Assessment of phosphorus transport pathways and loading in gilgel gibe-i catchment using swat model

Abstract

Eutrophication is a trouble that alter the ecological integrity of any water resources at global, regional and local scale including Ethiopia resulted by P that exported and loaded from agricultural-based catchment. Gilgel Gibe I watershed is agriculture –based catchment found in the south-western part of Ethiopia, in Oromia Regional State. This catchment is exhibited to this problem as a result of agricultural intensification with P concentration beyond the limit (0.86 mg/l).Therefore, the objectives of this study were to assess the transport pathways, quantify the amount of P load and to identify the prone sub-basins that were responsible for a significant P load utilizing SWAT model on this study area. To achieve this, a longitudinal desk study design was followed after all necessary input data was collected, analyzed and prepared. The GIS- Arc SWAT interface was applied to distinguish and classify the land use, soil and slope of the area and found the LU as Generic (AGLI) ( Agriculture, Agro-silvi-cultural, Agro-pastoral) 91.46%,Forest Evergreen (FRSE) ( Silvi-culture) 2.69%,Forest Mixed (FRST) (Forest with coffee under and coffee under tree),2.89%,Pasture (PAST) (Grass land), 2.75%,Urban (URBN) (Residential) 0.21% and dominant soil as Eutric Nitosols (Ne13-3b) 65%, Eutric Vertisols (Ne12-2c) 35%.Further the model performance was evaluated and found the model was satisfactory to be applied over the area for water resources management for sustainable social and economic development. Following this, the pathways of P was assessed and found that the organic form of (Org P) was the dominant exportation mechanism and accounts around 77.2% of paths. Similarly, the maximum total P load was also investigated and found as 4.4 × 10 ହ tons/year on 2009 and corresponding minimum total load was around 5.7 × 10 ସ tons /year and holds 4.4% load on 2007.The prone sub-basins which were significantly responsible for high P load wereidentified.Lastly, conclusions and recommendations would be offered based on study findings.