The effects of forest loss on precipitation and stream discharge in the upper catchment of Ewaso Ngiro south river
Kigomo, Mathew Kiura
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Forest loss is a direct driver of change to precipitation and stream discharge. These include changes in annual rainfall, temporal and spatial distribution of rainfall and rise and fall in stream discharge. This study focused on assessing the effects of forest loss on precipitation and stream discharge in the upper catchment of the Ewaso Ngiro South River. The destruction of forest in the upper catchment of Ewaso Ngiro South River is threatening hydrological functions, endangering water supplies for people downstream. It is also causing severe siltation, flooding, extreme low flows, disruption of socio-economic activities and ecological alteration downstream. The aim of the study was to assess how forest loss is driving and affecting precipitation and stream discharge in the upper catchment of Ewaso Ngiro South River. This was done by identifying and mapping forest cover; assessing trends in precipitation and stream discharge; and the correlation between forest cover, precipitation and stream discharge between 1975 and 2008. Based on satellite imagery forest cover areas were computed and correlated to precipitation and stream discharge data to establish direction and significance of association. The results revealed a decline i n forest cover and in the number of rainy days and a general increase in the mean annual rainfall between 1975 and 2008. Comparison of the mean annual rainfall for three ten year periods from 1979 to 2008 revealed that there was an increase in the mean annual rainfall from 1979 to 1988 and 1989 to 1998 and a decline from 1999 to 2008. Stream discharge for Ewaso Ngiro, Narok and Siayabei Rivers decreased between 1975 and 2008. Correlation between forest cover and mean annual rainfall revealed a negative association while forest cover versus the number of rainy days revealed a positive association. This implied that as forest cover decreased the mean annual rainfall increased but its distribution over time decreased. Correlation between forest cover and stream discharge revealed a positive association. This implied that forest cover and stream discharge declined in unison. Impacts of forest loss on precipitation and stream discharge have far reaching implications on the social, economic and ecological character of the upper catchment of Ewaso Ngiro South River. The study culminated in the development of an integrated river basin management plan that gives direction on coordinated conservation, management and development of water, land and related resources across sectors within Ewaso Ngiro South River basin.