Root-zone soil moisture (RZSM) impacts carbon uptake because water availability to plant roots is a prerequisite for activating photosynthesis -- plants without sufficient water available at their roots cannot become photosynthetically active. RZSM impacts carbon release through autotrophic and heterotrophic respiration processes, where vegetation and organisms in soil consume available organic matter and release CO2 to the atmosphere. The strength of this chemical reaction is determined by soil temperature and moisture profile.
Despite the importance of RZSM, it has remained an unobserved quantity other than localized, point-scale measurements at flux tower sites. Data from flux tower sites can define the functional relationships between carbon fluxes and ecosystem parameters (e.g. vegetation cover and soil moisture), but they cannot be used to meaningfully scale-up these relationships to the grid cell of a regional model (50 km), let alone the larger North American continental scale.
Remote sensing investigations have produced direct high-resolution synoptic views of RZSM using the NASA/JPL AIRSAR UHF radar, but have stopped short of producing spatially and temporally sustained observations. Indirect methods of obtaining RZSM from surface soil moisture observations through data assimilation are currently the only other alternative, but the resolutions are coarse and error propagation from satellite retrievals into assimilation systems is a problem. AirMOSS will provide the first temporally and spatially sustained direct observations of RZSM data sets for quantifying its control over carbon fluxes in North America.
The AirMOSS investigation seeks to address the following questions
- Quantitatively, what are the local-, regional-, and continental-scale heterogeneities of RZSM in North America?
- Quantitatively, how does RZSM control ecosystem carbon fluxes at each of these scales?
- By how much will the estimates of North American NEE improve with the accurate knowledge of both the mean and the variance of RZSM?