Using directional TIR measurements and 3D simulations to assess the limitations and opportunities of water stress indices D. Luquet a, * , A. Vidal b,1 , J. Dauzat c,2 , A. Be ´gue ´ a,3 , A. Olioso d,4 , P. Clouvel e,5 a CIRAD-AMIS, Agronomie/Geotrop, Maison de la Te ´le ´de ´tection, 500, rue J.F. Breton, 34 093 Montpellier Cedex 5, France b IPTRID, Viale delle Terme di Caracalla, 00100 Rome, Italy c CIRAD-AMIS, AMAP, TA 40/PSII, 34398 Montpellier Cedex 5, France d INRA, Unite ´ Climat-Sol-Environnement, Site Agroparc, 84914 Avignon cedex 9, France e CIRAD-CA, Programme Coton, Avenue Agropolis, TA70/03 34398 Montpellier cedex 5, France Received 22 March 2002; received in revised form 3 September 2003; accepted 6 September 2003 Abstract Multidirectional remotely sensed optical and thermal images acquired within a row cotton crop in Montpellier (France) were used to test the opportunities and limitations of an existing water stress index, the Water Deficit Index (WDI, based on the trapezoid approach). The WDI was applied with multidirectional crop surface temperatures (T s ) and reflectance data acquired on a row-cotton crop with different water and cover conditions from 11 different view angles in the east/west plane. This data set allowed a biophysical analysis of this index both inside and outside its validity domain, initially limited in terms of T s measurements in a [  20j, +20j] view angles interval around nadir. Results showed that the WDI was a robust approach, since its calculation is based on the relationship between crop cover and T s . However, it yielded some directional errors in the case of sparse crops even in its validity domain where the relative variation of WDI between oblique angles and nadir could reach 14% (and more than 40% for larger view angles). The same degree of variability was observed between WDI values estimated on a same plot at two different times in a given day from a nadir observation. In a large range of crop heterogeneity, hourly sunlit soil fraction presented a stronger influence on T s than the total soil fraction. However, by adapting the view angle to daytime measurements and crop structure, it seemed possible to overcome sunlit soil effects. These experimental results were tested and extrapolated using a 3D crop energy balance model (Thermo). It allowed simulations of directional T s measurements according to various sun/sensor angular configurations, crop structure, and water status characteristics. This confirmed the limitations of the trapezoid method both within and outside its validity domain. Moreover, Thermo allowed the computation of a ‘‘directional’’ WDI accounting for angular and hourly sunlit soil effects variability on T s . The interest of adapting the view angle to daytime measurements and crop structure was confirmed by comparing this ‘‘directional’’ WDI with the ‘‘theoretical’’ one (based on the original trapezoid approach). These results should encourage further development of water stress indices based on bidirectional thermal infrared and optical measurements to quantify and thus overcome sunlit soil fraction effects. D 2004 Elsevier Inc. All rights reserved. Keywords: Surface temperature; Directional effects; Sunlit soil fraction; Water stress index; 3D modeling; Row-cotton crop 1. Introduction The most established method for detecting crop water stress remotely is based on the measurement of surface temperature. Such a concept stands on the ecophysiological and biophysical signification of crop temperature (Jones, 1999a,b; Lu et al., 1998; Petersen et al., 1991). The interpretation of crop surface thermal infrared (TIR) signal as a water status indicator has been studied and documented for about 20 years (Garrot et al., 1994; Idso, 1982; Jackson, 1982; Jackson et al., 1981; Wanjura et al., 1990, 1984; Wanjura & Mahan, 1994). It has led to the development of 0034-4257/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2003.09.008 * Corresponding author. Tel.: +33-4-67-54-87-41; fax: +33-4-67-54- 87-00. E-mail addresses: luquet@teledetection.fr (D. Luquet), alain.vidal@cemagref.fr (A. Vidal), jean.dauzat@cirad.fr (J. Dauzat), begue@teledetection.fr (A. Be ´gue ´), olioso@avignon.inra.fr (A. Olioso), clouvel@cirad.fr (P. Clouvel). 1 Tel.: +33-4-67-04-63-38; fax: +33-4-67-63-57-95. 2 Tel.: +33-4-67-61-56-68; fax: +33-4-67-61-65-76. 3 Tel.: +33-4-67-54-87-39; fax: +33-4-67-54-87-00. 4 Tel.: +33-4-32-72-24-06; fax: +33-4-32-72-23-62. 5 Tel.: +33-4-67-63-62-52; fax: +33-4-67-63-62-57. www.elsevier.com/locate/rse Remote Sensing of Environment 90 (2004) 53 – 62