A Novel Geophysical Method to Monitor Ultra-Shallow Reservoirs: Mapping of Soil Moisture Content in Subsiding Peatlands to Forecast Drought Effects and CO2 Emissions

Managed peatland deterioration by microbial oxidation is a worldwide process that leads to the gradual disappearance of these highly organic and fertile soils. Peat oxidation is often the result of aerated top soils by artificially kept low groundwater levels, which enhances aerobic microbial breakdown of organic carbon. This process leads to both subsidence and large quantities of CO2 emissions. Furthermore, artificially drained peatlands are vulnerable to salinisation when proximal to coastlines, and fires when situated in dry and warm climatic zones. The decrease of the soil moisture content (SMC) and groundwater level (GL) during drought episodes can cause both subsidence and accelerated CO2 emissions, both of which are big issues in the Netherlands. Severe drought results in salinisation of groundwater and greenhouse gas emissions by oxidising peat, and therefore is a societal burden to the Netherlands and worldwide. Furthermore, in coastal lowlands drought accelerates subsidence, leading to relative sea-level rise. Both SMC and GL are proven to be mappable using geophysical methods. Multi-coil offset Electro-Magnetic Induction (EMI) has high potential to map peat properties for managed peatlands from apparent electrical conductivity (ECa). Studies show that by using EMI, it is possible to obtain ECa for different depth intervals, which are used to estimate soil organic carbon. Furthermore, EMI has a long application history of non-invasive SMC mapping, although obtained ECa also depends on other factors, such as variations in soil composition, density, and pore water conductivity (σw). TNO and SoilMasters, an expert EM company, employ a mobile electromagnetic mapping system to recover the soil moisture content, a crucial ingredient in these issues. These innovative Electro Magnetic Induction (EMI) measurements of the soil moisture content are used in combination with geodetic and CO2 measurements to forecast subsidence and CO2 emissions. We present a time-lapse pilot study for SMC and GL employing EMI in a managed peatland in the central peat-rich delta plain of the Netherlands, near the city of Gouda. Furthermore, we use soil moisture probe and GL measurements at fixed point locations to confront the obtained ECa. We also link the obtained calibrated soil moisture maps to observed subsidence, and CO2 NEE (Net Ecosystem Exchange) flux. The resulting highly correlated measurements and observations can be well explained by precipitation and drought patterns. This makes our findings good proxies for spatio-temporal prediction of subsidence and CO2 emissions for dry and wet spells by monitoring measurements of EMI.