Investigation Into the Spatial Variability of Dew at Field Scale.

The occurrence of free moisture on a crop canopy, whether in the form of intercepted precipitation or dew, has implications for both plant disease development and sensing of soil moisture by microwave remote sensing.  Research has been conducted on the variability of dew vertically within a crop canopy; however the horizontal spatial variability of dew at the field scale had not yet been examined.  A study was conducted in a maize and soybean field to investigate variations in dew duration and amount at locations in a 1 km² field with varying topography and soil characteristics.

At each of the sites, two leaf wetness sensors were installed at both 1/3 and 2/3 canopy height, providing insight into variations in dew duration between the four locations.  Above canopy temperature and relative humidity were measured, in addition to in-canopy temperature and relative humidity, measured at half-canopy height.  Soil moisture and temperature were measured continuously at each location at depths ranging from 1.5 to 60 cm.  Soil and canopy surface temperatures were measured using infrared thermometers.

In order to determine dew amount, both physical sampling and use of the Atmosphere-Land Exchange Model (ALEX) were utilized.  Sampling began at sunrise, and was conducted multiple times at each measurement location.  These samples were used to verify the modeled prediction of dew amount, utilizing modeled latent heat flux.  On several occasions additional samples were taken over the entire night at sunset, 11pm and 3am in order to monitor the progression of dew development.

We hypothesize that sites with higher soil surface temperature will have greater distillation and higher overall dew amount and duration.  The spatial distribution of soil surface temperature will govern the spatial distribution of dew.

Investigation of dew amount via manual measurements in a maize canopy revealed that the variability of dew amount between samples at one location exceeded the variability between locations within the field.   We found that based on the sampling procedure utilized in maize, we were not able to determine the spatial variability.  This season, in soybeans, the sampling procedure will be adjusted in order to eliminate some of the sample to sample variability, and these initial results will be presented.

The information gathered from this study provides new insight into the spatial variability of dew at field-scale, which can be used in mapping disease risk for integrated pest management, in addition to providing insight into possible sources of error in soil moisture measurements by microwave remote sensing and better understanding of land surface processes.