Estimation of leaf water content and specific leaf weight from reflectance and transmittance measurements

Abstract

Specific absorption coefficients for water and dry matter were estimated using a wide range of variation of fresh leaves. The coefficients were derived from the inversion of the PROSPECT leaf optical property model using reflectance and transmittance spectra measured over the 1 300-2 400-nm domain and the corresponding water content ) and specific leaf weight (mass of dry matter per unit leaf.area, g.cm (g.cm -2 -2 ). Results show that the estimated val- ues of the specific absorption coefficient for dry matter were not reliable in the strong water absorption bands, although there was agreement with previous studies in spectral regions where water contributed moderately to leaf absorption. We thus proposed to use the values derived by Fourty et al (1996) for dry leaves for the specific absorption coefficient of dry matter. Estimated values of the specific absorption coefficient of water were slightly higher than the values pro- posed by Curcio and Petty (1951) for pure water. We then investigated the possibility of estimating leaf water content and specific weight by inverting the PROSPECT model using concurrently or separately reflectance and/or transmit- tance spectra measured over fresh leaves and the specific absorption coefficients proposed by Fourty et al (1996) for dry matter, and Curcio and Petty (1951) for water. Results obtained on the training data set and on an independent data set show accurate and robust estimates of both water content (RMSE = 0.0025 g.cm ) and specific leaf weight -2 = (RMSE ) when reflectance and transmittance were used concurrently. When either reflectance or trans- 0.0016 g.cm -2 mittance measurements were used, the performances of water and dry matter content estimation decreased because of the relaxation of constraints in the inversion process. Possible applications of these results are discussed.