The Soil System

Soil exists at the boundary between the atmosphere and the Earth’s subsurface. It plays a critical role in the hydrologic cycle, in addition to serving as the location of most human activity. An examination below the Earth’s surface generally reveals a profile similar to that shown in figure 1-1 A. The first zone encountered is the soil zone. This soil has developed from parent material through biological and other factors of weathering. If time is sufficient, then horizons will have formed with differing physical and chemical properties. At greater depths the soil merges with additional unconsolidated material. Eventually, at still greater depths, bedrock is encountered. The dimensions of these various zones are highly variable. For example, the soil profile may exist on bedrock that is partially exposed at the soil surface. Conversely, the unconsolidated layer can be hundreds of meters thick, as is the case in many alluvial basins. The subsurface can also be described in terms of water regimes that exist.The hydrologic profile consists of the vadose zone and the phreatic zone. The vadose zone is from the ground surface to the permanent water table, and includes the root zone, the soil profile, and the capillary fringe, which is a tension-saturated zone bordering the water table. The water at the water table is at atmospheric pressure; above the water table the pressure is less than atmospheric pressure and below the water table it is greater. The system is unsaturated above the capillary fringe, meaning that not only is the water under tension, but that some of the pore space is filled with air. The extent of the capillary fringe is dependent on the porous material. Generally, itextends a few centimeters for coarse material, or perhaps a meter for fine materials. A more complete depiction would include further saturated regions in the vadose zone, such as those due to surface infiltration or due to impeding layers that result in a perched water. Historically, the term groundwater was used to denote water beneath the permanent water table, but it is now commonly used to describe all subsurface water.