Monitoring of hydrodynamics under Conservation Agriculture in southern
Africa using electrical resistivity imaging
Abstract
Southern Africa is facing unprecedented strains on its agriculture,
including a rapidly increasing population and demand for cereals. The
global issues of climate change, water scarcity, and soil erosion are
also affecting southern Africa, which expects a drier climate in the
future. A promising tool in the fight for food security is Conservation
Agriculture (CA), a technique based on minimum soil disturbance,
mulching using crop residues, and crop rotation and/or intercrops. CA is
promoted by organisations including the United Nations due to its
potential to increase crop yields in arid/semi-arid climates; increase
drought resilience; and increase infiltration of rainwater, reducing
flooding and erosion. Despite its benefits and promotion, little is
understood of the hydrodynamics of soils under CA cultivation. In order
to investigate these hydrological processes, we installed Electrical
Resistivity Tomography (ERT) monitoring systems (PRIME, developed by
BGS) at three agricultural research sites in southern Africa (Zambia,
Malawi, & Zimbabwe) under CA and conventional tillage systems. The
sites are also instrumented with soil temperature, moisture, and matric
potential sensors, as well as monitored groundwater boreholes, enabling
comparison between monitoring techniques and the tracking of water from
the ground surface to depth. ERT deployments for the respective sites
include surface 2D, shallow cross-borehole 3D, and surface 3D electrode
arrays. Each PRIME system is configured for twice daily data collection,
and uses data telemetry for remote data retrieval. ERT monitoring allows
us to monitor the hydrodynamics from the root zone, through the soil
profile and vadose zone, to the aquifer. Initial results show
variability between the sites, and heterogeneous nature of the vadose
zone within the sites. This heterogeneity has been shown to influence
preferential fluid flow pathways in the vadose zone. Monitoring over
rainfall events has shown a strong, rapid response of pronounced,
shallow wetting fronts, with limited changes at depth. We are beginning
the process of comparing the hydrodynamics between CA and conventional
plots, and the procedure of optimising data processing to enable better
imaging of soil moisture changes at depth in the presence of rapid near
surface changes.