Using SMAP and SMOS vegetation optical depth to measure crop water in vegetation

NASA's Soil Moisture Active Passive (SMAP) and European Space Agency's (ESA) Soil Moisture Ocean Salinity (SMOS) are two microwave remote sensing satellites. They were originally designed to measure soil moisture, but with an algorithm that already retrieves vegetation optical depth (VOD), they could also be used for vegetation measurements. VOD is the degree to which vegetation attenuates microwave radiation from the soil and may be an important product to quantify vegetation changes. SMAP and SMOS have some advantages to measure vegetation compared to existing practices. They can view the entire crop canopy as opposed to just the top layer, due to their ability to monitor soil moisture which is below the crop canopy. SMAP and SMOS also have on average a daily revisit time in the mid–latitudes. Knowing the location and amount of water in a crop canopy could be beneficial for remote sensing because as the crops grow and water becomes allocated differently, SMAP and SMOS are "seeing" water from many different sources(stems, leaves, ears, soil, etc.). These different sources of water will scatter radiation differently due to their varying sizes and shapes and accounting for water correctly could improve measurements of soil moisture and VOD. A challenge of using SMAP and SMOS is the need to know crop water on the ground for comparison to VOD from the satellites.Data from multiple field experiments were collected and analyzed to show where crop water is in different crop components at varying development stages. New empirical models that relate crop water to crop dry mass were also created with these in situ measurements. We will use this model to hopefully overcome the challenge of comparing satellite VOD to crop water. However, we need to verify that the model is accurate and actually telling us about crop water.To check accuracy of our new empirical model, SMAP and SMOS VOD were compared to crop water estimates from the Agricultural Integrated BIosphere Simulator (Agro-IBIS) at the South Fork SMAP Core Validation Site in Iowa. A crop model was used because it can obtain dry mass for multiple fields in the study area. This dry mass can then be converted to a crop water using our empirical model for comparison to SMAP and SMOS VOD. We find that SMAP and SMOS VOD are directly proportional to crop water. We also found the value of the proportionality constant (or "b-parameter") relating VOD to crop water at the satellite scale is about half as large as previous estimates. After finding that SMAP and SMOS VOD are directly proportional to crop water we wanted to validate SMAP and SMOS VOD with in situ data from the field campaign SMAP Validation Experiment 2016. We found that SMAPv2 VOD had the highest R2 value. The b–parameter was also shown to change over time and that other sources of water in the SMAP and SMOS pixel may need to be taken into account when calculating ab–parameter. Because L-band VOD is directly proportional to crop water at the satellite scale, and because we understand the relationship between crop water and crop dry mass, SMAP and SMOS have the potential to evaluate the large-scale performance of crop models in the Corn Belt on a near daily basis.