Influence of Land Cover Changes and Climatic Variability on Discharge Regime of Njoro River Catchment in Kenya
Mwetu, Kennedy Kitonga
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This study analyzed influence of land cover changes and climatic variability from 1973 to 2000 on discharge regime of Njoro River Catchment in Kenya. Analysis focused on hydro-meteorological data of 1977-1984 (Period I) and 1992-2000 (Period II), which are the periods found with available data and Landsat imageries for the same periods. Results show a downward trend of annual discharge that corresponded to increased deforestation. Forest area decreased by 25% while open fields combined with grasslands increased by 58% between the two periods. Discharge in Period I was 36% (0.23 m3/s or 82 mm/a) higher than Period II whereas increase of annual temperature (0.75˚C) between the two periods was significant (P < 0.05) but decrease in rainfall (24.87 mm/a) was not significant with effect of 25% deforestation. Statistical analysis of both four rainy and four dry years in the two periods showed that: 1) mean monthly discharge in Period I was not different (P < 0.05) and variability in decreases was higher (z-score: 4.60 > 3.98) in rainy years compared to dry years, and 2) mean discharge in Period II was different (P < 0.05) and variability in decreases was higher again in rainy years than dry years (z-score: 4.78 > 4.77). Influence of climatic variability alone accounted for 25% (0.06 m3/s) on reduction of discharge while human activity mainly deforestation accounted for 75% (0.17 m3/s) on reduction of discharge in Period II. This implies that land cover changes were largely responsible for decrease in discharge and therefore, intervention measures of restoring forest cover are recommended for Njoro River Catchment.