Hydrological responses to land degradation in the Northwest Benin Owena River Basin, Nigeria.

Adequate insights into how land degradation alters the hydrology of river basins will help forecast the hydrological impacts of land cover change dynamics, thereby providing unique information required for sustainable river basin management. This study attempts to utilize a multi-dimensional methodology involving the application of the physically based Soil and Water Assessment Tool (SWAT) hydrological model and Partial Least Square regression (PLSR) statistical model to evaluate the response of the hydrological components within the Northwest Benin Owena River Basin (NWBORB) in Nigeria to land degradation. Using the historic land cover maps of 1986, 2002, and 2015, covering the basin, the SWAT model was employed to simulate the hydrological components for each historic year. The PLSR model was utilized to establish a response relationship between land cover changes and hydrological component modification within the basin. Results showed that between 1986 and 2015, 18.56% of the forest was lost, with a major portion (16.19%) gained by plantation. Consequently, annual water yield and surface runoff increased by 18.28% and 17.26% respectively, while annual base flow and actual evapotranspiration decreased by 22.58% and 21.72% respectively. The reduction in forest is strongly correlated with an increase in plantation (-0.833), surface runoff component (-0.723) and water yield (-0.532). Also, expansion of plantation land is strongly correlated with surface runoff components (0.877) and negatively correlated with base flow component (-0.573). Variable Importance of the Projections (VIP) from the PLSR model showed forest (VIP = 1.23), Plantation land (VIP = 1.02) as the most important land cover variables altering the basins' local hydrology. The study clearly shows that forest preservation plays an important role in the regulation of water resources within a river basin. This methodology can be replicated for poorly gauged river basins provided the land cover maps and stream discharge data are available.