From the high mountains to the deltas
The excess meltwater that results from climate change induced mass loss of mountain glaciers is an important contributor to sea level rise (SLR). Up to now, large scale glacier models have been used to estimate the amount of generated excess meltwater and its transient contribution to SLR under the assumption that meltwater is added to the ocean instantaneously and in its entirety. However, other hydrological processes and water consumption during the transit from glacier to the ocean may affect the amount and timing of glacier runoff that eventually drains into the ocean. Some of the lost glacier ice may not reach the ocean at all or only at a much later stage.
In this project, we performed a first assessment of the impacts of the hydrological pathway of meltwater from the glacier snouts to the ocean in the Indus Basin. With its large glacier ice reserves, relatively arid climate and large irrigation scheme, this basin provides the optimal study for such an assessment. We coupled output from a detailed glacier model to the fully distributed hydrological model PCR-GLOBWB 2 and forced the models with bias-corrected historical and future climate data (ISIMIP3).
The simulated changes in hydrological stores indicate that the excess glacier meltwater is mostly used to meet irrigation water demands in the extensive irrigation scheme of the Indus Basin. Large increases in surface water irrigation due to excess meltwater availability primarily result in increases of evaporative fluxes. On the other hand, the increased availability of surface water results in reduced groundwater abstraction and a reduction of unsustainable groundwater depletion. Over the course of the 21st century, largely irrespective of climate scenario, approximately 15% of the excess meltwater does not enter the ocean directly on average. 11% of this meltwater contributes to a reduced reduction of the basin’s groundwater stores, and the remaining 89% evaporates and enters the atmospheric circulation. It is yet uncertain how much of the evaporative flux eventually enters the ocean and on which time scale.
We conclude that not all glacier mass loss contributes to sea level rise directly. Ignoring this leads to overestimation of future sea level rise. Further research is necessary to estimate the breadth of these effects at a global scale, but we hypothesize that this also plays a non-negligible role in other glacierized basins with semi-arid downstream regions and considerable distances between the glaciers and the ocean.