SOIL BACKGROUND EFFECTS ON REFLECTANCE-BASED CROP COEFFICIENTS FOR CORN

A previously developed reflectance-based crop coefficient (K(cr)) for corn, estimated from the normalized difference vegetation index (NDVI), has been shown to overestimate the basal crop coefficient (K(cb)) for corn by 24 % or more when used with a dark-colored soil. This overestimation occurs because the NDVI produces larger index values for the same vegetation amount over dark backgrounds. Thus, the purpose of this article was to investigate newer vegetation indices that have been developed to minimize soil background effects and to develop a reflectance-based crop coefficient for corn that applies over a wide range of agricultural soil reflectance. Two soils (light- and dark-colored) with red reflectance of 31 % and 13 %, respectively, were selected for this study. Reflectance data of the corn canopy were acquired with four combinations of these soils (light, dry; light, wet; dark, dry, and dark, wet) in trays inserted at the same place beneath the corn canopy. The soil adjusted vegetation index (SAVI), with an adjustment factor (L) set to 0. 5, was found to adequately minimize soil background influences from sparse to dense vegetation conditions. A linear transformation between the K(cb) for corn and the SAVI was used to convert the SAVI into the K(cr). The maximum difference for this K(cr) between the light, dry soil background and the dark, wet soil background (extreme cases) was less than 6 %. The K(cr) based on the SAVI corrects for a wet soil surface and requires no additional calibration to estimate the basal crop coefficient for corn grown on most agricultural soils.