Seasonal Variability of Flow and Nitrate Flux in Gilgel Gibe River, SouthWest, Ethiopia

Abstract

Nitrate is a limiting nutrient for plant growth and vital for crop production to increase agricultural productivity. However, its excessive presence in aquatic environment poses risk turning in to major aquatic ecosystem perturbation. Dissolved nitrate from lands to waterways is mainly exported via runoff and leaching. The transport of nitrate in a river is a function of the streamflow rate and its concentration, which render to the seasonal variation. Therefore, the aim of this study was to assess the seasonal variation of flow and nitrate flux. Streamflow data of the Gilgel Gibe River for the period of two years (2013-2014) were used. The baseflow is separated using Web-based Hydrograph Analysis Tool (WHAT). Dissolved nitrate concentrations measured on daily basis were also used. The effects of storm events on flow and nitrate transport were examined during dry and wet seasons. The river had shown high discharge rate from the mid June to late October. The baseflow index (BFI) was found 0.76 indicating that the streamflow is mainly controlled by groundwater discharge. Similarly, during the dry seasons, total nitrate input was considerably less than in wetter seasons. Nitrate concentrations, however, were unusually high in the first summer storm runoff after the dry season. The average annual nitrate loads varied from 13761.2 ton to 156.45 ton during the wet and dry seasons respectively. The regression curve for the nitrate load against streamflow (r2=0.88) has shown significant relationship (p-value=0.000). In the time interval studied, over 95% of the nitrate was transported from the watershed during the wet seasons of greater rainfall, which disclosed that seasonality and river flow are primary forcing functions when considering nitrate loadings in this watershed