Presentations

During the past four decades (1970s-2000s), land irrigation using river water has expanded in the semiarid Lake Chad Basin, Africa. Irrigation was shown to have an impact on groundwater resources in the Komadugu Yobe valley, a river shared by Niger and Nigeria: Piezometric surveys showed a pluridecennial (1993-2008) rise in the water table and this was interpreted as the result of irrigation return flow to the shallow (< 10 m) aquifer. In this region, agricultural land dedicated to irrigated cash crops (sweet pepper) increased dramatically at the expense of natural savannah; conversely, surface areas dedicated to rain-fed subsistence crops (millet and sorghum), economically less profitable, have decreased. Impacts of land clearing and irrigation on drainage and recharge were estimated in 2009 by drilling 4 bores down to 6 to 10 m deep through the unsaturated zone (2 irrigated croplands, 2 in natural savannah). Soil samples were analyzed for water content, matric potential (MP) and solute chemistry (major and trace elements, including Cl, NO3, PO4, K, F) to determine direction of water movement, recharge rates and predict changes in groundwater quality. Seven bores drilled in 2007 and 2008 (Favreau et al., 2008) supplemented the data set. Natural profiles have low MP (-200 to -5000 m) but high solute inventories (e.g., Cl: 40 to 60 kg/ha/m), indicating no water movement below the ~1 m herbaceous root zone. Profiles below rain-fed crops have low inventories (Cl: 12 kg/ha/m) and variable MP, depending on the land use history (high MP when cultivated, low MP when abandoned). Irrigated profiles have intermediate solute inventories (Cl: 24 - 77 kg/ha/m), locally enriched in anions concentrated in fertilizers (NO3, PO4, F) where clayey layers occur. Where sandy layers dominate, inventories are lower (Cl: ~20 kg/ha/m) and MP are high, showing drainage and recharge to the unconfined aquifer. Change from natural savannah to rain-fed cropping has a limited impact on groundwater quality (groundwater Cl content would increase by <10 mg/L). Conversion from rain-fed cropping to irrigation increases solute flux to the aquifer (sandy soils) may result in salt accumulation in clayey soil profiles. Both changes in land use result in increasing drainage and recharge rates. Reference: Favreau G., Scanlon B., Reedy R.C, (2008) Impact of land clearing and irrigation on groundwater recharge in the Lake Chad basin, Africa. In: Geological Society of America Abstracts with programs, 40, 6, 470.