The critical stage: When L-band radiometry sees both changes in soil surface roughness and vegetation in the US Corn Belt

Abstract

Satellite-based microwave remote sensing missions, such as NASA’s Soil Moisture Active Passive (SMAP), have greatly improved the ability to monitor the movement of soil, water, and nutrients within the landscape through its monitoring of global soil moisture (SM). L-band missions, such as SMAP, are frequently utilized for this purpose, as they enable microwave remote sensing of SM in all types of weather conditions, including dense cloud cover and precipitation. These missions, to date, have performed comparably with ground-based SM observations for various vegetation cover types. However, notable dry biases, where L-band missions are drier than ground truth measurements, persist in agricultural regions planted with row crops, including the U.S. Corn Belt. These dry biases coincide with months of the year when three components of agricultural systems are changing that are visible to SMAP: changes in soil surface roughness (h) due to farm management decisions and precipitation; changes in vegetation optical depth (VOD) due to vegetation water column (VWC); and changes in SM. Presently, SMAP assumes a static h value for each pixel globally, as it can only retrieve two variables. This causes changes in h to appear as VOD even when the soil is bare. It is believed that SMAP soil moisture retrievals can be improved during these transition phases of the year by identifying the timing of a “critical point,” or the time at which VOD retrievals are dominated by seasonal vegetation rather than h. This is hypothesized to be coincident with an observed minimum in VOD retrievals during the spring season, and to correspond with a specific corn development stage in the Corn Belt. In this thesis, the timing of the critical point is determined in SMAP retrievals for five crop reporting districts (NW, NC, NE, C, and SC) across the state of Iowa, which is largely dominated by row crop vegetation. These five districts are chosen due to variations in vegetation cover and farm management practices. The number of cumulative growing degree days (GDDs) between corn emergence and the critical point for all pixels within each crop district is computed. Furthermore, a corresponding corn development stage is computed. This is performed in these crop districts during four years of the SMAP mission (2015, 2016, 2018, and 2021), which experienced significant variations in springtime precipitation patterns. The critical point was found to occur after an average of 312 ◦C·days post-emergence, which corresponds with corn development stage V6. There is a significant standard deviation of greater than 80 ◦C·days between growing seasons, indicating that the timing of the critical point is not solely determined by springtime temperature patterns. It is understood that the variation in the timing of the critical point is due to inter-seasonal variations in precipitation and management activities, as well as in the spatial distribution of perennial vegetation within the Corn Belt. Rather than assuming a critical point in VOD retrievals throughout the Corn Belt, a critical stage, during which both h and VWC are changing simultaneously, is thought to be present. The results of this work can be used to determine the periods of time when springtime SMAP retrievals solely represent changes in either h or VWC, and whether or not both must be accounted for in L-band VOD retrievals throughout the spring and summer months. This study highlights factors that contribute to variations in springtime VOD retrievals in the Corn Belt, and suggests tools that could be implemented into the SMAP mission to develop a modified retrieval algorithm, including daily precipitation, vegetation cover, and farm management datasets. Improved retrievals of L-band SM using a partitioned approach to retrieving h and VWC separately during the spring and early summer will improve global weather and climate models, allow the use of L-band radiometry to monitor crop health and phenology, and enable the development of strategies to reduce the quantity of nutrients leaching into the world’s oceans