Abstract
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 thermocouples
and 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.
Original language | English |
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Title of host publication | American Geophysical Union, Spring Meeting 2005 |
Volume | 21 |
State | Published - 1 May 2005 |
Keywords
- 1818 Evapotranspiration
- 1829 Groundwater hydrology
- 5104 Fracture and flow
- 8010 Fractures and faults