The different effects of regional and local winds on dew formation in the Negev desert

Abstract With dew serving as an important water source for various small organisms and plants in deserts, knowledge regarding the spatial distribution of dew (which constitutes an important fraction of the non-rainfall water, NRW) is of prime importance. This is also the case for the Negev dew desert. According to the classical model, local nocturnal katabatic winds that descend down the slopes during the night to the wadi beds are responsible for the accumulation of cold air, subsequently triggering dew formation in the wadis. Nevertheless, NRW measurements that were conducted in a one-order drainage basin in the Negev during the dewy season (late summer and fall) yielded half the amount in the wadi bed in comparison to the hilltop, attributed to the sheltered position of the wadi from the cooling effect of the regional (sea-breeze) winds, which are not considered by the classical model. Hypothesizing that the classical model may however take place at wide wadi where the sea breeze winds are not sheltered, measurements of dew and temperatures were periodically carried out at the beds of a 5 m-wide narrow (NW) and a 200 m-wide (WW) wadi beds and at the hilltop (HT). The findings did not fully support our hypothesis. In comparison to the hilltop, and despite the mutual effect of the katabatic and the sea breeze winds on the wide wadi bed, also the wide wadi exhibited lower amounts of dew than that of the hilltop, with NRW following the pattern HT > WW > NW. The overwhelming effect of the sea-breeze winds was also supported indirectly by periodic NRW and temperature measurements during the winter during which the sea breeze does not commonly take place. Evidence suggests that whereas the classical model takes place during the winter during which the katabatic winds may play a central role in dew formation, the occurrence of the sea breeze (regional wind) during the late summer and fall overshadows the effect of the local katabatic winds. Our findings point to the possibility that the classical model may not adequately predict dew formation in regions subjected to sea-breeze winds.