2008 Joint Annual Meeting (5-9 Oct. 2008): Impact of Land Clearing and Irrigation on Groundwater Recharge in the Lake Chad Basin, Africa

In the Lake Chad basin, as in many semiarid African regions, land has been increasingly cleared for rainfed and irrigated agriculture. Seven boreholes (4 - 14 m deep) were drilled in November, 2007 at distances of 0.5 to 10 km from the Komadugu River (2 in natural savannah, 1 previously cultivated and abandoned, 1 cleared and noncultivated, 1 rainfed cropland, and 2 irrigated cropland). Soil samples were analyzed for water content, matric potential, and chloride concentrations to determine direction of water movement and recharge rates.

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.