Epsilon Archive for Student Projects

Abstract

On-site and off-site effects of erosion in the Lake Victoria basin are some of the main contributors to the actual situation of poverty which is facing its population, one of the densest in the world. The Nyando River Basin (Western-Kenya) was identified as a regional erosion hotspot and one of the main sources of sediment and phosphorous into the Lake Victoria (ICRAF, 2001). In 2004, within the context of the Western Kenyan Integrated Ecosystem Management Project (WKIEM, ICRAF, 2003) measures to reduce erosion and increment agricultural productivity, through enhanced vegetation cover and improved management, were implemented in some plots located in East-Nyakach (lower Nyando River Basin).
This study investigated the effect of tree-planting and exclusion of grazing livestock in infiltrability, soil bulk density and erosion, as well as the existent relationships between these variables four years after rehabilitation started. A comparative study between WKIEM treated plots, where trees were planted, and adjacent controls was carried out in 9 paired-plots. Infiltrability was measured using double-ring infiltrometers in six sample points per plot, and by rainfall simulations at 20 and 60 mm/h intensities in two sample points. Steady-state infiltrability was afterwards estimated by means of curve-fitting to Philip’s equation (Philip, 1957). Sediment generation rates (Mg ha-1 h-1) were calculated, for 20 and 60 mm/h rainfall intensities, by drying and weighing the amount of sediments present in the generated runoff after two hours of simulated rainfall. Four topsoil bulk density samples were collected in each plot and vegetation cover was assessed as percentage classes.
The results showed that the average steady-state infitrability was significantly higher, and the average sediment generation rate and bulk density significantly lower, in the treated plots than in the controls. The higher infiltrability rates in the treated plots as compared to the controls were significantly explained by the increment in vegetation cover and the reduction in soil bulk density. Vegetation cover also explained the variation in the amount of sediments generated, which were significantly lower under an enhanced cover. A negative relationship was found between the steady-state infiltrability and the sediment generation rate.
Tree planting practices constitute a feasible and functional management option to reduce erosion by decreasing surface runoff generation through enhanced soil infiltrability, and increasing vegetation cover.

Keywords: Lake Victoria basin, Nyando River basin, steady-state infiltrability, bulk density, erosion, gully, runoff, sediment generation rate, double-ring infiltrometer, rainfall simulator, trees, vegetation cover, agroforestry.