THE ROLE OF SURFACE-EXPOSED FRACTURES IN ARID ENVIRONMENT DURING NO-FLOW PERIODS

Many studies in the last two decades focused on exploration of flow and transport processes through surface-exposed fractures. Most studies focus on the impact of fractures as fast conduits for water, salts and contaminants during flood, intensive rain events or leaking from contamination sources. It is traditionally assumed that as long as the fractures are dry, their role for the hydrological cycle is negligible. This study focuses on the processes occurring within surface-exposed fractures during the dry season, especially in arid environments where the temperature differences between days and nights are typically high. Through a series of laboratory experiments and in situ field measurements we are showing that: (1) the cold nights in arid conditions create unstable state where the atmospheric air become denser than the fracture-air; (2) this unstable condition is responsible for the development of convective fluxes of moist air between the fracture-air and the atmospheric air above. Therefore, evaporation from fractures during the night is much higher than during the day; (3) these unstable conditions, indicated by Rayleigh numbers way above the threshold value of 40, are more significant during the winter; (4) the convective fluxes considerably enhance evaporation from the fractures. The convective fluxes between fractures and the atmosphere enhance not only evaporation and salt precipitation within fractures; it also enhances heat flux and greenhouse gas exchange rates. Additionally, it creates an effective bypass mechanism for contaminants accumulated in the upper section of the vadose zone to be transported laterally to the fracture surfaces and subsequently to groundwater. The enhanced salt precipitation within surface-exposed fractures, resulting from the fast evaporation due to convection, may also significantly reduce the life span of an hydraulically active fracture due to precipitation of low-solubility salts.