Estimation of groundwater recharge due to conservation agriculture practice

A review of groundwater recharge studies in Sub-Saharan Africa demonstrates a strong relationship between rainfall and recharge, but with considerable uncertainty due to significant impact of land cover and in particular land clearing and agriculture. This research project focuses on impacts of conservation agriculture (CA) practice on groundwater recharge. Conservation agriculture is being encouraged by governments over conventional methods in the understanding that CA practices such as minimum tillage, retention of crop residue and crop diversity improves crop resilience under dry conditions. However, there has been little consideration of the direct impact of such practices on groundwater. In this study, we setup three experimental sites in Zambia, Zimbabwe and Malawi to quantify recharge patterns under CA in comparison to conventional farming practice. Each site will be instrumented with soil moisture monitoring probes, a weather station, monitoring boreholes and electrical-resistivity tomography (ERT) equipment. Environmental isotopes and tracers (such as CFCs and SF6) and water chemistry will also be analysed. The monitoring will help to elucidate processes in the unsaturated zone around the plant root zone through to groundwater. Ultimately, this will help understand groundwater dynamics and fractioning below surface of CA field

Literature review – assessing groundwater recharge estimates under conventional tillage and conservation agriculture

The purpose of this review is to identify studies from across the world that evaluated the impact of conservation agriculture (CA) on potential groundwater recharge in comparison to conventional tillage (CT), taking into consideration the techniques that have been used in measuring the soil or groundwater fluxes. In this review, we quantify case studies in which direct and indirect methods have been used to calculate a direct or proxy value of groundwater recharge under the different agricultural treatments of CA and CT. This review revealed that CA systems have the potential to improve infiltration or deep drainage and therefore potential recharge to the groundwater as evidenced by 54% of the case studies, including all studies (n=5) in the SADC region, however significant proportion of studies, mainly from the Americas and Europe, also reported either reduced potential recharge or no significant difference under different treatments. A majority of these studies used infiltration rates as a proxy. This review demonstrates that consideration on the methods used in estimating infiltration rates is important when evaluating the impact of agricultural systems on groundwater recharge in different climate zones. Issues such as the infiltration measurement technique used, timing of the measurements within the season, rainfall intensity, and soil type, are some of the parameters that must be carefully stated in studies to allow the infiltration rates within and across treatments to be comparable. The review revealed a gap in the literature for studies that used direct methods of recharge estimation to evaluate the impact of CA vs CT treatments. Unsaturated zone techniques provide only estimates of potential recharge based on drainage rates below the root zone and in some cases, drainage is diverted laterally and does not reach the water table. Use of direct methods that allow collection of data from the saturated zone such as groundwater level fluctuations in monitoring boreholes and environmental tracers such as Cl and stable isotopes of water, would be greatly beneficial to further our understanding of groundwater recharge processes beneath CA and CT systems. However, direct observations are more challenging to acquire and do have limitations

Groundwater recharge in sub-humid drylands under different agricultural systems

Groundwater is a strategic long-term water resource used by an estimated 70% of the populations in sub-Saharan Africa for drinking, irrigation and a wide range of economic activities. Understanding groundwater recharge processes is key for effectively using and managing water resources. Very few studies have used direct groundwater observations to assess the impact of different farming systems on groundwater recharge processes. This study focused on assessing basement aquifer recharge in 4 instrumented catchments in Malawi (Chitedze), Zambia (Liempe and Kabeleka) and Zimbabwe (Domboshawa) within the SADC region between 2019-2022. Employing a range of methods, including direct field observations (groundwater hydrographs, precipitation data, stable isotopes, chloride mass balance and residence time tracer data), we quantify the amount of groundwater recharge as well as the timing and nature of recharge processes under both conservation and conventional tillage systems in these four study sites. Groundwater recharge was measured in most years across the study sites. The study reveals the strong climate controls on seasonal groundwater recharge volumes, the influence of low permeability layers in the unsaturated zone, and the likely magnitude of impact from different farming practices. Groundwater residence times are high (i.e. low fractions of modern recharge, interquartile range 1-5%, n=46), even in shallow piezometers, suggesting these unpumped systems may be highly stratified. The results provide an evidence-based suite of data that reveals much about key controls on groundwater recharge in basement aquifers in sub-humid drylands and will inform the development and management of such groundwater systems. Keywords: Groundwater recharge, basement aquifers, Southern Africa Development Community (SADC