Submitted to: American Geophysical Union
Publication Type: Abstract only
Publication Acceptance Date: 12/18/2009
Publication Date: 12/18/2009
Citation: Hornbuckle, B.K., Kruger, A., Rowlandson, T.L., Logsdon, S.D., Kaleita, A., Yueh, S.H. 2009. Uncertainty in SMAP Soil Moisture Measurements Caused by Dew [abstract]. American Geophysical Union. Dec. 14-18, 2009, San Francisco, CA. CD-ROM. Interpretive Summary:
Technical Abstract: Soil moisture is an important reservoir of the hydrologic cycle that regulates the exchange of moisture and energy between the land surface the atmosphere. Two satellite missions will soon make the first global measurements of soil moisture at the optimal microwave wavelength within L-band: ESA's Soil Moisture and Ocean Salinity (SMOS) mission; and NASA's Soil Moisture Active-Passive (SMAP) mission. SMAP is unique in that it will measure both L-band brightness temperature and backscatter. Changes in the water content of vegetation tissue, as well as transient liquid water within the vegetation canopy such as dew or intercepted precipitation, all affect the L-band terrestrial brightness temperature and backscatter. Although dew will often be present during the planned SMAP overpass at 6 a.m., the effect of dew on the L-band brightness temperature and backscatter is not completely understood. Some progress has been made in terms of the effect of dew on the L-band brightness temperature. For example, it is known that dew can either increase or decrease the L-band terrestrial brightness temperature (by up to 10 K) depending on the type of vegetation. This effect is significant but relatively small when compared to the sensitivity of L-band brightness temperature to soil moisture. On the other hand, NO measurements of the effect of dew on L-band backscatter have been reported. Considering the effect of dew on the backscatter at slightly shorter microwave wavelengths (X- and C-band), we hypothesize that there is the potential for an error of more the 5% in the estimate of soil moisture from the L-band backscatter when dew is present, which is unacceptable. We will present the first case study of the effect of dew on the L-band backscatter. We will use data collected by NASA's Passive and Active L-band System (PALS) over corn and soybean fields at the Iowa Validation Site on September 23 and 25, 2008. The conditions during this three-day experiment were ideal for deducing the effect of dew on the L-band backscatter. On the first day, cloud cover during the previous night prevented the formation of significant dew. On the final day, a large amount of dew was observed. This dew evaporated as PALS repeatedly collected data over the same flight lines until the vegetation was essentially dry. We will compare the remote sensing data from the first day (no dew) with the data on the third day (heavy dew) and analyze the time-series of data as the dew dried off in order to deduce the effect of dew on the L-band backscatter. We will use three different methods to characterize dew: manual measurements of dew amount; leaf wetness sensor measurements of dew duration; and estimates of dew amount and duration from a land surface process model. The result of our presentation will be an estimate of the uncertainty in SMAP soil moisture measurements that could be caused by dew.