Hydrogeologic Cycle
Ref. Handbook of Groundwater Development, Roscoe Moss Company, 1990 edition.
Depending upon climatic conditions such as temperature, humidity, and wind speed, most precipitation evaporates during and shortly after it occurs.
Most of the remaining water percolates to the vegetation root zone and is absorbed through osmosis and discharged or transpired as water vapor through plant leaves. The processes of evaporation and transpiration are known as “consumptive use,” because the water returns to the atmosphere as water vapor and is unavailable for further use at the earth’s surface.
Typically, depending upon many factors, only a very small percentage of the infiltrated water percolates deeper. It ranges from possibly 10% to 20% in coarse alluvial deposits to very little in clayey soils. This percolating water moves downward under the influence of gravity through a region defined as the “vadose zone.” In this zone the pores between grains are only partially filled with water, some of which is bound to grains by surface tension and molecular forces.
Unbound water continues downward until it reaches the lower boundary of the vadose zone, known as the “capillary fringe.” Here pore spaces are completely filled with water.
The thickness of the capillary fringe varies from a few inches to several tens of feet, depending upon the nature of materials forming the zone. Coarse-grained deposits, which have large pore spaces and a low ratio of surface area to volume, have little or no capillary fringe. Materials composed primarily of fine particles have a large surface area to volume ratio and may have capillary fringes of 50 ft or more.
Water molecules are bound to grain surfaces in both the vadose zone and capillary fringe by a process known as surface retention. Here, water within the pore spaces of the capillary fringe is under tension, and this zone is referred to as the “tension saturated zone.” Water held by molecular and surface tension forces does not drain by gravity and is said to be in “dead storage.”
If not held in dead storage, the percolating water will eventually reach the zone of saturation. Water reaching the saturated zone constitutes ground water recharge. Ground water recharge may be natural or artificial, and it replaces ground water removed from storage by pumping or natural discharge such as springs.
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