A disaggregating approach to describe overland flow occurrence within a catchment

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A parametrically parsimonious, data-based model was built on observations at hillslope and catchment scale to simulate the distribution of overland flow within a small East African Highlands catchment (Kwalei, Tanzania). A rainfall-flow Data Based Mechanistic model identified catchment effective rainfall and separated the discharge quick flow, interpreted as the combination of overland flow plus reinfiltration along the slopes, and the slow flow, interpreted as ground water displacement. Observations of overland flow occurrence along the slopes were used to derive probability distribution functions (pdfs) of overland flow in relation to effective rainfall for two pre-defined hydrologic response units (HRUs): perennial (HRU_1) versus other crops (HRU_2). At low effective rainfall, overland flow was more frequent in HRU_2, while at high effective rainfall overland flow in the two HRUs was similar. The pdfs were employed to disaggregate the quick flow into HRU overland flow depth. Reinfiltration was accounted for by assuming that only the overland flow generated in the lower part of the field would drain downslope. Effective reinfiltration length was about 4 m. Comparison of model simulations versus Gerlach trough measurements indicated that rainfall intensity was not accounted for sufficiently. The use of smaller time steps or, alternatively, of a rainfall intensity threshold could improve model performance. However, given the high variability of overland flow observed along the slopes and the limited dataset, model simulations were considered satisfactory. Though the model needs further testing on other datasets, the disaggregating approach represents an inductive alternative to the use of infiltration equations to model overland flow within a catchment.