Sustainable Water Resources
SWR Research Example: Africa
Impact of Land Clearing and Irrigation on Groundwater Recharge in the Lake Chad Basin, Africa
In the western part of the Lake Chad basin (semiarid Africa), natural savannah has been increasingly cleared for rainfed (dryland) and irrigated agriculture for the past 30 years. Rainfed crops receive ~350 mm over 4 months of the year. Irrigation with river water adds ~300 to 400 mm of water over 7 months. A total of 8 boreholes (4 to 14 m deep) were drilled between November 2007 and June 2008 at distances of 0.1 to 10 km from the river (Figure 1). Soil profiles below different land uses were analyzed for water content, matric potential, and chloride, fluoride, and nitrate concentrations. Downhole electrical resistivity (DC) and surface conductivity (EM-31) were used to spatially extrapolate borehole measurements.
Conversion from Savannah to Rainfed Crop
Natural savannah profiles below 1 m depth have low matric potentials (mean -200 to -1600 m) and large chloride inventories (40 to 60 kg/ha/m), indicating upward water movement, salt accumulation, and no recharge below the root zone.
Irrigation and Soil Texture
Irrigation water is transferred from the river through canals (Figure 2). One of hte primary crops is soft peppers which are flood irrigated (Figure 3). Profiles in irrigated fields (Figure 2) show variable matric potential and chloride inventories (24 to 77 kg/ha/m), attributed on the basis of resistivity logging to changes in clay content. EM-31 mapping showed resistivity contrasts of two orders of magnitude, suggesting that at the scale of the 0.5 km2 survey, ~1/3 of the area had clay layers at depth that hinder drainage. A savannah site that was cleared only ~1 yr prior to sampling, but had not been cultivated, showed high matric potential (mean -2.5 m) and a low chloride inventory (12 kg/ha/yr) below 1 m depth, similar to a nearby rainfed agriculture profile, indicating a rapid response to land use conversion. Estimated recharge is 210 mm/yr. High values of F and N in fertilizers, as well as atmospheric inputs and N-fixation in natural savannah, are likely explanations for the observed pattern related to land use. Low soil water concentrations in rain-fed profiles indicate that unsaturated zone salts may have reached the water table.
Conclusions and Perspectives
In the Lake Chad basin, there is no recharge under natural settings. Conversion from natural savannahto rainfed cropland increases recharge. Irrigation increases recharge relative to natural settings and also results in salt accumulation in the soil profile. Discontinuous, low permeability clay layers limit or prevent recharge in some areas.