Microlysimeters overestimate the amount of non-rainfall water – an experimental approach

Abstract

The notion that non-rainfall water (NRW) serves as an important water source for desert cyanobacterial biocrusts is wide-spread, although some publications reported very low values, substantially lower than the threshold required for cyanobacterial activity (0.1 mm). Recent publications highlighted however the possibility that microlysimeters (MLs), which are mainly used to measure NRW, may overestimate the amount of dew that is commonly formed at the upper 0.2 cm of the soil. These sources claim that the dew point temperature (Td) may be reached much sooner at the ML (which will result in higher NRW) in comparison to the intact soil (COT) due to heat loss through ML walls. In order to examine this possibility, a set of experiments were conducted in the Negev Desert, using small MLs. In comparison to COT, the findings indicate that: (a) significantly lower nighttime temperature (averaging 2.1 °C) characterized the ML, (b) even during days with very low NRW, relatively high amounts of 0.2–0.3 mm were recorded by the ML, (c) the 0–0.2 cm soil samples collected from the ML yielded significantly higher values than COT, and similarly, cloths attached to the surface of the ML yielded significantly higher amounts (2.05-fold higher) than those attached to COT, and (d) longer MLs or MLs filled with fine-grained soil (loess) yield higher amounts than short MLs or MLs filled with sand, respectively. The findings indicate that the amounts of NRW recorded by the MLs overestimate the amount of NRW recorded by COT, and that the amounts recorded by the ML depend upon its length and parent material. The findings may explain contradictory results published in the literature, and the wide-spread but apparently erroneous assumption that dew may serve as an important water source for cyanobacterial biocrusts.