s largest trans-boundary rivers, drains south from the eastern Tibetan Plateau through China and Southeast Asia to the South China Sea. We analyzed river discharge data, Aphrodite precipitation data, and MODIS fractional snow extent along the mainstem and sub-catchments (ranging in size from 10² to 10⁵ km²) in order to study the hydrological processes governing river flow across the climatological regimes traversed by the Mekong. We find that for stations along the mainstem of the Mekong River, both base flow and low flow increase linearly with basin area. For sub-basin stations there is no relationship between watershed area and these parameters, suggesting that discharge in these basins reflects local climatological conditions. All mainstem basins exhibit a well-defined hysteresis effect between rainfall and discharge through the year. We interpret the hysteresis as temporal groundwater storage within the system and that the increasing lag-time with distance downstream is due to increasing storage capacities along the river course. In addition, the peak monsoon discharge arrives up to two months later in the downstream stations; this delay cannot solely be due to internal travel time but is more likely due to different climatic regime and the larger groundwater storage capacity in the downstream lowlands. Surprisingly, specific baseflow discharge in winter seasons decreases downstream, allowing us to estimate increasing evapotranspiration induced by intensifying agricultural and aquacultural exploitation in the southern fertile areas and its impact on river flow. We will present a comprehensive conceptual description of the processes transferring precipitation to river discharge through several climatological regions and furthermore quantify sizes and transfer times of the different parts of the Mekong system.