Effect of Climate and Land Use/Land Cover Change on Groundwater Recharge in Osun Drainage Basin, Nigeria

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Sustainable management of groundwater resources on a Basement Complex (BC) aquifer can be better undertaken with the spatio-temporal knowledge of groundwater recharge. Groundwater residence time of BC aquifers is between three to six months. This makes the knowledge of the monthly groundwater recharge with emphasis on influencing factors imperative using the Osun drainage basin as a case study. The objectives of the study were to: (i) determine the spatio-temporal pattern of climate in the basin between 1976 and 2015; (ii) estimate the spatio-temporal pattern of land use/land cover change between 1984-2015; (iii) estimate the mean monthly water budget of the drainage basin; (iv) examine the effect of climate on the basin groundwater recharge; and (v) evaluate the effect of land use/land cover change on groundwater recharge. Data used for this study include rainfall amount, number of rainy days, temperature, relative humidity, windspeed, sunshine hour and potential evapotranspiration (PET). Others were satellite imageries, Digital Elevation Model (DEM), slope and aspect, soil texture, depth to water, and runoff discharge. The climate data were sourced from the Nigeria Meteorological Agency (NIMET) and National Centre for Environmental Prediction (NCEP), while PET was estimated using FAO Penman-Monteith method. Satellites imageries and DEM were acquired from USGS-EROS satellite image database and National Aeronautics and Space Administration, respectively. Soil map was extracted from the harmonized world soil database of the Food and Agriculture Organization (FAO). Depth to water was adapted from Federal Ministry of Water Resources master plan (2014), while the discharge data at Apoje station were collected from Ogun-Oshun River Basin Development Authority. The data were subjected to remote sensing and GIS analysis, hydrological modelling using WetSpass-M, Z-score, Mann-kendall, Regression and Spearman Ranks statistical analysis. The findings of study were that: i. eighteen dry years were discovered between 1976 and 2015based on rainfall amount, and seven (39%) of these occurred in the last decade (2006-2015) of record, ii. the most significant land use/land cover change occurred in forest and wetland that declined by about 59% and 85%, respectively between 1984 and 2015, while, built-up area increased by 235% during the same period; iii. water budget analysis indicated that about 27% of the rainfall received in Osun drainage basin resulted in groundwater recharge, while the remaining were lost through evapotranspiration (43%), surface runoff (21%) and interception (9%); iv. rainfall amount accounted for 95% of the variability in groundwater recharge in Osun drainage basin, with highest recharge occurring in September (0 - 212.74mm); and v. land use/land cover change accounted only 10% reduction in mean groundwater recharge between 1984 and 2015. The study concluded that the knowledge of the monthly spatio-temporal status of the groundwater recharge and the influencing factors are important. It is recommended that in preparing sustainable groundwater resources management plan for the basin the spatio-temporal status of groundwater should be considered. Furthermore, the monthly groundwater withdrawal should be regulated by Ogun-Osun River Basin Development Authority and relevant agencies.



Climate, Land Use, Land Cover Change, Groundwater Recharge, Osun Drainage Basin