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The Hydrologic Equation and Recharge
All of the water on the continents originates as precipitation. The hydrologic equation is a simple expression of the relationship between this precipitation and evaporation, infiltration, and streamflow:
precipitation = evaporation + infiltration + streamflow
Although it is difficult to measure, except through estimates from the hydrologic equation, infiltration is the amount of precipitation that soaks into the ground. Part of this infiltration soaks into the soil and moves slowly downward to a depth below the ground surface where all the open spaces in rock or soil are full of water. This water that saturates the soil and rock is called ground water.
Ground water occurs in the saturated zone of the earth's crust (Figure 2). The upper surface of the saturated zone is the water table. Above the water table the open spaces are partly filled by water and partly by air; thus, overlying the zone of saturation is the unsaturated zone (or zone of aeration, or vadose zone). Between the saturated and unsaturated conditions is an intermediate zone where water is held above the water table in tiny interconnected openings by adhesion, cohesion, and surface tension, that is, by capillary attraction. This capillary fringe does not readily release water to springs or wells, but movement of water and its dissolved constituents does occur. It is only a few inches thick in coarse sand and gravel deposits, but can be as much as ten feet thick in clay and silt deposits.
Water infiltrates at greatly differing rates from place to place and time to time. The chief variables that determine infiltration rate are soil moisture from previous precipitation, soil texture, vegetational cover, land slope, and frost penetration. Some infiltrated water never penetrates the soil deeply, but that which does and is added to the saturated zone is called recharge water, or simply recharge.
Cumulative values for precipitation, evaporation, and recharge during the course of a normal year are shown in Figure 3. Even though there is a continuous accumulation of precipitation, recharge of ground water occurs mostly during the late winter, spring, and fall months. This phenomenon is explained partly by the decrease in rainfall and increase in evaporation during the summer (Figure 4), but mostly by the fact that water cannot pass through the zone of aeration when the soil in this zone holds less than a particular level of moisture, often called the field capacity. Vegetation transpires water into the atmosphere and tends to keep the aerated zone relatively dry during the summer and early fall months. For most of the growing season, there is a soil moisture deficit; that is, the soil will absorb rainfall withouth letting any water pass through to a depth of more than a few inches. Water in the soil is held by strong tensional forces that are not overcome by gravitational forces until the soil is very wet. The soil will let gravitational water pass through only when the overall moisture content is large enough to satisfy the tensional attraction of the soil particles. Soil at field capacity contains all the water it can hold against gravity. Thus, any additional water will flow through a soil that is at field capacity. Other factors affecting recharge in Maine include frozen ground that restricts infiltration, and preciptiation that falls as snow and is therefore not immediately available for infiltration and recharge.
In the most general situation, recharge water migrates downward to the zone of saturation and forms a permanent water table, as depicted in Figure 2. Where there are intervening layers of poorly permeable material, such as clay lenses in a gravel deposit, downward movement of water is impeded, and a perched water table will form (Figure 5). Ground water whose upward movement is impeded because it is trapped beneath a poorly permeable layer becomes confined and forms an artesian water system (Figure 6).
Peak levels for ground water in the spring occur somewhat after peak streamflow because of the time required for downward percolation of the recharge water. The most important aspect of ground water levels, however, is that they decline at a very much slower rate than streamflow. The chief reasons for this much slower decline are that ground water discharges at a slower rate and is not affected as much as streamflow is by evaporation and transpiration by plants (Figure 4).
Last updated on March 25, 2009
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