A spatial and temporal evaluation of the SMAP cropland b-parameter across the U.S. Corn Belt

The liquid water contained within plant tissue per ground area is an indicator of plant biomass and related to plant transpiration. In the U.S. Corn Belt this plant water is dominated by water in crop tissue. Microwave remote sensing provides an opportunity to measure crop water at a 30 to 40-km spatial scale across the U.S. Corn Belt. Crop water has been shown to be directly proportional to L-band vegetation optical depth (VOD) through a proportionality constant called the b-parameter. NASA’s SMAP satellite uses a static b-parameter of 0.11 for cropland areas, however the b-parameter varies with crop type. This study answers the following research questions. First, does the value of the b-parameter value vary across the U.S. Corn Belt given the observed differences in the proportions of corn and soybeans grown across the region? Secondly, does the SMAP b-parameter value vary throughout the growing season given the significant change in the distribution of crop water within tissue types throughout the growing season? Since in-situ measurements of the b-parameter are higher for corn, we hypothesize that as the relative fraction of corn in a SMAP pixel increases, the b-parameter will increase. We test this hypothesis using satellite scale modeling of crop water and SMAP L2 DCA VOD measurements at 18 sites across the U.S. Corn Belt for the years 2015, 2016, and 2017. For each site year, the crop water for the satellite pixel is simulated using the Agro-IBIS crop model and is weighted according to the proportion of corn and soybeans grown in that location. We find that there is no statistically significant correlation between the relative corn fraction and the SMAP b-parameter across the U.S. Corn Belt and that the current SMAP cropland value of 0.11 is appropriate for the beginning of the growing season ($\text{growing degree days}\left(\text{GDD}\right)<10\bar{0}0{}^{\circ }\text{C}\cdot \text{day}$). However, we find that the b-parameter in the second half of the growing season ($\text{GDD}\ge 10\bar{0}0{}^{\circ }\text{C}\cdot \text{day}$), during the crop reproductive stages, has an average value of 0.17. The results from this study indicate that while there is no change in the b-parameter due to the proportion of crops growing in a satellite pixel, there may be a difference in b-parameter due to changing water distribution within the crop canopy due to the development of crop reproductive structures (ears and pods) during the second half of the growing season.