How to solve a problem like soil moisture monitoring – featuring Friston Forest

We have finally been able to install some of our baseline monitoring equipment, which we are using at one of our pilot sites for the PROWATER project!

PROWATER is a partnership project that the South East Rivers Trust is delivering locally with Kent County Council and South East Water. The project will investigate the opportunities for ecosystem-based adaptation to water scarcity and climate change.

As water is a scarce resource in South East England, we are interested in understanding the impact of different habitats and land management options on the availability of water. A lot of our public water supply in the South East comes from groundwater stored in underground reservoirs, also known as aquifers. These aquifers are normally recharged by winter rainfall. The quantity and quality of water recharged is highly influenced by how the land on top of the groundwater body is managed. PROWATER aims to understand how to quantify the benefits from those different management options, and how to reward those managing the landscape, like farmers and foresters, for making choices that protect those water resources.

Drilled probes

Our soil moisture profile probes have been drilled into the soft chalk that can be found if you scratch the few centimetres of topsoil from the surface of our pilot site near Eastbourne.

Here, they will continuously log the volume of water draining through the soil and chalk on its way to the groundwater body that collects water many metres below.

This groundwater body provides public drinking water (via South East Water abstraction) to residents of nearby settlements and beyond, but it also feeds into rivers and other groundwater-dependent ecosystems.

When it rains, water infiltrates into the well-drained soils of the area and makes its way past plant roots down into the chalk rock – either rapidly through cracks and fissures, or slowly through the tiny pores in the rock.

This process only really happens at certain times of the year, when plants don’t use much water – usually, that means the time between October and March, which explains why a lack of water in our region is often an issue of dry winters rather than summer droughts.

Drilling the probe in our scrub section © South East Rivers Trust

Improving understanding

Most of the land at our pilot site is owned by the water company, who manage the SSSIs on it (chalk grassland and chalk heathland) for the biodiversity they provide and to protect their water source from pollution.

The rest is rented to the Forestry Commission following the planting of a pine forest in the 1930s, which has since been succeeded by a plantation of beech trees. The thought at the time was that the forest might influence the microclimate in the area so that more rain would fall, which would mean more recharge to the groundwater.

What we are interested in is to better understand how the habitats that are on top of the chalk influence how much water actually infiltrates and drains towards the groundwater. We have spent a lot of time in the PROWATER project discussing what the best way of monitoring this would be, as there are a range of factors influencing this:

  • How much rain is caught or slowed by the vegetation cover
  • Demand for water from growing plants
  • Type and depth of soil (the thin soils on our site drain quickly and don’t hold much water)
  • Weather conditions (including wind and temperature that influence how much water plants use)
Richard Dyer, South East Water’s Biodiversity Lead, next to the new probe at Fore Down © South East Rivers Trust

How the probes work

Ultimately, only water that makes it past plant roots is likely to contribute to recharge in the groundwater body, so we decided to try to monitor water movement as far beyond roots as possible.

Soil moisture profile probes have sensors every 10 centimetres and log volumetric water content (i.e. the proportion of volume around them that is water rather than rock or air) every 15 minutes, to a depth of 120 centimetres.

These probes are tapered and can be drilled directly into the bedrock, which means they sit flush against their surrounding material and can monitor accurately. It also means there is little disturbance to the soil and rock profile, which is important to get a realistic picture of the processes going on, and is even more important if, like us, you are working on a protected site.

The probes are drilled into the ground to cause as little disturbance as possible and sit neatly against the soil © South East Rivers Trust

Tracking the water route

This data will allow us to compare the proportion of water that makes it from the very top of the soil profile to the bottom between different habitat types.

We are looking at protected chalk grassland and chalk heathland, as well as gorse, scrub and mixed woodland. By monitoring this for at least two years, and clearing scrub, gorse and woodland from some of the sites, we hope to see the impact on infiltration and potential recharge.

Alongside that, we will monitor the quality of water draining into the ground. This way, we will get a better, local understanding of the value of different habitats to water supply, which will help us make better management choices across the landscape to protect valuable habitats and increase our resilience to the impacts of climate change, such as drought.

You can read more about the PROWATER project here


The probes installed on Lullington Heath © South East Rivers Trust