| In order to fully utilize a microwave remote sensor as a monitor of agricultural crops, a good understanding of the way in which microwave energy interacts with vegetation is necessary. To achieve this level of understanding, mathematical models characterizing the vegetation canopy were developed and tested using data from experiments.; This investigation includes experimental measurements of canopy attenuation at 10.2 GH(,z) (X-band) for canopies of wheat and soybeans, experimental observations of the effect upon the microwave backscattering coefficient ((sigma)('0)) of free water in a vegetation canopy, and experimental measurements of (sigma)('0) (10.2 GH(,z), 50(DEGREES), VV and VH polarization) of 30 agricultural fields over the growing season of each crop. The measurements of canopy attenuation through wheat independently determined the attenuation resulting from the wheat heads and from the stalks. An experiment conducted to simulate the effects of rain or dew on (sigma)('0) shows that (sigma)('0) increases by about 3 dB as a result of spraying a vegetation canopy with water. The temporal observations of (sigma)('0) for the 30 agricultural fields (10 each of wheat, corn, and soybeans) indicates that fields of the same crop type exhibit similar temporal patterns.; Models previously reported were tested using these multitemporal (sigma)('0) data, and a new model for each crop type was developed and tested: the new models proved to be superior to the previous ones. The canopy-attenuation values mentioned previously, as well as those reported elsewhere, were used to set reasonable limits upon each model's estimate of attenuation. The influence of recent rain events (i.e., within two days of data collection) upon (sigma)('0) was found to be similar to that of free water in the canopy as determined by experimental measurements.; Finally, the need for and advantages of deterministic models were explored. One such model for computing the absorption loss due to leaves (L(,l)) was applied to the multitemporal (sigma)('0) data from the 10 corn fields with good results.; An examination of the way in which L(,l) varies with frequency and incidence angle and its related sensitivity to changes in soil moisture and leaf area index (LAI) concludes this investigation. |
