Retrieving the crop coefficient spatial distribution for cotton under different growth status with Landsat ETM+ image

Crop water requirement was important in the irrigation scheduling. The crop coefficient is a parameter for estimating crop water requirement by multiplying with the reference crop evapotranspiration. Crop coefficient used to be approximated by crop developing days. The method must have some defect because the crop coefficient is a parameter related to crop status, climate condition and surface albedo. All the factors relating to the crop coefficient are spatially diverse and remote sensing has advantages in obtaining the distributing parameters for vegetation and climate factor. Based on the Penman-Monteith equation, the reference crop evapotranspiration and potential evapotranspiration for cotton under different growth status was estimated with measured meteorological data, then the crop coefficient for cotton was retrieved from a Landsat ETM+ image. And the sensitivity of crop coefficient to the influence factors were analysed. The results showed that the crop coefficient retrieved from the ETM+ image was greater than those suggested by FAO and the crop coefficient was influenced and decided by NDVI that represents crop growth status, while surface albedo that has a very larger variance for the sparse vegetation cover has scarcely any effect on crop coefficient and the climate factors has litter influence on crop coefficient too; with the vegetation cover fraction developing, the climate factor has a much more positive effect on the crop coefficient.