An investigation of the extreme hydro-climatic characteristics and the underlying causes of river Nyando catchment, lake Victoria basin-Kenya
Ogutu, Ronald Sungu
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Extreme events, floods and droughts are natural occurrences which have caused numerous disasters world over. On a global scale the number of people affected and economic damages due to flooding and droughts are on the rise. Most of the deaths, post traumatic stresses and social and economic hardships can either be tremendously reduced or avoided altogether if the characteristics of the extreme events and the factors that cause and intensify them are understood and accordingly managed. The study picked the Nyando river catchment because of its vulnerability to these extreme events. Almost on an annual basis there are adverse reports on this area due to floods and to some extent on droughts. The purpose of the study was to come up with ways of reducing the effects of the extreme events. The objectives were to determine the causal factors of the extreme events, flow characteristics and to establish the effects of land cover and land use on extreme flows in the catchment. Data was collected from MWRD, KMD, LBDA, LVEMP, ICRAF, MP&ND and on site. Flow data was analyzed using Gumbel, Log Pearson-3 and General Extreme Value (GEV) fitting distributions. Rainfall data analysis was done to determine variability and reliability for Water Resources planning. Land cover and Land use analysis were done using ARCVIEW and ARCGIS from unclassified land sat images generated from P160R60, P161R60, P170R60 and P171R60 using GIS-IDRISI programme. The results indicate that flood frequency is on the increase, but contrastingly rainfall which is a major contributor does not show any appreciable corresponding rise. Rapid population increase in density of between 84 and 300 % from 1979 to 1999 has put a lot of pressure on the resources. Land use is more intense in terms of agriculture, deforestation for various activities, grazing and settlement. Deforestation in the upper catchment, especially around Mau, Tinderet and the Nandi escarpments is high thus altering the structure of the soils, and with the fragile nature of the area soils, soil erosion and flooding are enhanced. Steep topography of the middle zone compounded with deforestation and agricultural activities enhance both soil erosion and flooding implying less infiltration and less base flow soon after the rains meaning early and prolonged droughts. Gentle slopes of the lower basin promote sediment deposition and inundation which sometimes lasts even a month. The study provides a base from which various measures: preventive, mitigation, emergency response and rehabilitation of the flood management structures for both short and long term can be addressed to restore the hydrological balance of the basin. The study recommends an integrated watershed management to the basin, an all inclusive environmental management for sustainability. The study recommends further research on the impacts of these extreme events on the quality of water and the effect of global change on the extreme hydro-climatic characteristics.
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