Spatial Variation of ‘‘Non-Rainfall’’ Water Input and the Effect of Mechanical Soil Crusts on Input and Evaporation

Abstract

‘‘Non-rainfall’’ water is important to the soil water balance and ecology of arid ecosystems. A component of this ‘‘non-rainfall’’ water in the Namib Desert, fog, exhibits spatial variation implying variability in composition and significance of each vector (dew, fog, vapour adsorption) to ecology at different locations. The composition of ‘‘non -rainfall’’ water input directly into soil was investigated at two sites in the Central Namib Desert, Kleinberg and Gobabeb. Results showed spatial variation in composition between the sites, although vapour adsorption dominated input at both sites. Fog contributed more to total ‘‘non rainfall’’ atmospheric water closer to the coast (Kleinberg) compared to further inland (Gobabeb) but was lower than expected at both sites. Absolute values of fog input at both sites showed the opposite trend, Kleinberg 0.38 mm per night compared to Gobabeb 8.7 mm per night. This difference was attributed to the development of a mechanical crust on the soil surface at Kleinberg, which resulted in a significant reduction of vapour adsorption compared to Gobabeb. The crust also led to a significant reduction in evaporation from the sample at Kleinberg compared to the one at Gobabeb. Ecological differences between the two sites can be attributed to the development of the soil crust on the sample at Kleinberg and not on the sample at Gobabeb.