Time-Lapse GPR Aids Investigation of Depth and Timing of Crop-Available Soil Water Under Various Agricultural Practices

Abstract:

Soil moisture is an important driver of nutrient availability, soil health and crop growth. A major challenge for improved crop management is to representatively assess soil moisture conditions as the crop is growing under different management practices. Geophysical imaging is an effective method for quantifying subsurface hydraulic properties and moisture. Despite the success of geophysical imaging techniques for many shallow subsurface investigations, they are rarely used to study the effect of agricultural practices on soil moisture dynamics. Time-lapse ground penetrating radar (GPR) was used to evaluate soil moisture dynamics in three rainfed long-term agricultural cropping systems: sorghum (rotated with wheat and soybean) under no-till and conventional tillage, and soybean (monoculture) under conventional tillage. GPR transects were acquired daily, as ground conditions permitted, over the span of one month to capture temporal variations in soil moisture. Electrical resistivity tomography was also used to determine the bulk resistivity of the soil to a depth of 1 to 10 m and was interpreted based on Archie’s law modified to account for the clay content. Precipitation data were collected from a nearby hydrometeorological station and used as an input for constraining the water balance. Our results indicate time-lagged relationships between precipitation and GPR reflected signal intensity, which reveals the time scales of vertical soil-water movement and retention under different practices. The analysis suggests that time-lapse GPR can be an effective non-invasive approach to quantify soil moisture dynamics in agricultural settings, especially over large areas and where direct measurements are difficult.