The critical stage: When L-band radiometry sees both changes in soil surface roughness and vegetation in the US Corn Belt
Date
2025-05
Authors
Kavanaugh, Matthew Gerald
Major Professor
Advisor
Hornbuckle, Brian K
Vanloocke, Andrew
Archontoulis, Sotirios
Committee Member
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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
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