Impact of Land Use /Cover dynamics on Streamflow: A Case of Nzoia River Catchment, Kenya

Abstract

Degradation of watershed areas in Kenya’s basins is on the increase and is currently a major concern for the government. The main causes of watershe d degradation stem from the abuse and poor management of forests and soils, overgrazing, extension of settlements into watershed areas, and unsuitable felling of trees for fuel wood. Recent legislative reforms in the water and environmental sector have been introduced to stem these environmental negative trends. However, extensive quantitative hydrologic analysis is necessary for the assessment of the water balance of various basins to form a basis for policy actions . In this regard, modelling the hydrologic cycle at a local scale still remains the most important scientific method of research for the water balance assessment of basins. The study area chosen in this study is the Nzoia basin in Kenya. This basin is a typical example of a flood disaster prone basin experiencing increased flood related disasters due to the increased watershed degradation in the recent past. The Nzoia basin is situated between latitudes 1030’N and 0005’S andbetween longitudes 340E and 350 45’E and is the largest basin in Kenya’s Lake Victoria basin with an approximate area of 12,709km2 and a length of 334km to its outfall in to the lake. The Nzoia system has its sources in the forested highlands (Mt. Elgon, Cherangani Hills, Nandi Hills and Kakamega forest). The objective of this study is to simulate streamflow changes as a result of the land use/cover status as at 1973, 1986 and 20 00. Land use/cover data were based on Landsat images for these years. The runoff response as a result of the observed land use/cover change was test ed by keeping constant all input datasets in a SWAT model and varying the land use. The results from the model showed that with the expansion of the area under agriculture, the stream flow increases during the rainy seasons and reduces during the dry seasons, whereas when the area under forest cover is increased the peak stream flow reduces, but when the forest cover is reduced to almost zero there is an increased peak and mean stream flow in the basin. It is therefore worth noting that a decrease in surface runoff would be desirable, as this would also decrease the devastating effects of floods; the rapid expansion of urban centres in the lower parts of the catchment (Mumias, Bungoma, Rwambwa) can be said to be a major contributing factor to the annual devastating floods. The results also indicated an increasing trend in rainfall amounts in parts of the basin between the periods 1970 - 1998. A study of three rainfall stations (1BD02, 1DA02 and 1DD02A) has shown a significant increase in rainfall while one station, in the lower part of the catchment (EE01), has shown a significant decrease. The area under forest cover decreased betwe en 1970’s and 1986 by 6.4% in the northwest and south of the catchment. But between the 1980’s and the 2000’s there was an increase in area under forest cover by 41.3%. Agricultural land use showed an increase in areal coverage between 1970’s and 1986 by 6.7%, but in the year 2000’s the agricultural activities declined by 4.6%. The area under bushland/shrubland/riverine agriculture increased between the 1970’s, 1986 and the 2000’s by about 123.4% and 11.10% respectively. This could be as a result of an expansion in riverine agriculture