Does temperature-induced vapor flux trigger desert floods? The Negev example

High-magnitude floods characterize deserts, commonly explained by the high rain intensities coupled with the low vegetation cover, surface smoothness and the sealing capability of biocrusts. Nevertheless, runoff generation and the subsequent high-magnitude floods stand out when considering the high evaporation, the relatively dry desert surfaces and the high stoniness that characterize many of the surfaces. Also, the rain amount that stems from high intensity (convective) rains is not necessarily higher than in subhumid regions, and indeed, based on a study of runoff dynamics over biocrusted sand dunes in the Negev Desert, runoff was generated already at low to medium intensities of 9-12 mm/h, explained by the high-water absorbance of the extracellular polymeric substances (EPS) that is excreted by the variable microorganisms inhabiting the biocrust. In the Negev, convective intensities were found to primarily result from unstable daily atmospheric conditions following by daytime surface heating. Daytime heating, coupled with cool/cold nighttime temperatures, were also found responsible for the occurrence of temperature-induced vapor flux (TIVF) and the subsequent occurrence of wet-dry cycles that may last for up to 1-2 weeks following a rain event. We hypothesize that the wet cycles which stem from an upward vapor movement from the warm subsurface soil towards the cool soil surface during the night result in biocrust saturation, triggering in turn runoff generation. By wetting the surface (commonly already during the late afternoon), TIVF may trigger runoff in deserts. This is supported hereafter following a thorough four year-long analysis (1990-1994) of rain-runoff relationship that took place on a pair of biocrusted plots covering dunes in the Negev. Albeit the ~20% higher intensities measured during daytime that should have resulted in higher runoff, nighttime runoff amount, runoff flow duration, runoff yield per average rain intensities and runoff yield per the average intensity of each event were 1.8-, 1.5-, 1.5- and 3.1-fold higher during nighttime in comparison to daytime. It is suggested that in addition to the role played by runoff intensity and the surface properties, TIVF provides an additional mechanism that explains the high runoff generation already at low- to medium-intensities, also explaining, at least partially, the relatively high runoff yield and potent floods in the Negev and other arid regions.