Assessing differences in groundwater recharge flows under conservation agriculture and conventional tillage

Abstract

Under the pressure of growing populations and risks to food security by increased extreme weather events (droughts and floods), many countries, particularly in semi-arid and arid regions, such as sub-Saharan Africa 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 and mulching, and previous studies indicate positive effects of these practices on soil moisture balance. However, effects of CA on groundwater recharge, for example changes to quantities, timing or altered recharge pathways have scarcely been investigated in the sub-Sahara environment, 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 plot-scale 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. Preliminary data analysis indicates that differences in onset and pattern of groundwater recharge between conventionally tilled and CA treatment plots can be observed, which could be valuable information for example in informing best management such as timing of fertilizer and pesticide application. The results of this study can therefore inform policy and decision makers and further our understanding of the effects promoted land use changes may have on local groundwater resources