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Some factors affecting the evaporation of water from soil
Published online by Cambridge University Press: 27 March 2009
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In 1914 the result of a study of the evaporation of water from soil was published by B. A. Keen, who distinguished two factors governing the rate of evaporation. One factor was that the available surface from which evaporation takes place decreases as the moisture content decreases. Keen deduced an equation that expressed the operation of this factor, viz.
in which D is a constant, w = percentage of water by weight and s = the real specific gravity of the soil. He showed that this equation agreed with his experimental results with a reasonable degree of accuracy.
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References
page 121 note 1 Journ. Agric. Sci. 6 (1914), 456–75.Google Scholar
page 121 note 2 Really where p 1 = v.p. of the evaporating liquid and p 2 = the partial v.p. of that liquid in the atmosphere. This obviously reduces to
when the drying atmosphere is of zero humidity.
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page 131 note 1 [Note added Feb. 22nd, 1923.Google Scholar Since this waa written the work has been extended in various directions and to other materials. The results indicate that the curvature in the evaporation curve for clay soil is not the simple shrinkage effect described above. This point will be discussed in detail in a later paper and may necessitate a modification in some respects of the point of view developed in this section.—E. A. F.]
page 132 note 1 The first weighings were taken at about 57% water content after the first fine cracks had already appeared. Although these increased in number and width they produced no appreciable variation in the rate of evaporation as long as free water was evaporating. Thus the experimental time curve between 57% and 35% water showed no indication of curvature, the rate curve for this region being straight and horizontal. The rate of evaporation over this region, however, was not quite identical with that from the silty soil which showed no signs of cracking. Thus the silty soil lost 114 e.g. of water in 2·471 hours (in the region covered by the straight portion of the experimental time curve) as against 114·93 c.g. in the same time for quartz sand and 118·07 c.g. for the Rothamsted subsoil. The values should be identical in all three cases. The sand lost 0·815% more water than the silt and this may be regarded probably as an indication of the order of magnitude of the experimental error inherent in the method. The Rothamsted subsoil lost 3·57% too much water and the excess may be attributed to the slightly increased evaporating surface in this case due to the formation of the cracks.
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page 134 note 1 See experimental part.
page 135 note 1 The writer is indebted to Mr N. M. Comber for this sample and for details of its mechanical analysis.
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