Retrieval of surface parameters from microwave radiometry over open canopies at high frequencies

Methods for interpreting the microwave high-frequency emission of open canopies are proposed in this article. Three frequencies are considered: 23.8, 36.5, and 90 GHz. The microwave data were collected simultaneously with the canopy infrared temperature, and ground measurements of moisture, temperature, and bulk density of the soil together with water content and volume fraction of the vegetation. Soil emission is modeled using dielectric permittivity and roughness parameters previously determined over bare soil surfaces. Two kinds of vegetation canopies are considered: a sparse-growing sorghum and a wheat canopy composed of dense, large patches. The microwave mixed soil-vegetation emission is analyzed using three modeling approaches. At 23.8 and 36.5 GHz, a random continuous approach can be applied. The sparse canopy emission is efficiently described by considering the vegetation layer as homogeneous. Conversely, a mixing equation must be used for the patchy canopy. At 90 GHz, another technique is applied. It consists of including vegetation in soil roughness parameters. The problem with retrieving the canopy temperature and the near-surface soil moisture at high frequencies is addressed. For the sparse canopy, soil moisture retrieval is possible. The canopy temperature is retrieved with poor accuracy, however. Over the patchy surface, geophysical parameter retrieval with high frequencies alone seems much more difficult.