Estimation of wheat canopy resistance using combined remotely sensed spectral reflectance and thermal observations

A procedure is developed for determining actual canopy resistance ( r c ) from the combined use of remotely sensed spectral reflectance and thermal observations. The procedure entails three basic steps, the first being to establish the resistance for unstressed canopy conditions. This minimum canopy resistance ( r cm ) is derived from a spectral vegetation index and is then used to calculate the minimum canopy temperature ( T sm ) or canopy temperature which would be associated with the prevailing meteorological conditions and an unstressed canopy. Actual canopy temperature ( T s ) is obtained from remotely sensed thermal emissions and is the temperature associated with actual canopy resistance ( r c ). From a known (simulated) relationship between the ratios r cm / r c and T sm / T s , r c may be derived as the only unknown variable. Attention is focused on establishing a relationship between the resistance and temperature ratios for a wheat canopy by using models to simulate canopy reflectance, r cm , T s , and T sm . The modeled relationship is found to be stable for a range of leaf area index values, meteorological conditions, and times of data acquisition and compares favorably with an empirical relationship developed using field data collected over wheat.