The Impact Of Climate Change On The Streamflowresponsein The Northwestern Upper Indus Basinbased On A High Resolution Regional Climate Model(RegCM4.3)Simulations

South Asia has a diverse set of climates and the complex geographical features with extensive mountain ranges of Hindukush,Karakorum and Himalaya(HKH),has posed great challenges to regional climate models(RCMs)to reproduce observed climatology.The variability in South Asian summer monsoon(SASM)wields significant impacts on the economics,ecosystems,agriculture and water resources of the whole subcontinent of South Asia.Geographically,Pakistan is situatedon western side of South Asian monsoon system and the northwestern areas of Pakistan including the Upper Indus Basin(UIB),are extremely vulnerable to the SASM rainfall regime.The application of RCMs regarding climate change impact assessment in the UIB are rare.Usually three distinct regime could affect the climate of UIB including,glacial,nival and rainfall regime.Though the glacial and nival regime contribute significantly to streamflow in UIB at very high altitude,the rainfall regime is responsible for severe flooding during the monsoon season over the foothills of the Himalaya.Previous studies are either limited to the glacial regime or lacks to presents a detail investigation of RCMs parameterization before the application of climate change impact assessment.Moreover the climate change impact assessment on the Northwestern part of the UIB(NUIB)including the Kabul River,has not yet been investigated in the light of regional climate modeling.To study the climate change impacts,the sensitivity experiments are recommended byusing the RCMs over a particular region of interest.In order to attain the best model configuration over the South Asia region,the ability of ICTP Regional Climate Model version 4(RegCM4.3)is investigated by using two land surface schemes: the Biosphere-Atmosphere Transfer scheme(BATS)and the Community Land Model version 3.5(CLM3.5).For this purpose,six sensitivity experiments are conducted with three different cumulus convection schemes.RegCM4.3 coupled with CLM3.5 and mixed convection scheme option(MIX-CLM),produced better simulation than BATS.The cold winter bias and the intensities of wet-dry biases over the foothills of HKH and Central India are substantially reduced with MIX-CLM.In terms of seasonal variability,results suggest that different convection schemes behaved differently over sub-regionsof the domain.In spite of some deficiencies,the MIX-CLM scheme improves the model performance over the various parts of the domain.RegCM4.3 is further validated for present-day climate against the observations with best suited physical parameterization schemes options.To explores the possible changes in the SASM on the basis of regional and global climate model projections,RegCM4.3simulations are conducted at a 25 km fine horizontal resolution driven by GFDLESM2M(GCM)lateral boundary forcing under the moderate(RCP4.5)and the strongest(RCP8.5)representative concentration pathways.Temperature,precipitation and wind field datasets from the CRU,APHRODITE,ERAIM and the NCEPDOEreanalysis are used to assess the performance of the model for the reference time period(1976-2005).The regional characteristic features of SASM,like the low level jet stream and westerly flow over the northern Arabian Sea,are well captured by the RegCM4.3.The possibilities of excessive precipitation over the Western Ghats,include increased southwesterly flow in the wet period and the effect of model bias on the climate change.The spatial patterns of precipitation are decreased in intensity and magnitude as the monsoon approaches the foothills of the Himalayas.The projected changes in temperature under the RCP4.5 and RCP8.5 are highest from June to August in Noshera basin as compared to Besham and Khairabad.The annual cycle of precipitation over the aforementioned gauges show an overall increased streamflow under the both scenarios.The HEC-HSM hydrological model is used to generate the streamflow responses for climate change projection in the NUIB under RCP4.5 and RCP8.5 scenarios.The calibration of HEC-HMS hydrological model is performed to adjust the model parameters,so that the model could capture the variations in the observed flow.Three statistical indicators(NCE,R2 and PBIAS)are used to assess the performance of HECHMS over Besham,Noshera and Khairabad gauges in the NUIB.To generate the future streamflow responses(2070-2099)in NUIB,the bias corrected time series of temperature and precipitation from RegCM4.3 simulations are used as input to calibrated HEC-HMS.Though there are significant alterations in the seasonal mean flow,the average annual runoff volumeis found to be continuously increasing at the end of 2100.The increase precipitation as well as the streamflow in summer and winter in the NUIB,indicates that RegCM4.3 simulated climate change projections arerealistically transferred to HEC-HMS hydrological model.Maximum streamflow peaks,both in frequency and magnitude,are observed in July and August over the three gauges,posing possible excessive threats of flood risks during monsoon season of Pakistan.The percentage changes in the winter streamflow are more as compare to summer,revealing the possible effects of North Atlantic Oscillation(NAO)positive phase and El Nino Southern Oscillation(ENSO)warm phase on winter precipitation of Pakistan and Afghanistan.The analysis of flow duration curve for three hydrometric stations revealed that the flow occurrence probability could be greater under the projected scenarios relative to reference period.The increased streamflow is projected over all of the gauge stations under high(Q5)and medium(Q50)flow.This suggests that increased in high and medium flow is possibly related with occurrence of increase monsoon precipitation events in the NUIB.Overall,it is to be expected that the NUIB will confront more floods as a result of projected increase in medium and high streamflow,which demands better policies and managements towards the water resources of NUIB.Despite some caveats and uncertainties in the results,this study will significantly contribute to the CORDEX project and climate change impact assessment studies over South Asia,particularly over the NUIB of Pakistan.