P-band Radar Retrieval of Root-Zone Soil Moisture: AirMOSS Methodology, Progress, and Improvements

Abstract:

The AirMOSS mission seeks to improve the estimates of the North American Net Ecosystem Exchange (NEE)
by providing high-resolution observations of the root zone soil moisture (RZSM) over regions representative of the
major North American biomes. The radar snapshots are used to generate estimates of RZSM. To retrieve RZSM, we
use a discrete scattering model integrated with layered-soil scattering models. The soil moisture profile is represented
as a quadratic function in the form of az2 + bz + c, where z is the depth and a, b, and c are the coefficients to be
retrieved. The ancillary data necessary to characterize a pixel are available from various databases. We apply
the retrieval method to the radar data acquired over AirMOSS sites including Canada’s BERMS, Walnut Gulch
in Arizona, MOISST in Oklahoma, Tonzi Ranch in California, and Metolius in Oregon, USA. The estimated soil
moisture profile is validated against in-situ soil moisture measurements. We have continued to improve the accuracy
of retrievals as the delivery of the RZSMproducts has progressed since 2012. For example, the ‘threshold depth’ (the
depth up to which the retrieval is mathematically valid) has been reduced from 100 cm to 50 cm after the retrieval
accuracy was assessed both mathematically and physically. Moreover, we progressively change the implementation
of the inversion code and its subroutines as we find more accurate and efficient ways of mathematical operations. The
latest AirMOSS results (including soil moisture maps, validation plots, and scatter plots) as well as all improvements
applied to the retrieval algorithm, including the one mentioned above, will be reported at the talk, following a brief
description of the retrieval methodology. Fig. 1 shows a validation plot for a flight over Tonzi Ranch from September
2014 (a) and a scatter plot for various threshold depths using 2012 and 2013 data.