Hydrogeologic Cycle
While humidity, temperature, and wind speeds can affect evaporation rates, most precipitation evaporates shortly after it has settled to the ground.
However, much of the remaining water percolates through soil and into vegetation roots. It is then absorbed into the plants by means of osmosis and released as water vapor through the leaves. This process is called “consumptive use” because the transpired water is returned directly to the atmosphere without the ability for further use near the Earth’s surface.
Very little water percolates beyond this stage of consumptive use (up to 20% in the coarsest sediments and nearly none in dense clayey soils). The remaining percolated water is drawn by gravity into and through the vadose zone. Throughout the vadose zone, interstitial pores are only partially filled with water, which is bound to soil grains by intermolecular forces (known as Matric potential) and cohesion.
Excess water continues to be drawn down past the vadose zone and into the “capillary fringe,” where pore spaces are nearly completely filled with water.
Capillary fringes vary in thickness depending on the type and nature of the soils. Thicknesses can range from only a few inches to dozens of feet. Course sands and soils have little or no capillary fringes due to their low surface area to volume ratio, while finer deposits increase this surface area to volume ratio and as such may have capillary fringes exceeding 50 feet.
In both the vadose zone and capillary fringes, water molecules are adsorbed to soil particles through surface retention. In this region, known as the “tension saturated zone,” water between pore spaces is held in place under tension. Much of this water is not drained or drawn down by gravity, placing it in what is called “dead storage.”
Water that is not trapped in dead storage will percolate down into the saturated zone. This water is known as ground water recharge. This recharge replaces ground water removed by natural discharges such as springs or artificial pumping.
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