Physically-based Methods for the Estimation of Crop Water Requirements from E.O. Optical Data

The estimation of evapotranspiration ET represent the basic information for the evaluation of crop water requirements. A widely used method to compute ET is based on the so-called "crop coefficient" K_c, defined as the ratio of total evapotranspiration by reference evapotranspiration ET₀. The value of crop coefficient is related to canopy variables representing the crop growth stage such as canopy height, fractional vegetation cover and Leaf Area Index. Considering that these canopy variables influence the spectral response of vegetated surfaces, a direct correspondence between K_c and reflectance measurements can be established. On this baseline, two approaches have been compared across with field measurements: a first one, based on the correlation between the Near Difference Vegetation Index and the value of basal crop coefficient; a second one, based on the direct application of Penman-Monteith model by using reflectance-based estimates of canopy variables.