Development of reflectance-based crop coefficients for corn

Concurrent measurements of reflected canopy radiation and the basal crop coefficient (K_cb) for corn were conducted throughout a season in order to develop a reflectance-based crop coefficient model. Reflectance was measured in Landsat Thematic Mapper bands TM3 (0.63-0.69 um) and TM4 (0.76-0.90 um) and used in the calculation of a vegetation index called the normalized difference (ND). A linear transformation of the ND was used as the reflectance-based crop coefficient (K_cr). Basal crop coefficient values for corn were obtained from daily evapotranspiration measurements of corn and alfalfa, using hydraulic weighing lysimeters. The Richards growth curve function was fitted to both sets of data. The K_cb values were determined to be within -2.6% and 4.7% of the K_cr values. Reflectance-based crop coefficients are sensitive to periods of slow and fast growth induced by weather conditions, resulting in a real time coefficient, independent from the traditional time base parameters based on the day of planting and effective cover.