The amount of water in storage in the zone of saturation does not remain constant but fluctuates with the precipitation. Water is continually being discharged from the underground reservoir by seepage, wells, and springs. The water table lowers in times of drought and rises during times of precipitation.
The addition of water to the underground reservoir, termed recharge, may occur in several ways. Water available to wells in Marshall County originally fell as rain or snow within the county or surrounding area. Water reaches the zone of saturation in Marshall County by direct recharge from local precipitation, by recharge from streams and ponds, and by subsurface movement from outside the area.
Recharge from Local Precipitation
The normal annual precipitation in Marshall County is 28.30 inches. Of the total precipitation, a large part is consumed by evaporation and transpiration, a small part leaves the county as surface runoff, and a small part reaches the zone of saturation and becomes ground water. According to records of the U. S. Geological Survey and the Kansas State Board of Agriculture, the annual net runoff in the drainage area of Big Blue River above Randolph, Kansas, during the 10 years 1936 to 1945, inclusive, ranged from 0.55 inch in 1940 to 5.43 inches in 1945; the average for the 10-year period was 2.40 inches. The annual net runoff in the drainage area of Little Blue River above Waterville during the same period ranged from 0.52 inches in 1940 to 5.56 inches in 1945, and the average for the 10-year period was 2.30 inches. Comparison of these runoff figures with the precipitation records at Blue Rapids for the same period of time indicates a runoff of slightly less than 8 percent of the precipitation.
A large part of the precipitation in Marshall County occurs during the summer months when conditions are favorable -for a high rate of evaporation and transpiration, and the bulk of the water is probably lost in this way.
Factors favoring a high rate of runoff are steep slopes of the land surface, heavy downpours of precipitation, sparse coverings of vegetation, impermeable materials at or near the surface, and rainfall when the ground is frozen.
Recharge from Streams and Ponds
Streams and ponds may receive water from the underground reservoir, or they may add to the water in the ground-water reservoir. To add to the ground water, the surface of the water in the stream or pond must be above the level of the water table, and the material surrounding the body of water must be permeable enough to allow downward percolation of water to the water table. Streams that satisfy these conditions are called influent or losing streams (Fig. 5).
Figure 5--Diagrammatic sections showing influent and effluent streams (after O.E. Meinzer).
None of the streams of Marshall County are normally of the in-fluent type, because the surface of the water in the streams is generally below the water table. During times of prolonged drought, however, the water table may fall to such a low level that some of the major streams become temporarily influent. Heavy pumping from the alluvium bordering a stream may lower the water table near the stream sufficiently to induce recharge from the stream and thus recover water that would otherwise be lost as runoff.
Recharge from Outside Areas
Most of the ground water entering Marshall County by movement from adjacent areas probably fell as precipitation in the areas immediately surrounding the county. Ground water in some of the deep-lying formations, which are not generally exploited for water in Marshall County, probably entered the formation at the area of outcrop, which may be at a considerable distance. The amount of ground water entering the county by subsurface inflow is small and is probably about equal to the amount leaving the county in the same way.
Ground-water discharge is the removal, by any method, of water from the zone of saturation. Ground-water discharge in Marshall County is by transpiration and evaporation, by seepage into streams and ponds, by subsurface movement into adjacent areas, and by pumping of wells.
Discharge by Transpiration and Evaporation
Transpiration is the process by which water is taken into the root system of plants directly from the zone of saturation or from the capillary fringe just above it and is discharged into the atmosphere. The depth from which plants will lift the ground water varies with different plant species and different types of soil. Ordinary grasses and field crops will not send their roots more than a few feet in the search for water. If subsurface conditions are suitable alfalfa and certain desert plants are known to send their roots to greater depths. The water table is within easy reach of such deep-rooted plants nearly everywhere in Marshall County, and in the valley areas of the county the water table is probably within reach of the ordinary grasses and field crops.
Loss of ground water by direct evaporation can take place only where the water table is within a few feet of the surface and in Marshall County is probably limited to marshy lands and areas along streams.
Discharge by Springs and Seeps
A stream that stands lower than the water table may receive water from the zone of saturation, and is known as an effluent or gaining stream. This discharge of ground water may exist as a few large springs issuing from highly permeable sand and gravel beds or from fractured limestone beds, or it may occur as seepage along the entire course of the stream. Except for short periods of time during and immediately after rainfall, the perennial streams of Marshall County get their entire volume of water from springs and seeps, either within the county or in the areas upstream.
At present, the bulk of the ground water removed from the subsurface reservoir in the county is discharged by the processes of transpiration and evaporation and by springs and seeps.
Discharge by Movement to Areas Outside the County
Some ground water leaves Marshall County by movement into adjacent areas. The amount of ground water leaving the county by subsurface flow is small and is probably about equal to the amount entering the county in the same way.
Discharge by Wells
Although wells are the most obvious method of ground-water discharge in Marshall County, the quantity of water discharged in this manner is small. It is estimated that about 2,000,000 gallons of ground water is pumped daily in the county. The recovery of ground water from wells is discussed in the next section.
Kansas Geological Survey, Geologic History of Kansas
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Web version March 2004. Original publication date March 1954.