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Abstract: The objective of this paper is to report on the issues and proposed approaches in estimating the contribution of gully erosion to sediment yield at large catchment. The case study is the upstream of Pangani River Basin (PRB) located in the North Eastern part of Tanzania. Little has been done by other researchers to study and/or extrapolate gully erosion results from plot or field scale to large catchment. In this study multi-temporal aerial photos at selected sampling sites were used to estimate gully size and morphology changes over time. The laboratory aerial photo interpretation results were groundtruthed. A data mining tool, Cubist, was used to develop predictive gully density stepwise regression models using aerial photos and environment variables. The delivery ratio was applied to estimate the sediment yield rate. The spatial variations of gully density were mapped under Arc View GIS Environment. Gully erosion sediment yield contribution was estimated as a ratio between gully erosion sediment yield and total sediment yield at the catchment outlet. The general observation is that gullies are localized features and not continuous spatially and mostly located on some mountains’ foot slopes. The estimated sediment yield rate from gullies erosion is 6800t/year, which is about 1.6% of the long-term total catchment sediment yield rate. The result is comparable to other study findings in the same catchment. In order to improve the result larger scale aerial photos and high resolution spatial data on soil-textural class and saturated hydraulic conductivity – are recommended. [Copyright &y& Elsevier]

Abstract: This study intended to validate the Soil and Water Assessment Tool (SWAT) model in data scarce environment in a complex tropical catchment in the Pangani River Basin located in northeast Tanzania. The validation process involved the model initialization, calibration, verification and sensitivity analysis. Both manual and auto-calibration procedures were used to facilitate the comparison of the results with past studies in the same catchment. For this study, some model parameters including Soil depth (SOL_Z) and Saturated hydraulic conductivity (SOL_K) were assumed uniform within the study catchment and were therefore lumped comprising the huge computation resource requirement of the SWAT model. Results indicated that the same set of important parameters was identified with or without the use of observed flows data. Some of the parameters had physical interpretation and could therefore relate directly to hydrological controlling factors within the catchment. Despite swapping ranking importance of parameters, these results suggest the suitability of the SWAT model for identifying hydrological controlling factors/parameters in ungauged catchments. Results of calibration and validation at the daily timescale gave moderately satisfactory Nash–Sutcliffe Coefficient of Efficiency (CE) of 54.6% for calibration and 68% for validation while simulated and observed mean annual flow discharges gave an Index of Volumetric Fit (IVF) of 100%. The study further indicated the improvement of model estimation when more reliable spatial representation of rainfall was used. Although in this study SWAT model has performed satisfactorily in data poor and complex catchment, the authors recommend a wider validation effort of the model before it is adopted for operational purpose. [Copyright &y& Elsevier]