Gamma-Radiometric assessment of soil depth across a landscape not measurable using electromagnetic surveys
Soil depth affects plant available soil water storage capacity (PAWC). This storage capacity is important for grain yield, deep drainage, and nitrate leaching in dryland environments but is difficult to measure directly on a broad scale. Measurements of ECa provide a means of estimating soil depth at spatial resolution of <50 m when sufficient contrast in conductivity exists between the surface and the root-impeding subsurface layers. This work was conducted to address the inability of conductivity-based methods to estimate soil depth in commonly occurring sandy soil over lateritic cemented gravels/rock landscapes as both sandy soils and cemented gravels/rock have similar low conductivities. The gravel/rock layer is naturally enriched with radioisotopes of K, Th, and U. We developed the principle to estimate soil depth based on attenuation of radiation due to the thickness of the soil layer. The method was tested using ground and airborne gamma-radiometric surveys coupled with soil depth measurements. Site-specific gamma-emission was used to calculate component emissions from the soil and gravel/rock layers and in turn soil depth from its theoretical relationship with the component emission data. The root mean square error (RMSE) of soil depth estimates from flown and ground data were 7.9 and 4.9 cm, respectively. The maximum measurable soil depth was 45 cm due to attenuation. Plant available soil water storage capacity measured in the <90 cm soil layer was linearly correlated (r2 = 0.9) with soil depth. This opens the way to use gamma-radiometry to assess soil depth and PAWC in landscapes with shallow sandy soil over cemented gravels.