This product was developed to be used as input in the SMAP science data products (soil moisture). The vegetation water content (VWC) is calculated using following equation:

$VWC\quad =\quad (1.9134{ NDVI }^{ 2 }\quad -\quad 0.3215NDVI)\quad +\quad stemfactor\quad \left[ \cfrac { { NDVI }_{ max\quad }\quad -\quad { NDVI }_{ min } }{ 1\quad -\quad { NDVI }_{ min } } \right]$

where NDVI is Normalized Difference Vegetation Index, ${ NDVI }_{ max\quad }$ and ${ NDVI }_{ min\quad }$ are annual maximum and minimum NDVI and stemfactor is parameter accounting for  peak stem water content.

MODIS NDVI data was used as input.

The VWC maps was found to produce reasonable estimates of global VWC for the major vegetation classes. However, no information is available for global scale validation.

Reference:

Chan, S., Bindlish, R., Hunt, R., Jackson, T. & Kimball, J. Soil Moisture Active Passive (SMAP) Ancillary Data Report – Vegetation Water Content. (Jet Propulsion Laboratory, 2013).