Farming systems, runoff and contamination of water in Kamundi watershed in Malawi.

摘要

In Malawi, poor management of farmlands and use of agrochemicals result in acute forms of non-point source (NPS) water pollution. However, research data and information allowing evaluation of effects of NPS pollution and control measures on water quality are limited. This study evaluated effects of cropping systems and vetiver grass (Vetiveria zazaniodes) on reduction of non-point-source pollution from croplands. The crops evaluated were maize (Zea mays L.) and burley tobacco (Nicotiana tobacum) against bare plots as controls. Surface runoff samples were collected in field pits and analyzed for suspended sediments, NO3-N, total phosphorus and organochlorine (OC) pesticide residues to determine on-site material losses.;The results obtained showed higher material losses from the maize plots than the tobacco plots due to differences in crop canopy height development and structure. The losses were higher at the beginning of the season when the soil was dry, bare and still loose from previous tillage than at the end of the season when the soil became compacted. The results also showed that vetiver grass hedgerows can significantly reduce sediment and nutrient losses in surface runoff. The vetiver grass effect on pesticide residue loss reduction was, however, less definitive suggesting that trace levels of OC residues may not be sensitive to the cropping systems and management practices used.;A study of off-site effects investigated changes in stream discharge and water quality with respect to seasonal rainfall and smallholder farming activities. The results obtained showed that the variability in flows and pollutant loading in Mtemankhokwe Stream depended on seasonal rainfall/runoff patterns. Suspended sediment and total phosphorus pollution were dominant during high stream flows while nitrate (NO3-N) contamination was dominant during low flows, suggesting a concentration dilution effect downstream. Also, this phenomenon suggests that runoff and soil erosion control in the watershed can reduce non-point source pollution from croplands in watersheds. Overall, the establishment of a long-term research and monitoring program could improve our understanding of the seasonal changes in water quality and would also enhance detection of long-term changes in stream discharge.