Soil moisture (SM) retrieval in agricultural regions during the growing seasons is particularly challenging due to high spatial variability and dynamic vegetation conditions. The retrievals have been problematic even when the passive signatures at different spatial scales match well since they depend upon the accuracy of vegetation information such as the vegetation water content (VWC). The VWC used in the Soil Moisture Active Passive (SMAP) single channel retrieval algorithm (SCA) is derived from remotely sensed, climatologically-based Normalized Difference Vegetation Index (NDVI), which does not respond to real-time vegetation dynamics and is prone to saturation. This study explored the differences and seasonal trend in passive signatures and SM at satellite- and field-scales and investigated uncertainties in retrievals arising from different approaches used to estimate VWC from optical and radar indices. It used high temporal resolution, ground-based data collected during the SMAP Validation-Microwave Water, Energy Balance Experiment in 2016 (SMAPVEX16-MicroWEX) during a growing season of corn and soybean. Overall, the brightness temperatures (T_B) from SMAP matched well with the upscaled, ground-based T_B, with a root mean square differences (RMSDs) of about 5 K. In contrast, the SMAP SM retrievals were worse during rapid vegetation growth in the mid-season, with higher RMSDs compared to the upscaled in situ SM, than those in the late-season. In addition, the ground-based T_B from corn and soybean were similar in the early and the late seasons, while their emission differences were > 40 K in the mid season. This indicates the importance of accurate VWC information, particularly during the early and late growing seasons, to account for sub-pixel heterogeneities in agricultural regions. VWC obtained from five optical and radar indices were used in the SMAP SCA for soil moisture retrieval for the entire growing season of corn. The NDVI-based VWC provided SM retrievals that were consistently lower compared to those using in situ VWC, with a higher RMSD of 0.030 m³/m³ and a negative bias of 0.020 m³/m³ for VWC > 4 kg/m². The Normalized Difference Water Index (NDWI)-derived VWC resulted in lower SM retrieval RMSD of 0.022 m³/m³ when compared with in situ SM. Among the three radar indices, vertically polarized cross-pol ratio (CR_vv)-derived VWC provided similar RMSDs in retrieved SM as the NDWI-derived VWC during the growing season. The radar vegetation index (RVI)-derived VWC improved in the late season compared to the in situ VWC and resulted in SM retrievals with RMSDs similar to the CR_vv-derived retrievals. Results presented here suggest that SMAP SCA SM retrievals could be improved through the use of near-real time NDWI and CR_vv-derived vegetation information. Microwave data are available regardless of cloud cover, so the guaranteed availability of CR_vv to capture seasonal and interannual variability is advantageous.

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The NASA Soil Moisture Active/Passive (SMAP) and the ESA Soil Moisture and Ocean Salinity (SMOS) missions include microwave radiometers at L-band that provides global observations of SM at 36 and 25km, respectively, with a repeat coverage of every 2-3 days. Agricultural regions, with their highly dynamic vegetation and spatial heterogeneity are particularly challenging for soil moisture retrieval algorithms. The Microwave Water and Energy Balance Experiment was conducted as part of the SMAP Validation Experiment (SMAPVEX16-MicroWEX) during the summer of 2016 in a predominantly agricultural region in Iowa, USA. During SMAPVEX16-MicroWEX, ground-based observations of active and passive signatures were obtained every 15-30 minutes during a growing season of corn and soybean from May 23 through September 2, 2016. The field site was within the South Fork Watershed at the Sweeney Farms, near the city of Buckeye. The University of Florida L-band Automated Radar System (UF-LARS) observed the backscatter from corn. The brightness temperatures (TB) at the corn site were observed by the University of Michigan L-Band Radiometer (UMLMR), while those at the soybean site were observed by the University of Florida L-band Microwave Radiometer (UFLMR). Concurrent and colocated observations of soil, vegetation, and micro-meteorological conditions were also conducted at both the sites. The passive signatures from both the corn and the soybean sites were found to be similar during the early season, as both the fields were nearly bare terrains. As expected, the TB diverge during the mid-season, when the vegetation water content (VWC) of the corn is about 2 kg/m2. Interestingly, the TB of the two crops are similar again toward the end of the season, when VWC of the soybean crop reaches about 2 kg/m2. Preliminary modeling results show that physically-based emission models significantly underestimate vegetation opacity for a mature soybean canopy. These findings provide insights into retrieval algorithms for soil moisture in agricultural terrains.@en

In this study, the impact of spatial variability due to the heterogeneity of vegetation in the agricultural region on passive microwave signatures available at various scales are explored using the brightness temperature (T_B) observed from ground, air, and space. These observations were conducted during a growing season of corn and soybean in South Fork watershed, Iowa, as part of the NASA-Soil Moisture Active Passive Validation Experiment (SMAPVEX16). Both empirical and physically-based microwave emission models are used to understand the effects of vegetation on T_B for corn and soybean using ground-based T_B observations. The modeled T_B will be upscaled based upon the USDA crop layer map to compare with the T_B observed in the coarse scales.

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