Evaporation from a salt encrusted sediment surface - Field and laboratory studies

Abstract

Estimates of hydrologic budgets from arid zones are constrained by difficulties in evaluating evaporation loss from groundwater discharge areas, especially playa surfaces. Evaporation from a salt-encrusted playa surface (Lake Amadeus, central Australia) is estimated by field measurement of moisture loss from sediment blocks in plastic receptacles set into the playa. The evaporation process consists of two distinctively different evaporative patterns. E1 is a very low rate (70 mm/year, 2.4% of pan evaporation) from the salt-encrusted surface. E2 is a much higher rate which occurs after rain dissolves the surface salt crust. The total E2 evaporation is lower than the rainfall, indicating that a portion of rainfall recharges the playa brine. Therefore, the total E1(70 mm/year) can only be used as an upper limit of the net evaporation and the actual value may be significantly lower. In a laboratory analogue experiment, a very thin (2 mm) salt crust diminishes the evaporation to about 2% of that from a fresh water surface, even though the sediments underlying the crust remain saturated. When distilled water was added to the salt crust, the evaporation rate increased by nearly 20 times for a short period, then returned to the previous low rate. However, a portion of the distilled water infiltrated to the watertable, and became part of the brine supply to the sediments. Both the salt crusts of Lake Amadeus and those formed in the laboratory experiment are porous and buckled, and significantly drier than the underlying sediments. The significant reduction of evaporation from salt-encrusted sediment surface seems to be mainly due to the porous, buckled and dry nature of the crust which inhibits the removal of the vapour from the underlying sediments. The vapour pressure decrease of the brine has relatively less effects.