THE CLIMATE CHANGE IMPACT ON WATER RESOURCES OF UPPER INDUS BASIN-PAKISTAN

Abstract

PRECIS (Providing REgional Climate for Impact Studies) model developed by the Hadley Centre is applied to simulate high resolution climate change scenarios. For the present climate, PRECIS is driven by the outputs of reanalyses ERA-40 data and HadAM3P global climate model (GCM). For the simulation of future climate (SRES B2), the PRECIS is nested with HadAM3P-B2 global forcing. In the present day simulations, climatic means and interannual variability are examined and biases are identified focusing on the most important parameters (precipitation and temperature) for hydrological modelling. In this study, both the meteorological station observations and results of the PRECIS RCM are used as input in the HBV hydrological model in order to investigate the effect of PRECIS simulated precipitation and temperature on the HBV predicted discharge in three river basins of UIB region. For this, three HBV model experiments are designed: HBV-Met, HBV-ERA and HBV-PRECIS where HBV is driven by meteorological station data and by the outputs from PRECIS nested with ERA-40 and HadAM3P data respectively. The robustness and uncertainties ranges of these models are tested. The future water resources are quantified using the two approaches of transferring the climate change signals i.e. delta change approach and direct use of PRECIS data. The future discharge is simulated for three stages of glacier coverage: 100 % glaciers, 50 % glaciers and 0 % glaciers. The PRECIS is able to reproduce the spatial patterns of the observed CRU mean temperature and precipitation. However, there are notable quantitative biases over some regions especially over the Hindukush-Karakorum-Himalaya (HKH) region, mainly due to the similar biases in the driving forcing. PRECIS simulations under future SRES B2 scenario indicate an increase in precipitation and temperature towards the end of 21 st century. The calibration and validation results of the HBV model experiments show that the performance of HBV-Met is better than the HBV-ERA and HBV-PRECIS. However, using input data series from sources different from the data used in the model calibration shows that HBV-ERA and HBV-PRECIS are more robust compared to HBV-Met. The Gilgit and Astore river basins, for which discharges are depending on the preceding winter precipitation, have higher uncertainties compared to the Hunza river basin for which the discharge is driven by the energy inputs. The smaller uncertainties in the Hunza river ibasin as compared to Gilgit and Astore river basins may be because of the stable behavior of the input temperature series compared to the precipitation series. The robustness and uncertainty ranges of the HBV models suggest that regional climate models may be used as input in hydrological models for climate scenarios studies. In a changed climate, the discharge will generally increase in both HBV-PRECIS and HBV-Met in the 100 % glacier coverage stage up to 65% and 44%, respectively. At the 50 % glacier coverage stage, the discharge is expected to reduce up to 24% as predicted by HBV-PRECIS and up to 30% as predicted by HBV-Met model. For the 0 % glacier coverage under climate change, a drastic decrease in water resources is forecasted by HBV-Met is up to 96 % and by HBV-PRECIS is up to 93%. At 100 % glacier coverage, the magnitude of flood peaks is likely to increase in the future which is an indication of higher risk of flood problems under climate change. There are huge outliers in annual maximum discharge simulated with HBV-Met. This shows that the prediction of hydrological conditions through the delta change approach is not ideal in the UIB region. HBV-PRECIS provides results on hydrological changes that are more consistent with climate change. This shows that the climate change signals in HBV-PRECIS are transmitted more realistically than in HBV-Met. Therefore, the direct use of RCM outputs in a hydrological model may be an alternative in areas where the quality of observed data is poor. The modeled changes in future discharge and changes in peak flows under climate change are not conclusive because more research is needed to evaluate the uncertainties in this approach. Moreover, this technique needs to be tested with other RCMs and hydrological models preferably to river basins in other parts of the world as well.