Understanding Spatiotemporal Variations in Soil Moisture Associated with Tile Drains Using Electrical Resistivity Imaging
University of New Brunswick
Agriculture in the Canadian Atlantic provinces is influenced by a humid continental climate, hilly landscape, and soil derived from glacial till. As a result, many regions suffer from poorly draining soil. Tile drains have gained popularity since the 1970s for removing excess moisture and increasing field productivity. The goal of this study was to use time-lapse electrical resistivity imaging (ERI) to resolve, in space and time, how well tile drains help to remove excess water from sandy loam agricultural soils underlain by low permeability glacial till in the Saint John River valley at Fredericton, New Brunswick. One of the plots was equipped with tile drains at approximately 90 cm depth, while the other was not. Time-lapse ERI successfully inferred temporal changes in saturation in both fields. ERI-derived estimates of changes in water storage were in good agreement with those made using capacitive moisture sensors except immediately following intense rainfalls at the shallowest sensor locations (15 cm depth) where installation artifacts may have affected both methods. Results indicated that the tile-drained field (TDF) showed significantly faster drying immediately following rainfall, and that drying extended to greater depth after prolonged periods compared to the non-tile-drained field (NTDF). Up to 20% desaturation was observed after a week of drying in the TDF during the late summer period. Three different methods - soil water balance, moisture sensors, and time-lapse ERI - were used to estimate changes in water storage in the two fields during spring 2020. ERI-derived estimates of water gain during an intense rain event indicated that the TDF gained an amount nearly equal to the 25 mm rainfall, while the NTDF gained very little water, consistent with its higher antecedent water content and measurements of surface runoff. Following the rainfall event, with both fields near full-saturation, ERI indicated that the tile drains improved the field-scale drying by a factor of 3 after 48 hours. Beyond 48 hours, the effect of the tile drains was less noticeable as the two fields lost water at similar rates.