Soil moisture profile estimation from surface measurements at multiple frequencies

Abstract

It is possible to obtain estimates in situ of volumetric soil moisture content (SMC) as a function of depth using measurements of radio wave reflection at the soil surface at several discrete frequencies. The feasibility of the method was demonstrated through computer simulation by deriving empirical relationships between the number of frequencies, the frequency range, and the number of soil layers for which the SMC is estimated. The SMC profile was obtained to a depth of 1.5 m by inverting, through function minimisation, a simulation of the reflection coefficient from layered dielectric materials. The number of soil layers in which SMC could be resolved was found to be twice the number of frequencies used. The required bandwidth increased with the number of soil layers from 20 MHz for 6 layers to 140 MHz for 16 layers. Within some assumptions about the nature of the radio wave propagation, the theoretical accuracy of the estimate depended only on the quantisation error introduced by having to consider discrete layers of uniform soil with finite thicknesses. The method may have potential for enabling sensitive measurements of SMC profiles at lower cost and lower complexity than methods which use analysis of steps or pulses, such as time-domain reflectometry, and should be well suited for routine use by horticulturists and soil researchers.