Determination of crop evapotranspiration of table grapes in a semi-arid region of Northwest Mexico using multi-spectral vegetation index

The main goal of this research is to develop and to evaluate a relationship established between Normalized Difference Vegetation Index (NDVI) and crop coefficient (K_c) for estimating crop evapotranspiration (ET_c) of table grapes vineyards (Vitis vinifera L., cvs. Perlette and Superior) in the semi-arid region of Northwest Mexico. Two consecutive growing seasons (2005 and 2006) of continuous measurements of ET_c with the eddy covariance system were used to test the performance of the K_c-NDVI relationship. An exponential relation relating K_c to NDVI (R²=0.63) is proposed and tested here as the basis for calculating ET_c. The obtained results indicate that the K_c-NDVI approach estimates ET_c reasonably well over two growing seasons. The root mean square error (RMSE) between measured and derived ET_c from NDVI during 2005 and 2006 were respectively about 0.45 and 0.76mmday^-1. Some discrepancies between measured and simulated ET_c occurred when NDVI saturates at high values, causing the under-estimation of evapotranspiration.