Challenges in field approximations of regional scale hydrology

Abstract

Study region: Considering the availability of gauged data, the Karasnagala watershed of Attanagalu Oya located in the Gampaha district in the Western province of Sri Lanka was modeled with EPA SWMM 5. Study focus: This study analyses the effect of the catchment field approximations for accurate flood hydrograph prediction. Following an event based approach, 3 days Minimum Inter event Time (MIT) and 0 mm/day Minimum inter Event Depth (MED) were used as the threshold. Fifty events were separated from 1971 to 1982 period. Four major field approximation types were identified: stream geometrical parameters approximations, soil infiltration parameter approximations, approximation of watershed intermittent storages, and subcatchment delineation approximation. Soil parameter approximations and the stream network geometry parameter approximations were verified by the field observations. New hydrological insights for the region: Model calibration and verification revealed that EPA SWMM5 can be successfully used to develop regional Karasnagala watershed model with mean ratio of absolute error (MRAE) 0.289 for calibration, and 0.375 MRAE for verification. Incorporation of intermittent storages with optimized model layout obtained the best fitting of hydrograph recession MRAE 0.167. Subcatchment lumping with a 16 sub basin configuration showed the marginal increment of modeling error when compared with distributed modeling. Stream parameter approximations revealed that the head water streams/lesser order streams parameters sensitivity is higher than that of the higher order streams. In soil parameter approximations, saturated hydraulic conductivity of the soil was the most influencing parameter.