Mapping past, present and future dew and rain water resources for biocrust evolution in southern Africa

Abstract

Biocrust sustainability relies on dew and rain availability. A study of dew and rain resources in amplitude and frequency and their evolution is presented from year 2001 to 2020 in southern Africa (Namibia, Botswana, South Africa) where many biocrust sites have been identified. The evaluation of dew is made from a classical energy balance model using meteorological data collected in 18 stations, where are also collected rain data. One observes a strong correlation between the frequency of dew and rain and the corresponding amplitudes. There is a general tendency to see a decrease in dew yield and dew frequency with increasing distance from the oceans, located west, east and south, due to decreasing RH, with a relative minimum in the desert of Kalahari (Namibia). Rain amplitude and frequency decreases when going to west and north. Short-term dew/rain correlation shows that largest dew yields clearly occur during about three days after rainfall, particularly in the sites where humidity is less. The evolution in the period corresponds to a decrease of rain precipitations and frequency, chiefly after 2010, an effect which has been cyclic since now. The effect is more noticeable towards north. An increase of dew yield and frequency is observed, mainly in north and south-east. It results in an increase of the dew contribution with respect to rain, especially after 2010. As no drastic changes in the distribution of biomass of biocrusts have been reported in this period, it is likely that dew should compensate for the decrease in rain precipitation. Since the growth of biocrust is related to dew and rain amplitude and frequency, future evolution should be characterized by either the rain cycle or, due to global change, an acceleration of the present tendency, with more dew and less rainfalls.

One‐year‐long evaluation of non‐rainfall water available to soil biocrusts in the Negev Highlands

“…that the upper possible limit of vapour condensation for natural substrates (i.e., 0.45 mm; Monteith, 1957) was reached. Since $0.40-0.45 mm are also the upper limit of dewfall as obtained by model calculation at 1-m height above surface (Muselli & Beysens, 2021), it implies that similar near-threshold values of dewfall are apparently obtained at both heights above ground, whether at 1-m height or at the surface.…”

“…Amounts of up to 0.40–0.50 mm of NRW were reported in the Negev (Heusinkveld et al, 2006; Jacobs et al, 2000) or the Tabernas (Chamizo et al, 2021; Uclés et al, 2013) by microlysimeters, which denote, based on energy consideration, that the upper possible limit of vapour condensation for natural substrates (i.e., 0.45 mm; Monteith, 1957) was reached. Since ~0.40–0.45 mm are also the upper limit of dewfall as obtained by model calculation at 1‐m height above surface (Muselli & Beysens, 2021), it implies that similar near‐threshold values of dewfall are apparently obtained at both heights above ground, whether at 1‐m height or at the surface.…”

Physical and Chemical Characteristics of Dew and Rain in North-west Africa with Focus on Morocco: Past and Future Evolution (2005 – 2100)

“…This is a general observation (see Tables 3 and 4). A similar result was obtained in 4 countries in southern Africa by [30]. In addition, the timescale for mean and maximum time period between events is much larger for rain than for dew, a difference which can reach two orders of magnitudes.…”

“…Pue et al [29] introduced a Kriging-based Gaussian process for the prediction of soil water retention in tropical and temperate climates. Muselli and Beysens [30] used kriging to study the biocrust sustainability with dew and rain availability in southern Africa. Other studies combine Kriging models for the estimation of rainfall with Lagrangian approaches [31] or Bayesian [32].…”

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

“…Special attention was given by the scientific community to ways in which biocrusts deal with many rainless months and frequent droughts that characterize semiarid and especially arid regions. Similarly to rock‐ and cobble‐dwelling lichens (lithobionts) that were shown to utilize non‐rainfall water (NRW) for growth (Kappen et al, 1979; Lange et al, 1970), it was assumed that soil biocrusts increase dew deposition (Fischer et al, 2012; Jia et al, 2014; Li et al, 2021), and also utilize dew and fog that assist them to bridge the gap between long rainless periods and frequent droughts (Muselli & Beysens, 2021; Zheng et al, 2018). Numerous publications reporting high amounts of NRW, which surpass the minimum amounts required for biocrust growth, apparently supported the notion that biocrusts use dew for growth.…”