Accurate estimates of water losses by evaporation from shallow water
tables are important for hydrological, agricultural, and climatic
purposes. An experiment was conducted in a weighing lysimeter to
characterize the diurnal dynamics of evaporation under natural conditions.
Sampling revealed a completely dry surface sand layer after 5 days
of evaporation. Its thickness was <1 cm early in the morning, increasing
to reach 4?5 cm in the evening. This evidence points out fundamental
limitations of the approaches that assume hydraulic connectivity
from the water table up to the surface, as well as those that suppose
monotonic drying when unsteady conditions prevail. The computed vapor
phase diffusion rates from the apparent drying front based on Fick's
law failed to reproduce the measured cumulative evaporation during
the sampling day. We propose that two processes rule natural evaporation
resulting from daily fluctuations of climatic variables: (i) evaporation
of water, stored during nighttime due to redistribution and vapor
condensation, directly into the atmosphere from the soil surface
during the early morning hours, that could be simulated using a mass
transfer approach and (ii) subsurface evaporation limited by Fickian
diffusion, afterward. For the conditions prevailing during the sampling
day, the amount of water stored at the vicinity of the soil surface
was 0.3 mm and was depleted before 11:00. Combining evaporation from
the surface before 11:00 and subsurface evaporation limited by Fickian
diffusion after that time, the agreement between the estimated and
measured cumulative evaporation was significantly improved.