The effect of substrate properties, size, position, sheltering and shading on dew: An experimental approach in the Negev Desert

Abstract

Affected by the interrelationships between the air mass and the substrate properties, dew measurements entail numerous difficulties. Inattentiveness to the complexities involved may result in inconsistent and consequently unreliable data and thus in misleading conclusions. This is especially important in dew deserts such as the Negev where dew may be regarded as an important water source. In an attempt to highlight some of the factors that may affect dew condensation, an experimental approach was adapted during which dew condensation at different surfaces (smooth and ragged Plexiglas, bright limestone and dark flint cobbles), at various size substrata with identical axes (of 2.5, 5, 10, 20, 30 and 40cm) or different size axes (of 10×20, 10×30 and 10×40cm), and substrata at different heights and settings was measured. Some of the settings explored were also sheltered by a shrub, a cave, an 80-cm-tall cliff and neighboring cobbles.No significant differences were found in the dew amounts condensing on the limestone and the flint cobbles. Smooth Plexiglas surfaces yielded significantly higher amounts than a ragged Plexiglas surface. As for the substratum height and size, the dew amounts increased with height (from 0.7 to 10.0cm) and size (from 2.5×2.5cm to 40×40cm). However, an increase in the long axis (a-axis) to 20, 30 and 40cm while leaving the b-axis constant (at 10cm) resulted in only a slight increase in the dew amounts in comparison to a 10×10cm substratum. While confined to non-sheltered habitats, dew at partially sheltered habitats (such as the margin of a shrub or a cave, the vicinity of a cliff and on 7-cm-high cobble surrounded by 15-cm-high cobbles) was significantly lower than at the exposed habitats. The differences in dew amounts coincided with minute differences in the surface temperatures, explained as resulting from differences in the outgoing nocturnal longwave radiational cooling. The findings may shed light on the dew regime of lithobionts at different microhabitats and upon inconsistent values reported in the literature.