Determination of Watershed Bmps for Soil and Water Conservation using Agnps

Abstract

Non-point source pollution is a major source of contamination of water resources. I t occurs when surface runoff, resulting I"j'om a rainfall event, carries sed iments laden with plant nutrients, pesticides, human and anima l wastes, as well as other wastes into adjacent streams~ rivers or waterways. These pollutants, which re sull n'om human activities on a watershed such as agriculture, silviculture, mining and construction, have a major impact on the water quality especially in rural environments. The AGNPS model (Young et aI., 1989), is a computer-based watershed ailalyses model for the study of non-point source pollution, and can be used to prioritise potent ial water quality prob lems in ru ral areas. It was applied to a local watershed in the humid tropi cal environment, the Upper Nyaba watershed 111 Enugu State, Nigeria . This paper presents the resultsofthat model application. The Upper Nyaba watershed, with an area of2853.26 ha and for a 2-yeai, 80.01-mm rainfall event of 6 hours duration, produced at the watershed outlet a runoff volume of 32.26 mm and a peak runoff rate 01'46.121 n,",/s. Other results show that 413.8 ha or 14.5 percent of the watershed area "gave a high level 01' scdiment yield. Soil loss rates of between 2.23 x 10J and 4.46 x 10.1 Kglha occurred in 12.9 percenl of the watershed area, amounling to the erosion of 10.12 x 104 Kg of soil from an area of 370.3 11<1 in a si ngle rainLll1 event. At the watershed outlet, a totalsediment yield of2.05 x la' Kg was obtained, most of which was transported from the upland areas (upper-middle parts) of the watershed. Distributed soi l loss rates and sediment yields across the watershed were part of the model output. Relevant model parameters were twiddled in such a way as to reflect proposed changes in watershed management practices. AGNPS model estimates based on these input were used to investigate the ~ ftects of the changes, and to make recommendations on best management practices for water quality conservation . T he three model parameters lIsed to investigate best management practices were. the USLE conse rvation practice faetOL the fe rtili ser input factor, and the Manning's roughness coefficient f(lr over land 110w. Only the USLE pract ice laClOr had some effect a nd sediment-associated nutrient yields (about 35.6'Yo reduction). fertiliser application affected only the water- soluble nutrient yie lds. Using a mouldboard plough to cultivate the land reduced the sediment yield of the watershed s lightly, as compared to llsing a elise plough. These effects suggest contour farming as a best management practice on the watershed . It is concluded that, given basic measurable watershed data, AGNPS model has utility for fine-tuning water lluality management programmes in rural areas of the humid tropics. This modelling approach can hI! replicated in difterent localities espec ially in developing countries where historical data records are not available.