Mapping leaf area index through spectral vegetation indices in a subtropical watershed

Abstract

Leaf area index (LAI) has been associated with vegetation productivity and evapotranspiration in mathematical models. At a regional level LAI can be estimated with enough accuracy through spectral vegetation indices (SVIs), derived from remote sensing imagery. However, there are few studies showing LAI–SVI relationships in subtropical regions. The aim of this work was to examine the relationship between LAI and SVIs in a subtropical rural watershed (in Piracicaba, State of Sa˜o Paulo, Brazil), for different land covers, and to use the best relationship to generate a LAI map for the watershed. LAI was measured with a LAI-2000 instrument in 32 plots on the field in areas of sugar cane, pasture, corn, eucalypt, and riparian forest. The SVIs studied were Simple Ratio (SR), Normalized Difference Vegetation Index (NDVI), and Soil Adjusted Vegetation Index (SAVI), calculated from Landsat-7 ETM+ data. The results showed LAI values ranging from 0.47 to 4.48. LAI–SVI relationships were similar for all vegetation types, and the potential model gave the best fit. It was observed that LAI–NDVI correlation (r ²=0.72) was not statistically different from LAI–SR correlation (r ²=0.70). The worst correlation was obtained by LAI–SAVI (r ²=0.56). A map was generated for the study area using the LAI–NDVI relationship. This was the first LAI map for the region.

Acknowledgments

The authors would like to thank the State of Sa˜o Paulo Research Foundation (FAPESP) for financial support for this study (process 2000/09325-0) and for the fellowship to A.C. Xavier (process 98/14499-5). The authors also thank Professor Mauro A. H. Antunes from Rio de Janeiro Federal Rural University, Brazil, for helping with the atmospheric correction with the 6S model.