About Corn and Soybeans Water Content Mapping
Related page: Corn and Soybean Water Content Mapping Example
Gao (1996) proposed that satellites could detect liquid water in vegetation by applying NIR and SWIR bands through the normalized difference water index (Equation 1 from Gao, 1996):
The NDWI index was applied to Landsat 5 & 7 by Jackson et al. (2004; pdf) using band 5 which has a longer bandwidth (1.55 ɲɱ to 1.75 ɲɱ) than the MODIS bands used by Gao (1996). Landsat NDWI is written as Equation 2 in Jackson et al. (2004) as follows:
The NIR reflects vegetation relatively high while SWIR reflectance increases as vegetation water content decreases. An advantage of using NIR and larger bandwidth is that there is much less atmospheric scatter than smaller bandwidth (such as visible bands). Landsat bands 4 and 5 wavelength ranges are 0.74 - 0.90 and 1.55 - 1.75 μm, respectively (as previously mentioned, Landsat 5 or 7 band 5 is different than the 1.24 μm SWIR MODIS value from Gao (1996).
For Jackson et al.'s (2004) study, vegetation (crop) water content (VWC) data was taken from a previous Iowa study where biomass was removed then weighed wet and dry to determine water. It was found that NDWI was a better than NDVI as sensing water content. The following equations were developed by Jackson et al. (2004) (based on reflectance derived from atmospheric correction as described in Jackson et al. [2004] where apparent reflectance was first calculated then was further refined) and are to be applied to estimated vegetation water content with Landsat reflectance-based NDWI:
Testing results from Jackson et al. (2004) of the NDWI equation compared to NDVI are as follows:
References
Gao, B.C. 1996. NDWI - A normalized difference water index for remote sensing of vegetation liquid water from space. Remote Sensing of Environment 58: pp. 257-266.