Document Type

Article

Department

East African Institute

Abstract

Lake Victoria is of immense economic and environmental importance in the eastern and central African region. The lake is the largest freshwater body in the tropics and its basin supports one of the densest and poorest rural populations in the world. About 21 million people rely primarily on subsistence agricultural and pastoral production for their livelihoods. But pervasive poverty has hindered sustainable use of the land resources and there has already been considerable land degradation. There is little doubt that sedimentation and nutrient run-off, urban and industrial point source pollution and biomass burning, have induced the rapid eutrophication of Lake Victoria over the last fifty years. Invasion of water hyacinth and loss of endemic biodiversity are interrelated and compound problems for the lake environment and the welfare of its people. This project set out to assess the extent and causes of processes that contribute to pollution of the lake and to localize interventions for more sustainable land management. In a one year start-up phase, emphasis was placed on prioritizing river basins and assessing the land degradation problems in relation to nutrient and sediment inputs to the lake. This document summarizes progress on the proposed activities and the principal findings achieved under the start-up phase. The detailed results supporting these findings are given as an annex. A regional assessment identified the Nyando River Basin as a major source of sediment and phosphorus flow into Lake Victoria, and so much of the initial work was concentrated on this river basin while methods were being refined. The study identified severe soil erosion and land degradation problems throughout the Nyando river basin. Accelerated run-off and sheet erosion over much of the iii catchment area has led to severe rill, gully and stream bank erosion in lower parts of the river basin. The principal causes of erosion include deforestation of headwaters and overuse of extensive areas of fragile lands on both hillslopes and plains, coupled with loss of watershed filtering functions through encroachment on wetlands and loss of riverine vegetation. Associated with soil erosion, there has been substantial depletion of soil quality over much of the basin. Communities in the river basin are aware and concerned about water shortages and local land degradation but there is a low level awareness of the off-site effects. The lower parts of the river basin and the lake are particularly vulnerable to the return of a large rainfall event, such as experienced in the early 1960’s, which would cause catastrophic damage. The report outlines possible broad areas of intervention. The major requirements are to: (1) rapidly reduce pressure on vegetation cover over large areas of fragile land, including headwaters, shallow soils on hillslopes, and fragile soils on plains and the Nyando escarpment, (2) restore the filtering function of wetlands and riverine buffer strips, and (3) increase productivity of agricultural land that has high potential, particularly by removal of soil phosphorus and other soil nutrient constraints, which will help reduce pressure on fragile lands. The current catchment extension approach of the Soil and Water Conservation Branch has received considerable penetration in terms of number of participating farmers, but the efforts are unlikely to have significant impact on sediment and nutrient transfers to the lake. The Branch has had most impact in high population density areas largely within cash cropping systems. In contrast, it is in the marginal areas with extensive agropastoral systems that erosion risk is high. These are mostly communally-used grazing lands and government trust land where population density is fairly low and there is little incentive for investment in soil conservation. Complimentary approaches based on integrated watershed management are needed that jointly consider agricultural areas, forest areas and trust land, with highly active community participation. The emphasis must be on large area management of fragile lands and buffer zones around riparian zones, wetlands, and headwater catchment areas. Given the high cost of rehabilitation of marginal areas, priority should be given to assessment of their natural regeneration potential, in conjunction with policy and institutional innovation for their improved management. The project has been a focal point for the development of some exciting new methods for the quantitative assessment of land management problems that are of international significance. New remote sensing and ground survey tools were developed for rapid assessment of land and soil degradation and for precise spatial management of soils and vegetation. For example, critical target levels of dry season herbaceous vegetation cover depending on soil type and slope can be mapped. Progress towards achieving targets for buffer zone vegetation management can also be monitored. These tools are ready for deployment in the Nyando and other river basins.

Publication

International Centre for Research in Agroforestry

Share

COinS