Impact of Land Clearing and Irrigation on Groundwater Recharge in the Lake Chad Basin, Africa
Natural profiles have low matric potentials (mean > 1 m depth: -200 to -1600 m) and high chloride inventories (40 to 60 kg/ha/m), indicating no water movement below the ~ 1 to 3 m zone. The site that was previously cultivated and abandoned about 40 yr ago has matric potentials down to -3900 m and a low chloride inventory of 12 kg/ha/m, indicating previous flushing followed by current drying. The site that was cleared 6 months prior to sampling and never cultivated had high matric potentials to the water table (mean > 1 m: -2.5 m) and a low chloride inventory (12 kg/ha/m). A minimum estimate of the recharge rate is 210 mm/yr (velocity 7 m/yr, average water content of 3%). The rainfed cropland site had high matric potentials (mean > 1 m: -2.3 m) and a low chloride inventory (12 kg/ha/m), indicating drainage to 4 m depth. The 2 irrigated profiles showed variable matric potentials (mean > 1 m: -0.8 m and -80 m) and variable chloride inventories (24 and 77 kg/ha/m) attributed to spatial variability in drainage related to changes in clay content. Conversion of natural savannah to cropland should have a negligible impact on groundwater quality, increasing Cl by only 3 9 mg/L.