HYDROLOGICAL MODELS COMPARISON IN SIMULATING STREAMFLOW UNDER CHANGING CLIMATE: A CASE OF MOJO RIVER WATERSHED, AWASH SUB-BASIN, IN ETHIOPIA

Abstract

Quantification of climate change impacts on streamflow at catchment scale for the Modjo river watershed with area (1612.21 Km2) by soil and water assessment tool (SWAT) and Hydrologiska Byråns Vattenbalansavdelning model (HBV) were analyzed and compared. African domain CMIP5 output of precipitation and temperature (maximum and minimum) for (2041-2070 and (2071-2100) under intermediate scenario (RCP 4.5) and high emission scenario (RCP8.5) were applied. Linear scaling (multiplicative) for precipitation and linear scaling (additive) for temperature simultaneously both bias corrections and regional downscaling were applied to these forcing variables by CMhyd tool. Both hydrological models namely, Arc-SWAT and HBV were utilized to simulate streamflow in the study area. The models was calibrated from 1992-1997 and validated from 1998-2001 on monthly bases. In terms of hydrological modeling performance by SWAT models, Nash-Sutcliffe (NSE) = 0.85 and 0.90, R2=0.8 and 0.84, PBIAS=7.5 and 4.5 for calibration and validation period respectively, whereas for HBV performance NSE=0.78 and 0.80, R2=0.83 and 0.85, PBIAS =0.55 and 0.63 for calibration and validation period respectively. Future projections results as compared to base period (1983-2012) have shown that exhibit moderate increase in annual precipitation. Maximum increment is observed+7% and13.2% for RCP4.5 (2071-2100) and RCP8.5 (2041- 2070) projection respectively. Seasonal variation of precipitation showed an increase in dry season ONDJ and low in JJAS for both RCP4.5 and RCP8.5. The seasonal maximum temperature showed an increment in JJAS and low in ONDJ for both scenarios. The annual maximum temperature change in intermediate scenario (RCP4.5) increase from 1.74 0C to 2.15 0C and for high emission scenario (RCP8.5) increase from 2.35 0C to 4.17 0C. Minimum temperature showed an increment in dry season and low for wet season for both scenarios. The monthly maximum streamflow observed in month (Oct, Nov, dec, Jan, Feb, mar, appr and may) and minimum in (Jun, Jul, Aug, Sep) for both scenarios and models. Streamflow reduction observed in July (-34%) and July (-15%) for SWAT and HBV respectively. The seasonal streamflow change showed increase in ONDJ and minimum in JJAS for both models, but more values for SWAT. Annual monthly maximum extreme streamflow pattern difference showed reduction in all decade and more value in SWAT (-21%) whereas minimum extreme streamflow difference showed an increment in both models, but more value for HBV (+87%). In general the impacts of climate change cause an increase in dry and decrease in wet season streamflow for future periods due to in JJAS increases and decreases of maximum temperature and precipitation and reverse for ONDJ.The decreases in streamflow magnitude may probably have negative impacts in meeting the water resources projects and will require the implementation of appropriate mitigation measures.