The relation between stage‐one (atmosphere limited) and stage‐two (soil limited) evaporation is analyzed under the assumption that moisture profiles approximately preserve similarity during simultaneous drying and draining. The assumption is shown to be reasonable (through comparison with a numerical solution of Richards equation for homogeneous soils) under a wide range of conditions. The similarity assumption is used to derive a simple approximate expression for maximum soil evaporation (the exfiltration capacity) under the influence of gravity. The time compression approximation is used to relate stage‐one and stage‐two drying. It is demonstrated in many observed cases that the time at which stage‐two evaporation is reached is beyond the convergence limit (tgrav) of the often used one‐ or two‐term Philip type exfiltration capacity and consequently that stage‐two drying rates are better approximated by the gravity‐dominated asymptotic limit of the exfiltration solution. Combining the asymptotic limit of the similarity solution with the time compression approximation yields a simple diagnostic equation for estimating stage‐two drying: f=(8/π2)ēptd/t. The equation does not require in situ estimates of soil hydraulic properties and moisture state. It depends only on the average stage‐one evaporation ēp and the time between wetting (irrigation or rainfall) and the onset of stage‐two drying td. As has been demonstrated in the past, this time td may be estimated from the abrupt change that occurs at the transition from stage‐one to stage‐two drying in the ratio of daily averaged surface to air temperature or in the short wave albedo of bare soils. In essence, the approach here uses the readily obtainable (possibly via remote sensing) time td in place of information on vadose zone hydraulic properties and initial soil moisture conditions. The method is tested with considerable success against published evaporation data from laboratory soil columns, weighing bare soil lysimeters, agricultural bare soil plots, and a natural grassland.
Water Resources Research – Wiley
Published: Jan 1, 1997
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