Contribution of dew to the water budget and ecology of a grassland area in the Netherlands

The annual amount of dew input to the water budget in the midlatitudes has been neglected, possibly because direct dew measurements are very difficult and time-consuming. As The Netherlands has a very high frequency of dew events, a grassland area was selected to determine whether dew input could be significant. The study site is situated within the Wageningen University meteorological station. Dew measurement experiments were carried out in 2004. Data were used to calibrate a surface energy dew model, which was then applied to a 12-year data set. A mean annual dew amount of 37 mm was obtained with a standard deviation of 8 mm, while the mean annual precipitation was 830 mm with a standard deviation of 200 mm. Dew contributed about 4.5 % of the mean annual precipitation. The average number of dew nights per year was 250 with a standard deviation of 25 nights. Thus nearly 70% of all nights experienced a dew episode, which impacts significantly on the leaf wetness and possible vegetation diseases.