Dew variability within a small arid drainage basin in the Negev Highlands, Israel

Abstract

AbstractDew variability during the autumn dewy season within a small arid drainage basin in the Negev Highlands, Israel, is studied. Dew measurements were carried out at 18 stations on four exposures, using the Cloth‐Plate Method (CPM) and Duvdevani dew gauges. The study also included periodical wind and substrate temperature measurements.Dew variability within the drainage basin was high. Average daily dew values obtained by the CPM were between 0.07 and 0.31 mm, whereas dew duration ranged between 1.6 and 4.1 hours per dewy morning. Dew amounts monitored by the Duvdevani gauges were lower, between 0.09 and 0.20 mm. Both methods show, however, consistent variability and correspond to a similar pattern. Whereas near‐ground dew measurements were the highest at the hilltops and at the bottom of the sun‐shaded northern and western exposures, wadi bed stations and, especially, the south facing midslope station obtained the lowest dew quantities.The near‐surface dew patterns are not in agreement with the classical model of both Geiger and Oke, which predicts high dew quantities at the wadi beds (due to nocturnal down‐slope wind) and at the lee side of the prevailing wind, i.e. the south‐facing midslope station. The low quantities at the south‐facing midslope station is explained by the paramount role of surface temperatures, whereas variability in radiational cooling is seen as responsible for the high near‐ground dew quantities at the hilltops and the low quantities at the wadis.This conclusion is supported by dew measurements al 40 cm above ground. Dew measurements at 40 cm above ground at the south‐facing midslope station and at both wadi beds were significantly higher (p < 0.05) than at 0.7 cm above ground. Facilitating an efficient radiational cooling, and beyond the impact of the surface temperatures, dew measurements at this height correspond to the classical model, highlighting the important impact of surface temperatures and ventilation upon near‐ground dew condensation in an arid drainage basin.