THE TWO-STAGE PARADIGM FAILS TO DESCRIBE DIURNAL EVAPORATIVE DRYING OF DESERT SOILS AND SEDIMENTS

Seminal laboratory experiments on soil evaporation by Fisher (1923) showed two distinct stages of drying—a constant rate limited by external energy input, equivalent to evaporation from a free water surface (stage I), followed by an abrupt transition to an exponentially decreasing rate, controlled by profile hydraulics (stage II). The robustness of this paradigm was investigated in the laboratory by Constantz (unpublished data), who confirmed applicability from 5 to 45 C for various media (except pea gravel) while noting abrupt shifts in color of the evaporating surface at the I-II transition.

Field experiments on 3 surfaces in the Amargosa Desert (sand and gravel, silt, and incipient desert pavement) were conducted in May 2009 after a prolonged period (> 2 mo) without precipitation. Test rings 1 m in diameter were saturated with water and allowed to dry for ~2 d by evaporation and internal drainage. Evaporation was periodically sampled using a chamber device, while changes in soil color were recorded photographically. A pyranometer measured solar radiation. Evaporation from dry controls (unwetted rings) and wet controls (2.5-cm deep ponds) were measured with each test-ring measurement.

Evaporation from control ponds roughly tracked solar radiation, lagging by ~1 h. Soil evaporation initially exceeded traditionally assumed maximum (pond) rates by 38 to 144%, dropping monotonically when normalized to the latter. Field plots showed no evidence of stage I behavior, normalized or not. Raw rates from silt and pavement surfaces rose sharply throughout the first morning before dropping in the afternoon, while those from sand and gravel fluctuated around a generally decreasing trend. Distinct color changes were evident at the centimeter scale, but were masked by mottling at the meter scale. The course of evaporation in the field differs greatly from the two-stage model, apparently untested outdoors since development in the laboratory some 90 years ago.