Can a vegetation index derived from remote sensing be indicative of areal transpiration?

Monthly, maximum-value-composited normalized difference vegetation indices (NDVI), calculated from NOAA-AVHRR images, were correlated with annual tran spiration (TR) estimates (i.e. annual precipitation minus runoff) for seven watersheds in Pennsylvania. A moderate relationship between the standardiz ed variables was detected. The NDVI-TR relationship dramatically improved w ith improved watershed TR estimates. At the Little River watershed in Georg ia, where the water balances of two sub-catchments could be reliably estima ted over water cycles of variable length (about 2 months to 11/2 years), th e correlation coefficient between NDVI and TR was found to be 0.94 (a sampl e size of 13). The present approach avoids the common practice of applying arbitrary hydrological models to validate the NDVI-TR relationship and atte mpts to minimize the effects of possible spurious correlations between the two variables that may stem from well-defined annual cycles in both the TR process and the foliage development of vegetation. It is concluded here tha t NDVI seems to reflect temporal changes in areal TR in a humid environment under well-vegetated conditions. (C) 2000 Elsevier Science B.V. All rights reserved.