The relationship of evaporation in the falling rate stage to water diffusivity in soils

The relationship of evaporation in the falling rate stage to water diffusivity in soils was determined for disturbed and undisturbed samples of Lakeland sand, Norwood silt loam, and Miller clay. Columns of these soils were saturated, drained to field capacity, and then exposed to constant evaporational demand in a controlled environmental chamber. The evaporation rate was determined gravimetrically and plotted as a function of volumetric water content. Diffusivities of the same samples were resolved by a modification of the one step pressure plate outflow method developed by Doering. These values were also plotted as a function of volumetric water content. A graph of evaporation rate as a function of diffusivity at the same volumetric water content was then plotted and the results analyzed. A straight line relationship between the two factors was produced in every case indicating a simple dependency based on very few parameters. Undisturbed samples of the same soils were saturated, drained to field capacity, and then exposed to field conditions of evaporative demand. The evaporation rate of these samples was determined gravimetrically and plotted as a function of volumetric water content. The results were compared with the laboratory results indicated above. The data differed in absolute magnitude probably due to the difference in the level of evaporative demand. However, the laboratory experiment produced the same relative values between samples as observed for the field experiment. This evidence supports the use of laboratory data for predicting field results. ..