Ground Water, continued
The Water Table and Movement of Ground Water
Shape and slopeThe water table is defined as the upper surface of the zone of saturation, except where that surface is formed by an impermeable body (Meinzer, 1923a, p. 32). It may also be regarded as the boundary between the zone of saturation and the zone of aeration (fig. 6). The water table is not a static level surface but rather is generally a sloping surface which shows many irregularities caused by differences in permeability of the water-bearing materials or by unequal additions to or withdrawals from the groundwater reservoir at different places.
Figure 6--Diagram showing divisions of subsurface water. (From O.E. Meinzer.)
The shape and slope of the water table in Thomas County are shown on the map (pl. 1) by means of contour lines drawn on the water table. Each point on the water table along a given contour line has the same altitude. These water-table contours show the configuration of the water surface just as contours on a topographic map show the configuration of the land surface. The direction of movement of the ground water is at right angles to these contour lines--in the direction of the greatest slope.
The map shows that the water under the plains moves through the county in a general easterly direction, but that the direction of movement and the slope are somewhat different in various parts of the county. The average gradient, of the water table is about 12.5 feet to the mile. In the area south of Brewster and Levant the slope of the water table is as much as 30 feet to the mile and in an area south of Colby the slope is as little as 6 feet to the mile.
In general the contours show a remarkable evenness of water-table slope throughout the county. In the northern and particularly the northwestern part of the county the shape of the contours indicates a northerly component to the direction of groundwater movement. This is probably caused by the deeply entrenched stream valleys in Rawlins County to the north which afford points of groundwater discharge.
The only stream valley in Thomas County that seems to exert a noticeable influence on the shape of the water table is South Sappa Creek. This is probably because South Sappa Creek is the only stream that has entrenched its valley to the level of the water table, and this has been accomplished only in the area north of Gem. For several miles upstream from the point where the 3,000-foot contour line crosses the valley the contours under the valley are convex to the east, indicating a slight ridge on the water table; whereas downstream from this point to the Rawlins County line the contours are concave eastward, indicating a depression in the water table. This change in shape is due to the fact that the floor of the valley is slightly above the water table and along the valley surface water enters the groundwater body more rapidly than elsewhere in the county. At the 3,000-foot line the valley floor is just at the level of the water table, and downstream from that point, where the valley bottom is slightly below the general level of the water table, some water enters the valley as seepage. A stream whose channel neither gains nor loses with respect to ground water is in equilibrium with respect to the water table. Figure 7 shows these relationships.
Figure 7--Diagrammatic sections showing influent and effluent streams. (From O.E. Meinzer.)
Other than South Sappa Creek the only stream in the county that has reduced its valley bottom to the level of the water table is the South Fork of the Solomon River. In the first few miles of this valley west of the Sheridan County line and south of Menlo the stream is about at the level of the water table. All other streams in Thomas County flow at a level above the water table and receive no discharge from the groundwater reservoir. The relatively impervious nature of the surficial materials seemingly prevents water in large quantities from entering the ground from the channels as only one stream coincides for any appreciable distance with a "mound" or "ridge" on the water table. As can be seen on plate 1 this stream flows through a valley cut into the sand and gravel beds of the Ogallala formation which are relatively permeable and afford good infiltration possibilities. Stream channels that are above the water table (influent streams) do not gain water from groundwater seepage; thus they contain flowing water only during and immediately after rains.
The elevation of the low water levels in the major streams was determined instrumentally at selected points. These elevations are shown on plate 1 and give a direct comparison between the altitude of stream channels and the surface of the water table.
Relation to topographyOn the map (pl. 2) are shown the depths to water level in Thomas County. In preparing this map the more general irregularities of the topography were taken into account by using aerial photographs and a drainage map. As topographic maps are not available for any part of Thomas County, the depth to water map must be considered generalized and not presumed to be accurate in all details. As shown on the map, the depth to water level ranges from more than 200 feet below the surface in an area north and northeast of Brownville to depths of only a few feet along the lower courses of such valleys as Sappa Creek, Prairie Dog Creek, and the Solomon and Saline rivers. The water level under several large areas in the east-central part of the county is less than 100 feet below the surface.
In general the shape of the water table conforms to the regional topography but is little affected by minor or local features. That is, the water table is essentially a plane and in that respect conforms to the regional topography of the High Plains, but its shape is not affected by local features of topography, for it passes below the valleys of the area with no perceptible change (with the exceptions noted above). Thus local low areas in the topography result in a reduced depth to water level almost directly proportional to the depth of the surface features.
Fluctuations of the water tableThe water table does not remain in a stationary position but fluctuates up and down much like the water level in a surface reservoir. If the inflow to the underground reservoir exceeds the draft, the water table will rise; conversely, if the draft exceeds the inflow, the water table will decline. Thus the rate and magnitude of fluctuation of the water table depends upon the rate and magnitude at which the underground reservoir is replenished or depleted.
Factors controlling the rise of the water table in Thomas County are the amount of rainfall within the county that descends through the soil to the water table, the amount of seepage that reaches the underground reservoir from surface streams, and the amount of water entering the county beneath the surface from areas to the west. All these factors depend upon precipitation either in or immediately adjacent to the west edge of the county.
Factors controlling the decline of the water table are the amount of water pumped from wells, the amount of water absorbed directly from the water table by plants (transpiration), the amount of water lost from the groundwater reservoir by evaporation, and the amount of ground water passing beneath the surface into adjacent areas.
Changes in the water levels in wells record the fluctuations of the water table, which in turn record the recharge and discharge of the groundwater reservoir. In order to determine the character and magnitude of water-level fluctuations in Thomas County, 10 wells were selected for observation, and periodic measurements of the depth to water level in them were begun in July, 1942. Since the original measurements by me in 1942, measurements of water levels in these wells have been made by W.W. Wilson, Allan Graffham, and Howard Palmer. Complete records for these wells are published annually by the Federal Geological Survey (Meinzer and Wenzel, 1944, pp. 174-177). The numbers of the observation wells previously published and the numbers used in this report are given in table 3.
Table 3--Observation wells in Thomas County.
The fluctuations of the water level in six typical observation wells in Thomas County during the period of and monthly precipitation at Colby are shown in figure 8.
Figure 8--Hydrographs of six typical observation wells in Thomas County and the monthly precipitation at Colby. (Precipitation data from U.S. Weather Bureau.) A large version of this figure is available.
Kansas Geological Survey, Thomas County Geohydrology|
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Web version Nov. 2001. Original publication date Dec. 1945.