Leaf wetness: implications for agriculture and remote sensing
Information regarding leaf wetness duration (LWD) has been used in disease management schemes for decades by researchers in the plant disease and agricultural meteorology communities. LWD is currently measured predominantly by electronic leaf wetness sensors or through the use of a model that represents latent heat transfer. Studies have been conducted that examine the placement, orientation and treatment of leaf wetness sensors. Some studies have compared empirical and physical models to LWD measurements obtained from leaf wetness sensors. However, an article that summarizes all aspects of leaf wetness sensors and models, addressing the benefits and disadvantages, has not been provided to extension personnel that need to provide accurate information to growers regarding disease risk associated with LWD. It is recommended that LWD should be estimated using a relative humidity greater than or equal to 90% for operational use.
The vertical variability of dew has been examined for a variety of crops. Studies regarding the horizontal spatial variability of dew amount and duration has been limited to small areas, on the order of a few meters. Traditionally, information regarding LWD for disease warning systems has been obtained from a single sensor at a single point in a field. We sought to examine whether or not this provided accurate information regarding LWD, but also sought to determine if dew amount varies within a field. Our study examined how the spatial variability of both dew amount and duration differ within a field by examining locations that were hundreds of meters apart. Dew amount was measured manually, and simultaneously, at three locations within the field on seven mornings. The three sampling locations were chosen based on changes in topography and soil textures. Information regarding LWD was obtained by leaf wetness sensors placed at each of the three locations. It was hypothesized that there would be a significant difference in both dew amount and dew duration between the sites due to changes in the distillation contribution to the overall dew amount. The study found that there was high leaf-to-leaf variability regarding dew amount, and no variability between sites was seen. It was found that there was no significant difference in dew duration at the three locations.
The Soil Moisture Ocean Salinity (SMOS) satellite provides the first global estimates of soil moisture using microwave radiometry. This satellite makes passes at 6 pm and 6 am local solar time. The remote-sensing community have indicated that data from the 6 am pass time should be preferred over the 6 pm pass time for a variety of reasons, however
land-based studies of soil moisture using microwave radiometry have indicated that the presence of free water on canopy can cause errors in the estimations of soil moisture. Evaluation of the influence of dew on vegetative canopies for satellite measurements has not previously been possible. Our study examined a region in north-central Iowa, where
the land-cover is uniform consisting of row crops. We hypothesized that there would be no significant difference in brightness temperature or soil moisture between evening and morning SMOS passes. We examined the soil moisture product and found that there was a significant difference in soil moisture between evening and morning SMOS passes
for days when precipitation had not occurred after noon prior to the evening pass, nor during the time period between the evening and morning pass time. The soil moisture product is obtained from measurements of brightness temperature, however no significant
difference in brightness temperature was seen between evening and morning passes. We indicate that there may be issues with retrieved values of optical depth during the SMOS processing phase that is resulting in errors in soil moisture measurements. We also highlight the possibility that decreases in the polarization index (a normalization of
brightness temperature) could falsely indicate a decrease in soil moisture when it maybe the result of an increase either of the volumetric water content of a vegetative canopy or the presence of free water on the canopy surface.