By Santosh Nepal and Arthur Lutz

The changes happening in Himalayan Rivers has been widely discussed in last decades which ranges from single catchment to large river basins. These river basins are dependent on snow and glacier melt which has been largely used for agriculture, hydropower, household and industries. Providing water and other important ecosystem services to millions that reside in the upstream and downstream regions of these basins, the importance of understanding long-term climate impact on these basins is crucial. Specifically, the impact on water resources in form of water quality and quantity requires large scale hydrological modelling of these river systems. This in turn provides information on not only the hydrological characteristic of these basins, but provides for sectoral water management also such as irrigation and hydropower in the downstream regions.

The climate projection data plays an important role to understand these changes by applying hydrologic models at a river basin scale. New projected climate change data (CMIP5-Coupled Model Intercomparison Project Phase 5) provides new insight about the future climate change. While previous studies have focused on water availability in general for the entire region and catchment level studies, the impact on hydrological extremes and seasonal shifts are still to be understood in the HKH region. The HI-AWARE program builds on earlier experiences and further developing our understanding on the impacts of climate change on water availability and examining hydrological extreme and seasonal shifts. This would help better understanding the impact on water resources sector (such as hydropower, water availability, agriculture and disaster) which might be useful to design adaptation measures by policy makers.

As part of HI-AWARE, ICIMOD and FutureWater researchers collaborated for large scale hydrological modelling in the Himalayan region to understand climate change impact on glacier dependent river basins. The large scale modelling aims at understanding the hydrological regime and climate change impact assessment of the river systems originating from the high-altitude areas of the Himalayan region. The team (Photo 1) met during the first two weeks of June 2016 to discuss model improvements, and tailoring of the model in Himalayan river basins. The team included: Dr. Walter Immerzeel, Dr. Arthur Lutz and Mr. Wilco Terink (FutureWater) and Dr. Santosh Nepal and Mr. Sonu Khanal (ICIMOD).

The team photographed at Future Water office (left to right: Sonu Khanal, Arthur Lutz, Wilco Terink, Walter Immerzeel, and Santosh NepalFor climate change impact assessment, a hydrological model (SPHY, which can represents cryospheric and hydrologic processes) has been applied. The model has been calibrated and validated in the Indus, Ganges and Brahmaputra river basins. In each river basin, two sub-catchments, one in upstream and the other in downstream, have been chosen in the same river stretch (Figure 1). The model has been calibrated in three steps for different water balance components. First, using the glacier mass balance data, the glacier melt parameters will be optimized. Then, the snowmelt parameters will be optimized by comparing the MODIS (moderate resolution imaging spectroradiometer) snow cover data with snow extent estimated from SPHY model. Both of these steps will be carried out in upstream catchment where snow and glacier melt processes are dominant. Similarly, in the downstream gauging station, the rainfall-runoff parameters will be calibrated and validated. In this way, the models will be validated in different scales representing dominated cryospheric and hydrological processes. 
During the two-week stay, the SPHY model was set up in upstream catchments in Hunza (Indus), Marshyangdi (Ganges) and Sunkosh (Brahmaputra) catchments. The models were calibrated from 2001-2004 in Hunza, 1992-1996 in Marshyangdi and 2000-2005 in Sunkosh catchments. The initial results of the simulated and observed hydrographs are provided below (Figure 2 and 3). The simulated discharge overall represents the observed hydrograph, but more fine tuning is required. Moreover, a sensitivity analysis was also initiated which will help in understanding the different sensitivity parameters in the catchments.

Figure 1: Study area with upstream and downstream catchments (from left to right: Hunza, Marsyandi and Sunkosh).  
The initial steps of the large scale modelling has begun and we are planning to carry out the following activities to complete the assessment in coming days:
i) Improvements in glacier processes to represent glacial mass conservation
ii) Optimization of model parameters using glacier mass balance and snow cover data
iii) Sensitivity and uncertainty analysis
iv) Calibration and validation of SPHY models using discharge data
v) Apply the climate projection data to understand future hydrology
vi) Impact of climate change on water availability, and extremes in different seasons

After the end of the assessment, we hope that the study will provide crucial information about how the hydrological regime of the Hindu Kush Himalayan region will change by the end of the century. Particularly, we will be able to know:
i) How the melt pattern will change by the end of the century?
ii) Will there be a seasonal shifts of hydrological regime by the end of the century?
iii) How the change in hydrological extreme will impact downstream water availability?

Figure 2: Simulated and observed hydrograph of the Hunza river basin.
Figure 3: Simulated and observed hydrograph of the Marshyangdi river basin.

Works Consulted:

Lutz, A. F., Immerzeel, W. W., Shrestha, A. B., & Bierkens, M. F. P. (2014). Consistent increase in High Asia’s runoff due to increasing glacier melt and precipitation. Nature Climate Change Nature Climate Change, 4(7), 587–592. http://doi.org/10.1038/nclimate2237

Immerzeel, W. W., Beek, L. P. H. V., & Bierkens, M. F. P. (2010). Climate Change Will Affect the Asian Water Towers. Science, 328(5984), 1382–1385. http://doi.org/10.1126/science.1183188

Immerzeel, W. W., Beek, L. P., Konz, M., Shrestha, A. B., & Bierkens, M. F. (2011). Hydrological response to climate change in a glacierized catchment in the Himalayas. Climatic Change, 110(3-4), 721-736. doi:10.1007/s10584-011-0143-4

Nepal, S., Krause, P., Flügel, W., Fink, M., & Fischer, C. (2013). Understanding the hydrological system dynamics of a glaciated alpine catchment in the Himalayan region using the J2000 hydrological model. Hydrological Processes, 28(3), 1329-1344. doi:10.1002/hyp.9627

Shea, J. M., Immerzeel, W. W., Wagnon, P., Vincent, C., & Bajracharya, S. (2015). Modelling glacier change in the Everest region, Nepal Himalaya. The Cryosphere,9(3), 1105-1128. doi:10.5194/tc-9-1105-2015