Submitted to: IEEE Transactions on Geoscience and Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2011
Publication Date: 4/18/2012
Publication URL: http://handle.nal.usda.gov/10113/58004
Citation: Rowlandson, T., Hornbuckle, B., Brammer, L., Patton, J., Logsdon, S.D. 2012. Comparisons of evening and morning SMOS passes over the Midwest United States. IEEE Transactions on Geoscience and Remote Sensing. 50:1544-1555. Interpretive Summary: Satellites track soil water content for each morning and evening in central Iowa. The satellite data can potentially evaluate soil water conditions on a regional scale. This study showed that on most days there was no difference between the morning and evening measurements, either for the satellite measurements or for the direct soil measurements. For a few days, the satellite soil water content was less in the morning due to the water in the crops. This information is of interest to scientists who want to understand the accuracy of soil water determined from satellite measurements. As accuracy of satellite information improves, the information would eventually be of interest to policy planners concerning regional effects of drought and excess water on crop growth.
Technical Abstract: This study investigates differences in the soil moisture product and brightness temperatures between 6 pm and 6 am local solar time, when the SMOS passes for a region in north-central Iowa. This region consists of 69 SMOS pixels and has uniform land-cover, consisting of maize and soybean row crops. The comparison was restricted to periods with no rainfall after noon prior to the evening pass, and no rainfall between the evening and morning passes. It was found that there was a significant difference in the soil moisture between evening and morning SMOS passes, with the soil moisture being lower in the morning for a majority of the days. The difference between the soil moisture measurements on some days exceeded the allowable error for SMOS of 0.04 m3/m3. In-situ and model results indicate this discrepancy, measured brightness temperature was converted to a polarization index (PI), and evening and morning values were compared. It was found that there is no signficant difference between evening and morning measurements of PI. We found that there was a large spread in data at low incidence angles. The PI values also indicate that there may be a decrease in soil moisture for the morning, but we suggest that given the in-situ and model measurement that this could be attributed to an increase in the volumetric water content of vegetation.