Measurement and simulation of evaporation from a red earth. I. Measurement in a glasshouse using a neutron moisture meter

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Abstract
The evaporation of water from, and redistribution of water within, intact monoliths (23.6 cm diameter, 60 cm depth) of red earth were studied in a glasshouse under a wide range of evaporative conditions. A neutron moisture meter was appropriately calibrated and used to document changes in the distribution of soil water. This is a novel use for such equipment. Strongly curved and generally different calibrations were required for each depth. Prolonged exposure to highly evaporative environments resulted in the removal from the profile of 90% of water available at matric potentials of between -0.01 and -1.5 MPa within 3 months. Bare soil evaporation was not controlled solely by soil hydraulic parameters as the profile dried, but was influenced by evaporative demand throughout the drying cycle. Only two stages of evaporation were discerned. The first stage, when evaporation from the soil surface was similar to free water evaporation, was virtually non-existent under highly evaporative conditions. The second stage was characterized by a continuous exponential decrease in evaporation. By the end of the drying cycles, evaporation was still decreasing exponentially, with a half-life averaging 34 days. Empirical predictive relationships for the dependence of evaporation on soil water parameters in the surface 10 cm, and evaporativity, were established. These relationships are tested in the following paper.