Satellite L-band vegetation optical depth (L-VOD) contains new information about terrestrial ecosystems. However, it has not been evaluated against the geophysical variable that it represents, plant water, the mass of liquid water contained within vegetation tissue per ground area. We quantitatively assess the seasonal variation of three L-VOD products at the South Fork Core Validation Site in the Corn Belt state of Iowa where L-VOD is directly proportional to crop plant water. We use three satellite-scale crop plant water estimates: in situ measurements; a normalized difference water index (NDWI) calibrated with in situ measurements; and a crop model. We find that overall the L-VOD satellite products are 0.02–0.09 Np (0.4– ${1.7} \,\,\text {kg} \cdot \text {m}^{-2}$ ) lower than the three estimates. We show that overestimation of L-VOD can be attributed to dynamic soil surface roughness, and hypothesize that crop plant water observations will require the incorporation of this effect into retrieval algorithms.

It is believed that the cosmic-ray neutron sensor can be used to measure vegetation. We compare biomass water equivalent (BWE) to neutron counts from a cosmic-ray neutron sensor. We believe that the thermal neutrons will have a highly correlated inverse relationship with BWE. This is due to the ambient energy of thermal neutrons and the ability of hydrogen to capture them easily. Our results for soybean in 2017 showed that there was better correlation between raw fast neutron count (R ² = 0.61) and BWE than raw thermal neutron count (R ² = 0.21). It would be ideal to explore this relationship of fast neutrons to BWE, with the addition of other hydrogen pools at the surface.