Assessment of NDVI and agrometeorological indices for major crops in central Greece
Major agricultural crops such as vineyards and apples can be monitored by comparing normalized difference vegetation index (NDVI) with several agrometeorological indices. The agrometeorological indices used in this study are heat unit (HU), photothermal unit, vapor pressure deficit, phototemperature nyctotemperature and cumulative precipitation. The NDVI curve of temporal variability for vineyards have shown an unexpected fluctuation with continuous high and low values, whereas the raise in chlorophyll concentration due to first spring sprouting should correspond to NDVI raise. No particular relation between cumulative rainfall and NDVI is detected, because the vineyards of the studied area are systematically irrigated. An inverse correlation between degree-days and NDVI especially during the second 10-day interval of August was also detected. For apples, a positive relationship between the HU parameter and NDVI, was observed. The amount of cumulative degree-days was also important at the end of natural ripening, which signifies the importance of the HU to apple crops. The whole study has shown that maximum NDVI can be used for crop monitoring in Greece. Furthermore, NDVI is representative of phenological changes of the examined crops, namely vineyards and apples. There is a positive and strong relationship between NDVI and some agrometeorological indices and parameters, especially HU and cumulative rainfall throughout the growing season of 1994. This finding was mainly due to the critical significance of basic meteorological parameters to plant's physiology and phenology, such as air temperature, relative humidity and rainfall. (C) 2002 Elsevier Science Ltd. All rights reserved.