Mornings with the highest non-rainfall water during 2 years of measurements in the Negev–Ecological implications

By providing water during the long dry rainless season, non-rainfall water (NRW), principally dew and fog, is commonly thought to serve as an important water source for arid soil biocrusts, which require water threshold of 0.05 (for lichens) and 0.1 mm (for cyanobacteria) for growth. Such values are commonly reported in the literature. Moreover, values of >0.45 mm/d (considered as the practical theoretical threshold for dew) are also reported. Following an analysis of limited (29 days) dewy mornings, it was recently reported that dew is incapable of forming on the soil surface. Being of limited scope, we therefore analyse days that yielded the highest amounts of NRW during 2 years (2021–2023) of measurements (22 days with NRW ≥0.15 mm). We conducted measurements at 1 m above ground, cobbles, rocks, and the soil surface, and compared them to the cloth-plate method. Additionally, periodical temperature measurements were conducted, and the dewpoint temperature (T_d) was calculated. Our findings indicate that (a) dew yield never surpassed 0.4 mm, (b) while the water threshold amounts were reached on the rocks and especially on the cobbles, these thresholds were seldom reached on soil (a single morning with 0.1 mm and 3 days with ≥0.05 mm), (c) in comparison to air temperatures at 0.05 m and especially at 1 m-height above ground, surface nocturnal soil temperatures were 2–3°C warmer, thus eliminating possible vapour condensation. Our data cast doubt on values of >0.45 mm reported from various deserts. Additionally, while supporting the view that NRW may provide water for the growth of rock-dwelling lichens, NRW in the Negev does not provide the soil biocrusts with sufficient water for growth.