Detecting vegetation leaf water content using reflectance in the optical domain Pietro Ceccato a, *, Ste ´phane Flasse b,1 , Stefano Tarantola c,2 , Ste ´phane Jacquemoud d,3 , Jean-Marie Gre ´goire a,4 a Global Vegetation Monitoring Unit, TP 440, Space Applications Institute, Joint Research Centre of the E.C., 21020 Ispra (VA), Italy b Environmental Sciences Department, Natural Resources Institute, The University of Greenwich, Chatham Maritime, Kent ME4 4TB, UK c Methodologies for Information Analysis Unit, TP 361, Institute for Systems, Informatics and Safety, Joint Research Centre of the E.C., 21020 Ispra (VA), Italy d Laboratoire Environnement et De ´veloppement, Universite ´ Paris 7-Denis Diderot, Case Postale 7071, 2 place Jussieu, 75251 Paris Cedex 05, France Received 22 February 2000; received in revised form 6 October 2000; accepted 13 January 2001 Abstract This paper outlines the first part of a series of research studies to investigate the potential and approaches for using optical remote sensing to assess vegetation water content. It first analyzes why most methods used as approximations 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 water content in terms of Equivalent Water Thickness (EWT) at leaf level. Using laboratory measurements, the radiative transfer model PROSPECT and a sensitivity analysis, it shows that shortwave infrared (SWIR) is sensitive to EWT but cannot be used alone to retrieve EWT because two other leaf parameters (internal structure and dry matter) also influence leaf reflectance in the SWIR. A combination of SWIR and NIR (only influenced by these two parameters) is necessary to retrieve EWT at leaf level. These results set the basis towards establishing operational techniques for the retrieval of EWT at top-of-canopy and top-of-atmospheric levels. D 2001 Elsevier Science Inc. All rights reserved. Keywords: Leaf water content; Fuel moisture content; Optical domain; Shortwave infrared 1. Introduction During the last decades, the repeated occurrence of severe wildfires affecting various parts of the world has highlighted the need to develop effective monitoring tools to assess and eventually mitigate these phenomena. Research on biomass burning has progressed from monitoring active fires using satellite data (Malingreau & Tucker, 1988; Flasse & Ceccato, 1996) to studying impacts of biomass burning on the environment (Levine, 1996). To understand biomass burning processes, it is essential to monitor the parameters that influence these processes, such as meteorological vari- ables, amount of biomass, and vegetation water content. Remote sensing is advantageous for the detection of vegeta- tion water content since it could provide an indication of fire occurrence risks and burning processes from a local to global scale. The first step towards establishing an operational tech- nique to retrieve vegetation water content using remote sensing is to clearly identify and demonstrate where the potential lies. This study investigates the first level at which water content influences a radiometric response, i.e. at leaf level. It initially discusses the suitability of existing remote sensing methods for assessing vegetation water content in the context of biomass burning and clarifies definitions that are commonly used. It then docu- ments the physical basis supporting the use of remote sensing to directly detect vegetation water content in terms of Equivalent Water Thickness (EWT), and discusses a 0034-4257/01/$ – see front matter D 2001 Elsevier Science Inc. All rights reserved. PII:S0034-4257(01)00191-2 * Corresponding author. Current address: Environmental Sciences Department, Natural Resources Institute, University of Greenwich, Central Avenue, Chatham Maritime, Kent ME4 4TB, UK. Fax: +44-1634-883232. E-mail addresses: p.ceccato@greenwich.ac.uk (P. Ceccato), s.p.j.flasse@greenwich.ac.uk (S. Flasse), stefano.tarantola@jrc.it (S. Tarantola), jacquemo@ccr.jussieu.fr (S. Jacquemoud), jean-marie.gregoire@jrc.it (J.-M. Gre ´goire). 1 Fax: +44-1634-883232. 2 Fax: +39-332-785733. 3 Fax: +33-1-44-278146. 4 Fax: +39-332-789073. www.elsevier.com/locate/rse Remote Sensing of Environment 77 (2001) 22 – 33