SMAP Vegetation Optical Depth is Directly Proportional to Crop Water in the US Corn Belt

Abstract

NASA’s Soil Moisture Active Passive (SMAP) is a satellite L–band radiometer whose primary mission is to measure soil moisture. However, it also retrieves vegetation optical depth (VOD), the degree to which vegetation attenuates microwave radiation. We believe that VOD has the potential to monitor crop growth in the US Corn Belt. To show its value, we compare SMAP VOD to satellite–scale estimates of crop productivity created using the Agricultural Integrated BIosphere Simulator (Agro– IBIS) and observed weather at the South Fork SMAP Core Validation Site in the Corn Belt state of Iowa. We find that SMAP VOD is directly proportional to crop water, the mass of liquid water in crop tissue. We discovered this relationship by using new empirical models that relate crop water to crop dry mass, the standard output for Agro–IBIS as well as all other crop models, created with in situ data spanning multiple years and stages of crop development. The value of the proportionality constant (or b–parameter) relating VOD to crop water at the satellite scale is half as large as the SMAP value. Because L–band VOD is directly proportional to crop water at the satellite scale, SMAP has the potential to evaluate the large–scale performance of crop models in the Corn Belt on a near daily basis.