Paper No. 9-7
Presentation Time: 9:45 AM
ASSESSING DIFFERENCES IN GROUNDWATER RECHARGE PATTERNS UNDER TILLED AND ZERO-TILL SYSTEMS USING A COMBINATION OF WATER LEVEL OBSERVATIONS, HYDROCHEMISTRY, AGE TRACERS (CFCS, SF6), AND ISOTOPES
BRAUNS, Bentje, British Geological Survey, Keyworth, NG12 5GG, United Kingdom, BANDA, Kawawa, Department of Geology, School of Mines, University of Zambia, Lusaka, P.O. Box 32379, Zambia, LAPWORTH, Dan J., British Geological Survey, Wallingford, OX10 8BB, United Kingdom, MACDONALD, Alan, British Geological Survey, Edinburgh, EH14 4AP, United Kingdom, MUDIMBU, Daina, Geology Department, Faculty of Science, University of Zimbabwe, Harare, Zimbabwe, NAMAONA, Willy, Lilongwe University of Agriculture and Natural Resources, Lilongwe, Malawi, OWEN, Richard, Department of Civil Engineering, School of Engineering, University of Zimbabwe, Harare, Zimbabwe and SINDA, Mabvuso, Department of Soil Science, School of Agricultural Sciences, University of Zambia, Lusaka, P.O. Box 32379, Zambia
Under the pressure of growing populations and risks to food security by increased extreme weather events (droughts and floods), many sub-Saharan countries have promoted a shift to climate-smart agricultural systems such as conservation agriculture (CA). The core principles of CA, along with crop diversification/rotation, comprise minimum tillage or zero-till and mulching, both of which may affect water retention in the upper soil layers. One of the observed benefits of CA is increased crop resilience in periods of drought, which is important to improve food security in this water scarce region. However, effects of CA on groundwater recharge, for example changes to quantities, timing or altered recharge pathways have scarcely been investigated, and need consideration because they may affect not only groundwater renewability, but potentially also contaminant transport and therefore groundwater quality.
In this study, we introduce a multi-disciplinary approach at three sites in Zambia, Zimbabwe and Malawi using a suite of investigation methods to characterize recharge patterns under CA and conventionally tilled plots. In our presented setup, high-resolution (30 minutes) groundwater level and weather data are complemented by monthly groundwater chemistry (major, minor, and trace elements) and stable water isotope analysis from shallow monitoring boreholes over a monitoring period of multiple (2—3) recharge seasons. Annual repeat measurements of residence time tracers (CFCs and SF6) add additional information on groundwater age. Information on water movement in the vadose zone are obtained via in-situ soil moisture measurements, and time-lapse electrical resistivity tomography (ERT) enables an integrated assessment of groundwater recharge processes. Preliminary data analysis indicates that differences in timing and pattern of groundwater recharge between conventionally tilled and CA treatment plots can be observed. The results of this study can inform policy and decision makers and further our understanding of the effects promoted land use changes may have on local groundwater resources.
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