Crop coefficients are determined empirically as the ratio of the actual ET of a crop to the ET of a reference crop which is usually alfalfa or grass. The normalized time base (percent of time from planting to effective cover and elapsed days after effective cover) associated with the use of these crop coefficients does not always allow matching the crop coefficient in the irrigation scheduling model with actual crop growth in the field. This is especially the case when abnormal climatic conditions occur (such as a cold, wet spring) that slow plant growth during the initial vegetative development of warm climate crops such as corn. In this case actual ET could be overestimated and, if corrections are not initiated within the irrigation scheduling model, the error may compound as the season progresses and eventually result in overirrigation of the crop. The technique proposed in this paper uses reflected canopy radiation as a means of effectively monitoring crop growth and ultimately uses this information to derive reliable crop coefficients which can be used as inputs to irrigation scheduling models.