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

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

Longitudinal analysis of a long-Term conservation agriculture experiment in Malawi and lessons for future experimental design

Resilient cropping systems are required to achieve food security in the presence of climate change, and so several long-Term conservation agriculture (CA) trials have been established in southern Africa-one of them at the Chitedze Agriculture Research Station in Malawi in 2007. The present study focused on a longitudinal analysis of 10 years of data from the trial to better understand the joint effects of variations between the seasons and particular contrasts among treatments on yield of maize. Of further interest was the variability of treatment responses in time and space and the implications for design of future trials with adequate statistical power. The analysis shows treatment differences of the mean effect which vary according to cropping season. There was a strong treatment effect between rotational treatments and other treatments and a weak effect between intercropping and monocropping. There was no evidence for an overall advantage of systems where residues are retained (in combination with direct seeding or planting basins) over conventional management with respect to maize yield. A season effect was evident although the strong benefit of rotation in El Niño season was also reduced, highlighting the strong interaction between treatment and climatic conditions. The power analysis shows that treatment effects of practically significant magnitude may be unlikely to be detected with just four replicates, as at Chitedze, under either a simple randomised control trial or a factorial experiment. Given logistical and financial constraints, it is important to design trials with fewer treatments but more replicates to gain enough statistical power and to pay attention to the selection of treatments to given an informative outcome

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

Longitudinal analysis of a long-term conservation agriculture experiment in Malawi and lessons for future experimental design

Resilient cropping systems are required to achieve food security in the presence of climate change, and so several long-term conservation agriculture (CA) trials have been established in southern Africa – one of them at the Chitedze Agriculture Research Station in Malawi in 2007. The present study focused on a longitudinal analysis of 10 years of data from the trial to better understand the joint effects of variations between the seasons and particular contrasts among treatments on yield of maize. Of further interest was the variability of treatment responses in time and space and the implications for design of future trials with adequate statistical power. The analysis shows treatment differences of the mean effect which vary according to cropping season. There was a strong treatment effect between rotational treatments and other treatments and a weak effect between intercropping and monocropping. There was no evidence for an overall advantage of systems where residues are retained (in combination with direct seeding or planting basins) over conventional management with respect to maize yield. A season effect was evident although the strong benefit of rotation in El Ni˜no season was also reduced, highlighting the strong interaction between treatment and climatic conditions. The power analysis shows that treatment effects of practically significant magnitude may be unlikely to be detected with just four replicates, as at Chitedze, under either a simple randomised control trial or a factorial experiment. Given logistical and financial constraints, it is important to design trials with fewer treatments but more replicates to gain enough statistical power and to pay attention to the selection of treatments to given an informative outcome

The appraisal of pedotransfer functions with legacy data; an example from southern Africa

Predictions of soil hydraulic properties by pedotransfer functions (PTFs) must be treated with caution when they are used in an application domain which differs from the domain of their original development and calibration. However, in some settings, scientists may have little alternative but to use PTFs calibrated elsewhere. In this paper we consider how legacy data can be used to evaluate PTFs in new regions, paying particular attention to the challenges that arise when, as is often the case, the legacy data are not obtained by independent random sampling, and may be clustered at multiple scales. We undertook this work in southern Africa (Zimbabwe, Zambia and Malawi) where PTFs have been little-used, despite the scarcity of direct measurements of the soil properties of interest. We evaluated the extent to which existing PTFs provide a useful tool for the prediction of soil moisture content at field-capacity (−33 kPa) and permanent wilting-point (−1500 kPa) at different spatial scales. Soil legacy data for Zambia, Zimbabwe and Malawi were collated from various sources and PTFs from temperate and tropical domains were evaluated. We examined error variance components of predictions at within-profile, within-site and between-site scales; and estimated their mean errors. In general the better-performing PTFs (with respect to bias and the size of the error variance components) were ones calibrated with data from a tropical domain. This was most apparent at −1500 kPa. However, not all PTFs calibrated with data on tropical soils performed well, and predictions from some PTFs calibrated over a temperate domain were better at −33 kPa. The observations were spatially clustered, with data from different depth intervals in the same profile, from profiles in the same experimental site or farm, and from clusters across the region. This enabled us to show, with an appropriate mixed model analysis, that PTFs which effectively capture regional-scale variation may be less useful for predicting variation within a profile. We propose that such studies, based on legacy data, and with a suitable linear mixed model, should be used to screen PTFs of any provenance before their wider application