Interpretation of In Situ Observations in Support of P-Band Radar Retrievals

Current estimates of Net Ecosystem Exchange (NEE) at regional and continental scales contain significant uncertainty. It has been shown that root-zone soil moisture (RZSM) content has a first-order effect on NEE. The objective of the NASA Airborne Microwave Observatory of Subcanopy and Subsurface (AirMOSS) project is to provide measurements to estimate RZSM using a P-band airborne radar. Flight operations are conducted over representative sites of the nine major North American biomes which have been instrumented with soil water content sensors in various locations and over various depth layers which will be correlated to the P-band radar return signal. The hypothesis of the AirMOSS project is that integrating spatially and temporally resolved observations of RZSM into ecosystem dynamics models can significantly reduce the uncertainty of NEE estimates and carbon balance estimates. The calibration of the sensor systems is described and results are demonstrated for multiday up to annual time series at various sites. The success of fitting four models (e.g., power, first-, second-, and third-order polynomials) to the RZSM content with depth is tested by using in situ data collected in Tonzi Ranch, CA, USA, in 2014. Under the testing condition, all models satisfy the mission goal most of the duration. The second- and third-order models perform better during the dry season than the simpler power and first-order models. However, during periods of rapidly changing values of RZSM content, none of the models are able to meet the root-mean-squared error objective.