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Title:   Author  item
                                            KABELA, E - SAVANNAH RIVER NAT LABitem
                                            HORNBUCKLE, B - IOWA STATE UNIVERSITYitem
                                            Cosh, Michaelitem
                                            Anderson, Marthaitem
                                            GLEASON, M - IOWA STATE UNIVERSTIY
  Submitted to: Agriculture Forest Meteorology
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 8/1/2008
                                    Publication  Date: 1/4/2009
                                    Citation: Kabela E., Hornbuckle, B., Cosh, M.H., Anderson, M.C., Gleason, M.L. 2009. Dew frequency, duration, amount, and distribution in corn and soybean during SMEX05. Agriculture Forest Meteorology. 149(1):11-24.Interpretive Summary: Satellite remote sensing of soil moisture is influenced by many factors at the surface, among them leaf wetness, or dew.  The magnitude and importance of this influence, especially with the upcoming launches of soil moisture specific satellites which overpass significant agricultural regions during the early morning hours, requires some assessment of leaf wetness impact on the remote sensing signal.  The first step to investigate this process is to develop a method of estimating leaf wetness amount via modeling and monitoring.  Using a simple atmospheric model, modeled leaf wetness amount was compared to field measured leaf wetness amount during a focused experiment on leaf wetness in conjunction with SMEX05 (Soil Moisture Experiments in 2005).  There was good agreement with the model and the observed quantities of leaf wetness and future satellite estimates of soil moisture would benefit from inclusion of this methodology in their algorithms.
                                Technical Abstract: Dew affects the brightness temperature of vegetation and backscatter from vegetation at microwave wavelengths. Must this effect must be taken into account in order to avoid corrupting remotely–sensed observations of important ecosystem variables such as soil moisture? As a first step towards answering this question, we report the frequency and duration of dew events, the total amount of dew in the canopy, and the distribution of dew within the canopy for two different types of crop canopy, corn and soybean, during SMEX05, a twenty–one day field experiment conducted during June and July, 2005, in Iowa, USA. We observed dew to be present more than 50% of the time in both corn and soybean at common satellite overpass times of 1:30 and 6:00 CST. Dew was most likely to be present between 12:00 and 6:30 CST, and as late as 9:00 CST. Two different methods to scale the liquid water measured on single leaves to the entire vegetation canopy produced similar results, and we observed dew amounts that were comparable, and in some cases higher, than those that have been shown to affect the microwave brightness temperature and backscatter. The distribution of dew within the canopy among the top and bottom of a leaf and (for corn) the leaf collar may influence its effect on remotely–sensed measurements. We found that this distribution is different for light, moderate, and heavy dew events. A modeling approach will be necessary to estimate dew at larger spatial scales associated with satellite remote sensing. The Atmosphere–Land Exchange (ALEX) model, a land surface process model that accounts for both dewfall and distillation, produced estimates of dew amount and duration that were in agreement with manual observations and observations made with leaf wetness sensors.


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