Effects of human settlement in reservoir catchment area_ Case of Mpira Dam

Abstract

Effects of human settlement within the catchment of Mpira Dam were investgated to measure contribution of human activity around Mpira Dam towards nutrient loading in the water. The study estimated the levels of nutrients from natural mineralisation and runnoff from land, examined possible correlation between demographic trends and nutrient loading in the water and possible relationship between removal of vegetative cover in the catchment area and nutrient loading in the water. Secondary data were collected on nitrate (N) and phosphorus (P) concentrations for Mpira River and dam outlet and on population density for the area for the past 20 years. Historic and current satelite images were employed to determine changes in vegetative cover. Data on inorganic fertilizer utilisation were also obtained. Differences between wet season and dry season N and P concentrations were statistically insignificant, 4% and 70% for Mpira River and Dam respectively. Population within the area remained around 6,000 from 1985 to 1995 and rose steadily to 9,000 from 1995 to 2013. There was a very strong positive corelation of r = 0.84, between population and nitrate concentration in the river and dam water. Land under vegetative cover reduced from 3,890 ha to 2,903 ha within the period 1975 and 1984, but rose to 3,723 ha by 1994. This could be attributed to afforestation interventions introduced in the area as an effort to reverse catchment degradation in 1985 but from 1994 the land cover continued to reduce to 3,397 and 2,064 in 2005 and 2013 respectively. Annual nitrate concentration averages tended to increase with decreasing vegetative cover area while it was not that clear for phosphorus. Mean annual means of 0.001 mg/l nitrate and 0.0003 phosphorus were obtained between 1990 and 2005 but increased to 0.02 mg/l nitrate and 0.01 mg/s phosphorus between 2006 and 2013. This increase coincided with a subsidised fertilizer distribution program by the Malawi Government to benefit farmers in Mpira area. Strong positive links were observed between population and nitrate concentration, amount of nitrogenous fertilizer and nitrate concetration in Mpira River while a strong negative link was apparent between vegetative cover and nitrate concetration. The study shows that population growth and respective human activities have increased the nutrient concentrations in Mpira River and Mpira Dam.