Is There Enhanced Evaporation In Surface-Exposed Fractures Due To Convection Of Water Vapor?

Diffusive fluxes limit the potential amount of evaporation from surface-exposed fractures and other discontinuities crossing the land surface. However, if density differences between the air above a fracture and the moist air within the fracture voids exist, convective flux might be developed. If so, evaporation could increase by up to several orders of magnitude, dependent on temperature differences and the size of the aperture. Theoretical calculations suggest that under typical high deserts conditions convection is likely to occur during the night while diffusion is likely to control evaporation from fractures during the day. Field experiment carried out in the Negev desert of Israel showed that the amount of salts accumulated within a fracture in six months is much higher than what could be explained by diffusion alone. This further support the existence of additional mechanism that enhances evaporation. The major objective of this work is to experimentally explore the existence of convective condition in natural fractures in the field. A surface exposed fracture in the Negev desert of Israel was instrumented in a way that the temperature and relative humidity within 120 cm deep fracture are constantly monitored. These parameters are also collected 20 cm above land surface, just above the fracture surface. To explore the potential of convective flux of moist air in larger discontinuities (e.g., karsts systems), a large diameter uncased borehole, 55 m depth, was also instrumented with thermocouple sand relative humidity probes. Preliminary results indicate that: (1)convective conditions exist at least in the upper part of a fracture from early evening to late morning (the existence of convection cell deeper in the fracture is still unclear); and (2) convective conditions exist almost 24 hours a day (excluding at noon for a short period) in the large borehole. These field measurements suggest that large amounts of salt could accumulate within surface-exposed fractures due to enhanced evaporation controlled by convection. Subsequently, these salts could find their way to the groundwater, bypassing the thick vadose zone.