A Comparison between Active and Passive Sensing of Soil Moisture from Vegetated Terrains

A comparison between active and passive sensing of soil moisture over vegetated areas is studied via scattering models. In active sensing, three contributing terms to radar backscattering can be identified: 1a) the ground surface scatter term; 2a) the volume scatter term representing scattering from the vegetation layer; and 3a) the surfacevolume. scatter term accounting for scattering from both surface and volume. In emission, three sources of contribution can also be identified: 1b) surface emission, 2b) upward volume emission from the vegetation layer, and 3b) downward volume emission scattered upward by the ground surface. As ground moisture increases, terms 1a) and 3a) increase due to increase in permittivity in the active case. However, in passive sensing, term 1b) decreases but term 3b} increases for the same reason. This self-compensating effect produces a loss in sensitivity to change in ground moisture. Furthermore, emission from vegetation may be larger than that from the ground. Hence, the presence of vegetation layer causes a much greater loss of sensitivity to passive than active sensing of soil moisture.