Can geophysics help feed people in a changing climate?

As climate change affects the globe, the regional impact in southern Africa is that of a drying climate with more frequent droughts (Dai, 2012). Combined with an increasing population, this poses challenges to future food supply (van Ittersum et al., 2016).

Conservation agriculture (CA) is a promising tool to help combat the factors affecting food security in the region. This is an agricultural technique consisting of minimal soil disturbance (no tillage), mulching the soil with crop residues (the remains of the crop after harvest), and crop rotation and/or intercropping to diversify the system (FAO, 2016). CA has been shown to increase both yields and the resilience of crops to drought when compared to conventional agricultural methods and has been found to increase water infiltration into soils, reducing runoff and erosion (McGarry et al., 2000; Palm et al., 2014; Pittelkow et al., 2014; Steward et al., 2018; Thierfelder and Wall, 2009; Verhulst et al., 2010).

Whilst we are aware that CA can improve yields and drought tolerance and that it can influence water infiltration, little is known about hydrodynamics below the ground surface, i.e. the way the water moves through and is stored within the soil profile and how it travels to the groundwater table. The CEPHaS project, described in detail in a blog by project lead Prof Murray Lark, aims to provide some answers to fill this knowledge gap.

Questions the project hopes to answer include:

• how does CA affect the quantity of water stored in the soil profile?
• how is water uptake by crops influenced by CA?
• how does CA alter recharge rates for groundwater?

My own research is on near-surface geophysics, specifically developing and applying electrical resistivity tomography (ERT), a ground imaging technique. ERT entails the use of an instrument to pass an electrical current through the ground via cabling and metal electrodes, and the measurement of voltages produced by the resultant electrical field in the vicinity of that current. By collecting many measurements in one dataset and processing the data with specialised software, it is possible to model (or image) the electrical resistivity of the subsurface in either two or three dimensions and at a high spatial resolution.

The resultant electrical resistivity models may be influenced by factors such as soil composition, porosity and water content. Through the repetition of ERT measurements over time, it is possible to see the changes in resistivity of the ground; assuming the soil composition has remained constant, any difference in resistivity is likely caused by variations in the water content of the soil, therefore it is possible to image soil moisture content over time (hydrodynamics). This ability to study the hydrodynamics of soils makes ERT ideally suited to helping to answer the research questions of the CEPHaS project.

At BGS we have developed a low-cost, low-power, ERT monitoring instrument called PRIME. Due to its low cost we have been able to purchase several instruments, enabling each to be permanently installed at an individual site. The low power requirements mean that we can install them in remote locations with just a solar panel and some batteries to provide the required energy. Consequently, we have been able to collect data with a higher temporal resolution than has been previously possible. We are collecting measurements twice daily in our three CEPHaS observatories in Zambia, Malawi and Zimbabwe.

In the CEPHaS project, we are using a combination of techniques to try to address our research objectives, through both the novel technique of ERT monitoring and more traditional methods:

• collecting weather data and monitoring the crops themselves to calculate evapotranspiration
• installing point sensors in the upper metre to monitor soil water content and soil suction (the capillary suction caused by the porosity of the soil).
• drilling boreholes to tens of metres and instrumenting them with piezometers to monitor the level of the groundwater table in order to study groundwater recharge

These traditional point-sensor methods provide high accuracy and a high frequency of measurements over time (temporal resolution), but their low spatial resolution means that they struggle to capture heterogeneities common in the subsurface and are not always able to show the cause of any rise or fall in groundwater levels.

We are using these more traditional methods together with ERT monitoring as the strengths and limitations of each complement one another.  While the collection of twice-daily measurements is pushing the boundaries for ERT monitoring, it is far less frequent than data collection by point sensors. Meanwhile, the high spatial resolution collected by ERT monitoring allows us not only to better understand heterogeneities between point sensors, but also to bridge the gap between the soil sensors in the upper metre and those sensors found in the boreholes at depth.

Fly through of the modelled ERT data at the Malawian CEPHaS observatory. BGS © UKRI.

The aerial photograph in the video shows the agricultural test plots, while the coloured cuboids beneath the ground surface show the modelled electrical resistivity. Each of the resistivity cuboids are 1 m square in plan view, and 3.1 m deep. In the initial, static phase, warmer colours equate to higher resistivities, while cooler colours signify lower resistivities. After this, the modelled ERT shows changes in resistivity over time, with blue colours depicting decreasing resistivity and red showing resistivity increases. The decreases in the resistivity are caused by the wetting fronts created by several rainfall events penetrating to depth.

In addition to the research objectives of CEPHaS, the project possesses a very strong capacity-building element. The training and experience that has been provided throughout the project means that colleagues at the University of Zambia, Lilongwe University of Agriculture and Natural Resources and the University of Zimbabwe are amongst the most advanced users of PRIME technology outside of BGS. Not only this, but the equipment installed in the three CEPHaS observatories is to remain after the end of the project, allowing project partners to continue with important research and bid for new projects.

The CEPHaS project aims to answer important questions surrounding the hydrodynamics of CA by using complementary methods, both traditional and novel. With our findings, we want to assist policymakers and potential CA farmers with making informed decisions about the uptake of this promising agricultural method. We hope that the dissemination of our findings, and the increased research capacity the project has built, will leave a legacy long beyond the project end.

Russell Swift

Geologist and geophysicist

BGS Keyworth

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