Modeling of Nutrient Loading and Transport Pathways using SWAT model in Fincha’a watershed, Western Ethiopia

Abstract

Nutrients, particularly nitrogen (N) and phosphorus (P), are needed for plant growth and healthy ecosystems. In excess, however, they can impair surface water systems giving rise to a range of water quality problems like blooms of algae and the water hyacinth, depletion of oxygen levels and even suffocation or death of aquatic organisms and also increased nitrogen and phosphorus application on the land has enlarged N and P nutrients burdens to the lake through runoff and leaching. The objective of the study is modeling Nutrients (Nitrate-Nitrogen and phosphorus) loading and transporting in the Fincha watershed using Soil Water Assessment Tool (SWAT). The model was calibrated and validated using flow of Fincha’a gaging station. Sensitivity analysis showed curve number, Ground waterDelay, Moist bulk density (Sol_BD)and Threshold depth of water in shallow aquifer required for return flow (GWQMN) were the most sensitive top four parameters. The model was calibrated using stream flow data from 1988 to 2002 and validated from 2003 to 2011.TheR 2 and NSE values were used to examine model performance and the result indicates 0.93 and 0.79 to R 2 and 0.60 and 0.59 to NSE during calibration and validation respectively. This shows that there is good agreement between observed and simulated stream flow. From the simulation the annual total average of P and N were 17.5kg/ha/year and 78.6 kg/ha/year respectively. Surface runoff, lateral flow and percolation to the ground were the main transporting pathways for both Phosphorus and Nitrogen which depends on rainfall pattern, duration and intensity. Organic Phosphorus and organic N were dominantly transported through surface run off whereas NO3 was dominantly transported via percolation to ground water. The highest annual total P and total N load were contributed by sub basin 2, 4, 7 and 6,20,1 respectively. These Subbasins were mainly located in Jimma Rare Woreda, Jimma Geneti and Horo.