Evaporation from soils and transpiration

Early in the course of our studies on water‐relations of soils and plants, it was noted that the amount of water transpired by growing plants was tremendously greater than that evaporated directly from the surface of the soil, and that, under the conditions prevalent in California, evaporation‐losses are too small to warrant concern. These results are based not only on comparison of the losses of water through trees or plants growing in tanks with losses of water from uncropped tanks but also on the results of sampling the soil for moisture‐determinations for fallow and cropped areas in the field. The conclusion that losses of moisture are occasioned by the upward movement of moisture from the lower moist layers of soil and dissipated into the air is based upon the familiar teaching that moisture is capable of moving through the soil in all directions through capillary forces. It is reasoned that the moisture exists in the form of films around the so 11‐particles and in wedge‐shapped masses of water between the soil‐particles at the points of contact with each other, being held partly by the attraction of the non‐water particles for the water and partly by the molecular attraction of the liquid itself. Adjoining particles with different thicknesses of moisture‐films surrounding them hold different amounts of water in the adjacent capillary spaces.