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