Dynamics of surface runoff and soil loss under varied landuse practices in Rwizi Catchment, Lake Victoria Basin
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Runoff and soil loss is reported to be widespread and increasing in Rwizi catchment in the Lake Victoria Basin. However, their magnitude and dynamics with respect to rainfall characteristics and land use practices is insufficiently understood. Cognizant of the changing climatic conditions in the Lake Victoria Basin, knowledge on rainfall characteristics differentially affecting runoff and soil loss on diverse slope segments under varied land uses is important. The study assessed the dynamics of runoff and soil loss under predominant land uses and slope segments in Rwizi catchment. This study established rainfall events / amounts and their effects on runoff and soil loss; established the rainfall induced sediment production at a sub-catchment scale and determined nutrient losses and monetary value. A total of thirty six (36) runoff plots were established using a split plot design. The design involved four land use practices and three slope positions with three replications of plots under farmer management. The land uses were grassland, tree plantations, mulched banana and unmulched banana plantations. The hillslope segments entailed lower slope segment, middle slope segment and upper slope segment. Rainfall was measured using automatic weather stations and manual rain gauges. In order to estimate the extent of risk to soil erosion at sub catchment level, the Revised Universal Soil Loss Equation (RUSLE) was used. The amount of the essential (NPK) nutrients lost in the sub catchment was established and its monetary value established. The results show that daily rainfall depth ranged between 1.2mm to 52mm; monthly rainfall depth ranged between 9mm to 167mm. Seasonal rainfall amounts ranged between 90mm (in the first season for 15 rainfall events) and 586.8mm (in the second season with 30 rainfall events). Therefore, annual rainfall received in 2014 was 680mm. The annual runoff mean varied in land uses from 8.36±6.03m3/ha/yr in mulched bananas, 8.37±8.04m3/ha/yr in grasslands, 10.65±6.25 m3/ha/yr in Unmulched bananas and 10.94±7.63m3/ha/yr in tree plantations. The magnitude of runoff was found out to be statistically sensitive (p<0.05) to rainfall amounts. However, there was no statistical difference between runoff (p > 0.05) and land use practices or slope positions. The Unmulched bananas recorded highest annual soil loss (0.5±0.55t/ha) closely followed by tree plantations with 0.39±0.43t/ha, mulched bananas with 0.25±0.27t/ha and grasslands with 0.22±0.21t/ha. It should be noted that these soil loss amounts were all within the tolerable soil loss rates as compared to the values reported by previous studies in LVB. There existed statistical difference between soil loss (p <0.05) and land use practices and slope positions. Using RUSLE model, the extent of risk toxiv runoff and soil loss at the sub catchment level was in range of 5t/ha to 50t/ha. Spatially, areas with the highest risk to erosion were over grazed area and those with annual crops with poor surface cover especially on steep slopes. Annual Nitrogen amounts were lost in the order of mulched bananas (300kg/ha) followed by tree plantations (220kg/ha), unmulched bananas (200kg/ha) and grasslands (160kg/ha). Annual Potassium nutrient losses were lost in mulched bananas (109.2kg/ha) followed by unmulched bananas (82.8kg/ha), tree plantations (40.4kg/ha) and 32.8kg/ha from grasslands. Annual Phosphorous nutrients losses were 149.1kg/ha in mulched bananas followed by grasslands (75.8kg/ha), unmulched bananas (52.2kg/ha) with the lowest loss (15.8kg/ha) from tree plantations. According to the findings, the vulnerability of the landscape to soil loss is due to land use practices. While the recorded annual soil loss magnitudes are within the tolerable thresholds, sustainable maintenance of appropriate land use practices is still critical in minimizing soil loss on the fragile slopes of the Rwizi catchment.