Crop Coefficients Derived from Reflected Canopy Radiation: A Concept

Abstract

ABSTRACT SIMILARITIES between the crop coefficient curve and a vegetation index showed potential for modeling a vegetation index into a crop coefficient. Therefore, the possibility of directly estimating the crop coefficient from measured reflectance properties of a crop/soil scene was investigated. Reflected canopy radiation in the 0.63 to 0.69 jwm and 0.76 to 0.90 jjim band widths was measured normal to and two meters above corn (Zea mays L.), and the normalized difference (ND) vegetation index was computed. The seasonal ND curve was curvilinear and resembled the basal crop coefficient (K^b) curve for corn. Leaf area index and canopy shading were 3.2 and 77.6%, respectively, when the ND reached its maximum valvue. A linear transformation of the ND was developed by equating the ND at effective cover and for dry, bare soil at the experimental site to the K^b at effective cover and for dry soil evaporation, respectively. This transformation produced a seasonal curve very similar to the basal crop coefficient curve and was named the basal spectral crop coefficient (K^J. Crop coefficients derived from spectral measurements are independent of the usual time base parameters, planting date and effective cover date, associated with traditional crop coefficients. Thus, the basal spectral crop coefficient is a real-time crop coefficient that permits the crop to express its response to weather, management practices, and stresses.