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 was measured continuously at each of the four locations.

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. In addition to samples taken at sunrise, on several occasions, samples were taken at sunset, 11pm and 3am in order to monitor the progression of dew development.

Leaf area index (LAI) was measured throughout the growing season at 1/3 and 2/3 canopy height at each of the measurement locations. This information was utilized to investigate how dew measurements taken at 1/3 and 2/3 canopy height should be scaled in order to determine a value of dew amount for the entire canopy to be used in comparisons for ALEX.

In addition to the investigation into the spatial variability across a field, dew duration was monitored in a soybean canopy at 10 cm increments beginning 10 cm above the soil surface in order to determine the dew duration profile within a soybean canopy. Manual samples of dew were taken at each layer within the profile to determine if the duration profile and the dew amount profile coincide.

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. The location hypothesized to have the longest dew duration resulted in the lowest mean dew duration over the growing season in maize.

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 remote sensing in the microwave frequency.