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Table of Contents

Abstract

Introduction

Geography

Geology

Ground Water

Water Bearing Formations

Well Records

Well Logs

References

Plates

Ground Water, continued

The Water Table and Movement of Ground Water

Shape and slope

Figure 6--Diagram showing divisions of subsurface water. (From O.E. Meinzer.)

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.)

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 topography

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 table

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.

Well No. in this report	Well no. in Meinzer and Wenzel (1944)		Well No. in this report	Well no. in Meinzer and Wenzel (1944)
25	12	60	26
37	9	71	7
43	21	72	2
53	4	82	33
54	13	91	1

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.

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