An investigation into the soil loss problem in the Upper Ewaso Ng'iro North Basin

Abstract

This study was undertaken to quantify the amounts of soil loss taking place in the different catchments in the Ewaso Ng'iro basin. The extent of erosion and changes in cover conditions in the Siakuu river catchment covered predominantly by the Mukogodo forest were assessed for the duration between 1967 and 1997. The study area comprised predominantly semi- arid environment with most of the areas experiencing high population growth rates and increasing pressure on land and water resources. The problem of soil loss in the Upper Ewaso Ngiro basin was studied using suspended sediment yield data and changes in land cover. Suspended sediment which comprises over 95 % of the total amount of sediment discharged in the rivers was used as means of quantifying the soil loss taking place in the basin. The effect of the bedload in the rivers was assumed to be negligible in the study. Samples were collected from 21 river gauging stations located in different rivers within the basin and used to develop sediment rating equations and also to determine the annual sediment yields from their respective catchments. The average sediment production rates per square kilo metre within all the catchments were evaluated and comparison done for the different catchments. Values of sediment production rates for Archer's Post, Teleswani, Likii and Lower Logilado catchments were found to be the highest ranging from 219 to 610 tones per square kilometre per year. Soil erosion and vegetation cover changes over the last 30 years in the Siakuu river catchment covered by Mukogodo forest were investigated. Remote sensing techniques were applied to estimate change in forest cover between 1967 and 1991. Fieldwork was carried out in 1997 to review the situation on the ground. GIS techniques were used to estimate the areas under different cover categories. It was also used to analyse changes occurring in the various cover categories over the years under study. Canopy cover was observed to have improved slightly over the 25 years span with the area under dense treeland kind of vegetation increasing from about 60 to 65 %. It was evident that vegetation succession had taken place in most parts of the forest. Some marked reductions in cover condition were also observed in some isolated areas. Gullies were used as an indicator of erosion activities within the forest environment. They were mainly described in terms of size and condition mainly highlighting their activity. The study found that despite the fact that gully erosion had been analysed in another catchment under intensive farming before using aerial photographs, the same could not be repeated under a tree canopy. This was confirmed when gullies were found to exist below some well established tree cover captured in the aerial photographs.