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UNESCO-IHE Institute for Water Education, Delft (2014)

Water quality assessment of agricultural runoff in the upper Mara River Basin, Kenya

Fouchy K .

Titre : Water quality assessment of agricultural runoff in the upper Mara River Basin, Kenya

Auteur : Fouchy K .

Etablissement de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science (MS) 2014

Topography, soil properties and vegetation cover influence the quality of water drained from the land. In the agricultural dominated region of the upper Mara River Basin (MRB) in Kenya, rural communities are vulnerable to changes in surface and sub -surface water quality, as they depend on the direct use of water for domestic purposes. The aim of this study was to quantify the variability in surface and sub-surface runoff quality in a two hectare mixed-crop farm in the upper MRB. Field work was conducted in May and June of 2014. The generation of surface runoff and sediment transport on the agricultural hillslope was studied using a rainfall simulator. Six minute long micro-scale rainfall simulations were undertaken for 780 cm2plots on eight different land covers of the farm : Bare ground plowed (BGP), Grass Mulched (MU), Footpath(FP), Maize (M), Sorghum (SG), Napier grass (Pennisetum purpureum) (N), Pasture (P) and Non-grazed mixed grasses (NGG). Two simulations were always run consecutively and are refered to as dry (D) and wet (W) condition runs. Sediment mass (g), concentration (g/L) and soil loss (g/m2) were quantified from the plots. Runoff pH, electrical conductivity (EC), and major cation and anion concentrations were also measured. Water quality of different sources along the Kapsosurwa stream was also tested. Results of this study indicated that surface runoff represented 85.3% of sprinkled water for Footpath, however zero surface runoff was measured on Bare ground plowed and grass mulched. Soil loss on Footpath averaged 28.4 g for a 6 minutes simulation, which was 2 to 3 times higher than on Maize or Sorghum and nearly 5 times higher than on Napier grass. Changes in chemical composition of runoff samples were observed over the 6 minute simulation period, including a significant increase in Potassium concentration in water running over Pasture. Moreover, ion chemistry varied significantly between different water sources : stream, spring, river and well. Elevated nitrate concentrations were found in spring samples, indicating pollution of these water sources. The results of this study were used to propose activities for improving water and food security on the farms. These may be implemented as part of MaMaSe, a new 4-year development initiative in the basin.

Sujets  : water quality assessment ; surface runoff ; river basins ; Kenya


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