Measurement of soil moisture content under physical crust by millimetre‐wave backscattering

Summary This paper presents a multi‐sensor remote sensing study of physically crusted soil with various moisture contents by using millimetre‐wave backscatter radiation. The backscattered radiation was used to generate an empirical model to estimate soil moisture at the thin surface crust scale by using a set of soils with known moisture content as a reference. The model was then used to assess the moisture content of a new set of soils from the same group. Real‐time measurement of soil moisture in the top layer beneath the soil crust is impractical at best and impossible using traditional methods. The proposed method, therefore, paves the way to assessing soil moisture content immediately beneath the surface crust as well as in a micro‐profile at depths of 0–3 cm. It can be combined with other active and passive remote sensing methods to provide a micro‐profile measurement of soil moisture to the depth of the entire upper root zone (0–30 cm) and of the plough layer (0–20 cm) under undisturbed conditions. The method is based on the emerging technology of millimetre waves, which provide improved spatial resolution of the subsurface concurrent with surface mapping. Further development and use of the method described herein will make it simple and practical to analyse and understand the processes governing the soil–water interface. The major processes for which this information is important are soil crusting, water budgets (infiltration, runoff and evaporation) and soil erosion.