MICROWAVE BACKSCATTERING AND EMISSION MODEL FOR GRASS CANOPIES

Microwave radar and radiometer measurements of grasslands indicate a s ubstantial reduction in sensor sensitivity to soil moisture in the pre sence of a thatch layer. When this layer is wet it masks changes in th e underlying soil, making the canopy appear warm in the case of passiv e sensors (radiometer) and decreasing backscatter in the active case ( scatterometer). A model for a grass canopy with thatch will be present ed in this paper to explain this behavior and to compare with observat ions. The canopy model consists of three layers: grass, thatch, and th e underlying soil. The grass blades are modeled by elongated elliptica l discs and the thatch is modeled as a collection of disk shaped water droplets (i.e., the dry matter is neglected). The ground is homogeneo us and flat. The distorted Born approximation is used to compute the r adar cross section of this three layer canopy and the emissivity is co mputed from the radar cross section using the Peake formulation for th e passive problem. Results are computed at L-band (1.4 GHz) and C-band (4.75 GHz) using canopy parameters (i.e., plant geometry, soil moistu re, plant moisture, etc.) representative of Konza Prairie grasslands. The results are compared to C-band scatterometer measurements and L-ba nd radiometer measurements at these grasslands.