Hydrological Response to Changing Climate for Water Availability- A Case Study of Rawal Dam

Abstract

Hydrological modeling at catchment scale has been used in this study to analyze the effect of climate change on Rawal Dam, one of the main sources of water supply to twin cities of Rawalpindi and Islamabad. A suitable hydrological model had to be applied to simulate the hydrological response of Rawal Dam catchment to historical precipitation and later on, the calibrated model had to be utilized to assess climate change and its impact on reservoir inflows. HEC-HMS was used to effectively apply a rainfall-runoff hydrological model which distributed the watershed of Rawal Dam into 4 sub-basins each having different characteristics. Two climate stations i.e., Murree and Rawal Dam covered the whole catchment area of the reservoir. The simulation period of model was divided into calibration (1993-1996) and validation (2001-2004) periods. Parameters of the model were adjusted after calibration on a daily time step and the calibrated model was then utilized for validation of the simulation. The model provided a good fit between observed and simulated reservoir levels. Climate change projections of precipitation under Representative Concentration Pathways 8.5 and 4.5 were then used, after bias correction, as input to calibrated model to assess potential impacts of climate change on Rawal Dam. The analysis were performed on seasonal basis for three time windows 2011-2040 (2025s), 2041-2070 (2055s) and 2071-2100 (2085s), collectively which covered the current century. Baseline data from 1983-2010 was used. Projected precipitation at Rawal dam, Murree and basin wide were assessed. The results show increase in precipitation under both emission scenarios. Precipitation and as a result inflows projected to increase in all seasons i.e., Winter (Nov-Feb), Summer(May-June), Monsoon (July-Aug) and Autumn (Sep-Oct) except for Spring (Mar-Apr) where it is projected to decrease. RCP8.5 depicted a 12%, 10%, 18% increase in precipitation over Rawal dam catchment during 2025s, 2055s and 2085s respectively while RCP4.5 showed 21%, 6%, 8% increase in precipitation during the same time windows. RCPs differed in their results during start and end of the century while for mid of the century their results remained same. It was established that increase in flows is expected in future. Moreover, it was also found out that the calibrated model can be used satisfactorily for further hydrological impact studies on the reservoir.