Evaluation of SWAT’s surface runoff estimation using field observed data in the Upper Blue Nile Basin – Ethiopia Yihun Dile a,b , Raghavan Srinivasan c , Louise Karlberg b , and Johan Rockström a a Stockholm Resilience Center, Stockholm University, Stockholm, Sweden b Stockholm Environment Institute, Stockholm, Sweden c Texas A&M University, Texas, USA
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Evaluation of SWAT’s surface runoff estimation using field observed data in the Upper Blue Nile Basin – Ethiopia
Yihun Dilea,b, Raghavan Srinivasanc, Louise Karlbergb, and Johan Rockströma
aStockholm Resilience Center, Stockholm University, Stockholm, Sweden
bStockholm Environment Institute, Stockholm, Sweden cTexas A&M University, Texas, USA
estimates surface runoff from single precipitation events in agricultural watersheds developed in 1954 by the SCS of the USDA based on thousands of infiltrometer tests carried out from 1930s and 1940s it is adopted for urbanized and forested watersheds
Advantages simplicity, practicality, and predictability reliance on single parameter responsiveness to watershed properties
Disadvantages marked sensitivity to CN fixing the initial abstraction ratio at 0.2 (i.e. Ia=0.2S) equating the ratio of actual retention (F) to potential retentionto (S) to the ratio of actual runoff to potential runoff temprate vs tropical watershed conditions
Curve Number Method
The Curve Number – A theoretical Background
0 < S < ∞
100 < CN < 0
,
Assuming
After F=P-Q and including initial abstraction Ia
Mico-watershed Area (ha)
Elevation(m.a.s.l
Land use type Distance from Met-
station (meters) Max. Min. Gigudeguad 4.63 1920 1879 crops 726
Model setup threshold area of 0.5ha is used for watershed delineation zero percent threshold area is used to define HRUs teff was the dominent agricultural crop in the micro-watersheds
High 5-days antecedent rainfall + Low rainfall intensity
Low 5-days antecedent rainfall + High rainfall intensity
slope=0.1 (R2 =0.14) slope=0.46 (R2 =0.97)
Conclusion
CN method performed reasonably well in simulating surface runoff in the Upper Blue Nile Basin Calibration can further improve the agreement between simulated and observed flows Five days antecedent runoff is not a good indictor to adjust the curve number Rather rainfall intensity or soil moisture as implemented in continues models is a better approach to adjust the curve number