Detecting vegetation leaf water content using reflectance in the optical domain

This paper outlines the first part of a series of research studies to inves tigate the potential and approaches for using optical remote sensing to ass ess vegetation water content. It first analyzes why most methods used as ap proximations of vegetation water content (such as vegetation stress indices , estimation of degree of curing and chlorophyll content) are not suitable for retrieving water content at leaf level. It then documents the physical basis supporting the use of remote sensing to directly detect vegetation wa ter content in terms of Equivalent Water Thickness (EWT) at leaf level. Usi ng laboratory measurements, the radiative transfer model PROSPECT and a sen sitivity analysis, it shows that shortwave infrared (SWIR) is sensitive to EWT but cannot be used alone to retrieve EWT because two other leaf paramet ers (internal structure and dry matter) also influence leaf reflectance in the SWIR, A combination of SWIR and NIR (only influenced by these two param eters) is necessary to retrieve EWT at leaf level. These results set the ba sis towards establishing operational techniques for the retrieval of EWT at top-of-canopy and top-of-atmospheric levels. (C) 2001 Elsevier Science Inc . All rights reserved.