Even in a dew desert: Dewfall does not provide sufficient moisture for biocrust growth – Evidence from direct measurements and a meteorological model

Knowledge regarding the distribution of non-rainfall water (NRW) and particularly dew has great ecological importance. Nevertheless, the role of NRW as a water source for biocrusts is controversial, with most scholars attributing a central role to NRW as water source for biocrusts. Estimating the amounts of NRW and particularly dew is indeed challenging due to the interrelationship between vapor, temperature and wind. Recently, the use of NRW by biocrusts was also suggested following a meteorological model. Here we compare the amounts obtained by model calculation to the amounts of NRW as measured at 1 m-height above ground (used for meteorological models), and on cobbles, rocks and the soil surface in the Negev. While field measurements at 1 m-height above ground (averaging 0.144 mm/day) showed a fairly good fit (r = 0.52) with model calculation (averaging 0.147 mm/day), and the NRW measurements surpassed the threshold for net photosynthesis for lichens (0.05 mm) and occasionally for cyanobacteria (0.1 mm) on the rocky surfaces (averaging 0.093 mm/day for the cobbles and 0.042 mm/day for the rock slabs), it was substantially below these thresholds at the soil surface (averaging 0.009 mm/day). Our data imply that the amount of NRW reaching the soil surface is too low to provide the soil microorganisms sufficient water for net photosynthesis. With 0.5 mm/day marking the maximum theoretical dew value, the 16.9-fold lower amounts obtained by us at the soil surface in comparison to 1 m-height above ground (used by meteorological models) indicates only 0.03 mm/day at the soil surface, substantially below the required amounts for net photosynthesis by biocrusts. When estimating the amounts of NRW available for biocrusts, meteorological models must be adjusted in accordance with height above ground.