The Role of Surface-Exposed Fractures in Land-Atmosphere Dynamics

Throughout the past two decades, most studies that explored flow and transport processes through surface- exposed fractures, focused merely on the role of these fractures as fast conduits for water, salts and contaminants during intensive rain events, flooding or leakage from contamination sources. Conventional wisdom has assumed that as long as fractures are dry, their role in the hydrological cycle is negligible. This study, however, explores the processes occurring with in surface-exposed fractures during the dry season, and shows that their role in hydrological and atmospheric cycles is not negligible. As a result of a study that incorporated theoretical work, laboratory experiments and in situ field measurements, we show the following new mechanism: (1) Cold nighttime temperatures, common to arid environments, create unstable air stratification within fractures where atmospheric air becomes denser than fracture air; and (2) This unstable condition results in convective venting of air between the fracture and the atmosphere. Convective venting leads to the following phenomena: greater exchange of atmospheric gases; higher nighttime evaporation rates vs. daytime; lateral transport of solutes toward fracture surfaces and precipitation as salt crusts; and higher heat loss from the vadose zone. Conditions necessary to trigger convection are more prevalent during the winter. Salts that accumulate on the fracture surface may be transported downward during infiltration events and thus contribute to aquifer contamination. This bypass mechanism is especially important in low permeability zones. Convective venting may also impact the life span of hydraulically active fractures by the accumulation over time of low solubility salts. The mechanism of convective venting of air-filled surface exposed fractures is relevant and important to various aspects of mass and energy transfer between the atmosphere and the vadose zone.