A role for soil organic carbon in passive remote sensing of soil water content from SMAPVEX12

English Abstract:

A field soil water content (SWC) sampling campaign (SMAPVEX12) was held in southern Manitoba, Canada in 2012 for algorithm development improvement prior to the 2015 launch of the SMAP satellite. Although current passive remote sensing algorithms for SWC interpretation do not directly include soil organic carbon (SOC) as an input variable, SOC was included in the SMAPVEX12 ground sampling. During SMAPVEX12, the Passive Active L-Band System (PALS) instrument recorded passive microwave reflectance over the southern Manitoba agricultural and forest fields at altitudes of 2750m on repeated dates from June 7 to July 19, 2012. The area included agricultural fields over 3 soil texture groups, and covered a range of soil wetness and crop growth stages of common grain crops. In addition to ground SWC measurement with resistance probes, soil cores were gathered and oven dried for determination of soil parameters. The dried samples were further used to measure soil organic carbon (SOC) by loss on ignition. PALS underestimated SWC in wet conditions and overestimated SWC in dry conditions compared to the measured ground SWC over 39 grain fields on 11 sampling dates with root mean square error of 0.041 m³ m^-3. A regression of the anomalies between PALS and measured SWC on soil variables, indicated SOC explained more variability than sand, clay or bulk density. The dominant effect of SOC was strongest in dry conditions and on loam soils when the relative anomalies were computed. While SOC has not previously been considered to play a role in remote sensing of SWC, this analysis, suggests that in soils with significant organic matter from agriculture or forest, SOC should be further investigated for a role in the dielectric constant reflectance and in SWC variability.

Acknowledgment: Part of this work was carried out at Jet Propulsion Laboratory, California Institute of Technology under contract with National Aeronautics and Space Administration.