How Water Flows in Soil

Abstract

From the very beginning of their studies, soil scientists noticed two characteristic values of soil water content. One was when water moving down from the nearly saturated topsoil starts to drastically slow down even though the soil contains still a substantial amount of stored water. If evaporation was prevented, the change of rate of drainage was reflected by the soil water content, which did not appear to change for several days after the initial decrease. Today, it is a common practice to observe such a characteristic by first surrounding a small, flat soil surface of about 1 m² with a small dike having a height of about 10 cm. Water is next steadily ponded on the area isolated from its surroundings until we guess that the soil is water saturated to the depth we wish to study. With the surface no longer ponded with water and covered by a folio in order to prevent evaporation, a limited portion of the soil water flows quickly and readily from the topsoil into the subsoil. The soil water content in the top layers decreases, while the deeper layers are gaining water. In a few days, the soil water content in the topsoil looks as though it is not changing – indeed the downward flux is ten to hundred times smaller than it was when water first started to drain immediately after its surface was not ponded with water. This soil water characteristic was originally called capillary capacity and later renamed to field capacity. It can be estimated using undisturbed soil samples within metallic cylinders of 100 cm³ volume maintained at approximately 10⁴ Pa negative pressure = 100 cm of water column. A rough value of capillary capacity could be read from the soil water retention curve (Fig. 8.11).