Estimation of vegetation water content for corn and soybeans with a normalized difference water index (NDWI) using Landsat Thematic Mapper data

A mid-infrared Normalized Difference Water Index (NDWI) is proposed for the estimation of vegetation water content (VWC). As part of a large-scale hydrology experiment (SMEX02) an extensive VWC data set was collected for corn and soybeans over a portion of the growth cycle. Landsat Thematic Mapper (TM) data for several dates were used to develop two indices, the well-known NDVI and the NDWI, for each ground sampling site. Since multiple dates and satellites were used, atmospheric correction and cross calibration was performed on each TM image. Two atmospheric correction models, 6S and MODTRAN, and an empirical method, produced comparable results. Comparisons of VWC and the indices showed that the NDVI saturated at a lower VWC than did the NDWI. This indicates that the NDWI is a better index for use in estimating the VWC over a full crop growing cycle. Relationships were developed between the VWC and NDWI for corn and soybeans. The error with this approach was approximately 0.5 kg/m² for corn and 0.2 kg/m² for soybeans.