IMPACT OF LAND USE AND CLIMATE CHANGE ON WATER BALANCE - A CASE STUDY OF ROKEL-SELI RIVER BASIN IN SIERRA LEONE

Abstract

Sierra Leone, a country in West Africa, has experienced substantial economic growth in recent years. It has a special significance in the West African history, though the ruinous effects of the civil war continue to be felt. The Rokel-Seli River, which is the largest river in the country, stretches across the entire northern region before joining the estuary of the Sierra Leone River that in turn joins the Atlantic Ocean. It has a basin area of 10946km2 which infringes four major districts (Koinadugu, Bombali, Tonkolili and PortLoko Districts) having 31% (2,159,119) of the total country population. This basin is characterized by a heterogeneous forest-savanna mosaic and experiences a humid tropical climate with annual rainfall averaging 3000mm and mean monthly temperature of 25.78oC. ). There are two main seasons: Rainy/wet season (May to October) and Dry season (November to April). There are several small traditional villages in the area with rice cultivation in wet depressions and harvesting of non-timber forest products such as oil palm nuts. This river basin is of critical importance to the country’s economy as it supplies water to the Bumbuna hydroelectric power scheme as well as water for the agriculture, fisheries, mining and transportation and for ecological purposes. Keeping the long term water resources planning in mind, it is desired to study water balance of Rokel-Seli River Basin due to impact of land use and climate change. In this study, an attempt has been made to study the past variation of rainfall and to identify the trend. The long term trend has been detected using the MK and MKK test(s) for historical and projected (future) time series in terms of monthly, seasonal and annual basis. Further, shift change point has been detected for break point identification using SNHT and MWP test(s). Further, rainfall has been forecasted till 2050s under different climate scenarios with various CMIP5 emission conditions, i.e., RCP-2.6, RCP-4.5 and RCP-8.5. Climate change scenarios will help the future planning of water resources in the Rokel-Seli river basin (RSRB). The forecasted results are at 95% level of significance. Further, the runoff at the catchment outlet has been estimated and the effect of rainfall variability on the runoff estimates is highlighted. Moreover, the study also investigate the morphometric charactieristics of the basin using geospatial techniques in order to prioritize the basin interms of soil conservation measures. Through the morphometric analysis the basin is more prone to soil loss on the mid and downstream of the basin. The landuse land cover classification of the basin has been reclassified into five classes with most part of the basin covered with pasture (41%) and iv | P a g e forest(40), only 15% of the basin is under agriculture and 3% residential areas with just 1% as water bodies. The water balance over the basin have been simulated using SWAT model for a period of 14 years (2000-2014). The water balance component of the RSRB was analysed that out of 2180mm precipitation 23% flows out as runoff, 30% as ground water and 47% as evapotranpiration. The present study highlights the rainfall variability over the Rokel-Seli River Basin in Sierra Leone. The adopted analysis provides key information of basin’s water availability and hence this work offers benchmark information that can be used to increase the capacity of long-range water resource planning and management, land use planning, agricultural water development and conservation, and industrial water use over the next several decades at basin level. The results of the study also helps in the assessment of the future impact of climate change over the basin which affects changes in the hydrologic cycle of the basin