The Water Table
Shape and Slope
The water table is defined as the upper surface of the zone of saturation. The water table is not a static, level surface, but rather it is generally a sloping surface that has many irregularities caused by differences in permeability of the water-bearing materials or by unequal additions of water to and withdrawals from the ground-water reservoir at different places.
The shape and slope of the water table in Morton County are shown on the map, plate 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 topographic contour lines show the shape of the land surface. The direction of movement of the ground water is at right angles to the contour lines in the direction of the downward slope.
The map (pl. 1) shows that the general direction of movement of the ground water in Morton County is to the east, but that the direction of movement and the slope varies considerably from one part of the county to another. The maximum slope is in the north-western part of the county and is about 40 feet to the mile. The minimum slope is in the northeastern part of the county and is about 7 feet to the mile.
Irregularities on the water table may be caused in several ways. In places where conditions are exceptionally good for recharge from precipitation or intermittent streams, the water table may be built up to form a mound or low ridge from which the water spreads out, but this spreading is very slow because of the frictional resistance offered by small interstices through which the water must move. In material of low permeability these mounds or ridges may be very sharp, but in very permeable material the slopes generally are gentle. Depressions in the water table indicate places where ground water is being discharged and may occur along streams that have cut below the normal level of the water table or in places where considerable water is withdrawn by wells or plants. The contour map indicates that the water table slopes toward the Cimarron River valley from both sides and that ground water is discharging into the river. The permeability of the water-bearing materials also affects the slope of the water table. If the water is moving through very fine-grained sediments the frictional resistance to the movement of the water is great, causing comparatively steep slopes. Where the sediments are coarse and very permeable the resistance to movement is slight and the slopes are more gentle. In the northwestern quarter of the county, where the slopes of the water table are steepest, the water is passing through the fine-grained Cockrum sandstone. East of Richfield the water moves into the more permeable sands and gravels of the Ogallala formation, and the slopes become much more gentle. Similarly, in the southwestern and southeastern parts of the county the ground water moves through the Ogallala and the slopes of the water table are gentle. The steeper slope of the water table in the south-central part of the county is probably due to the less permeable character of the Ogallala sediments in that area.
The contour lines on the map indicate a broad arch or ridge on the water table in the northwestern part of the county. This may be due in part to recharge from the ephemeral streams in that area and from rainfall on the areas in which the Ogallala and younger formations crop out. It is possible, however, that the lines represent a general easterly movement of the waters, the flexure of the con-tours toward Cimarron River being caused by the discharge of ground water into the river.
The discharge of ground water from wells causes local depressions of the water table known as cones of depression and excessive pump-ing of many wells may depress the water table over a large area. As shown by the map, however, there are at present no widespread depressions in the water table caused by pumping in Morton County.
Relation to Topography
The general slope of the upland surface in Morton County is between 7 and 15 feet to the mile and the slope of the water table ranges from 7 to 40 feet to the mile, but the slope of the land surface along stream valleys in this county may exceed 200 feet to the mile. The map, plate 2, shows the depths to water level in Morton County. The lines separating the shaded areas are isobath lines that connect points of equal depth to water level. As shown on the map, the depths to water level in Morton County range from less than 30 feet in the northwestern corner of the county and along some of the stream valleys to about 225 feet along the southern boundary of the county. The shallowest ground water is found in the alluvium of the Cimarron River valley and in the Cockrum sandstone in the northwestern part of the county, and the deepest water is found in the Ogallala formation in the southern part of the county.
For the purpose of detailed description, Morton County may be divided into six areas based upon the depths to water level: (1) northwestern area, (2) northeastern area, (3) Cimarron valley area, (4) Richfield area, (5) area south of Cimarron River and (6) southern area.
Northwestern area--The northwestern area comprises about 35 sections in the western part of T's. 31 and 32 S., R. 43 W. In this area the water table is 30 to 75 feet below the land surface and in the stream valleys the depth to water level generally is only 30 to 40 feet. In the northern part of this area the principal water-bearing beds are the Cockrum sandstone and the Cheyenne sandstone, but in the southern part the Ogallala formation supplies most of the wells. Wells in this area range in depth from about 31 to 215 feet, the deeper wells penetrating the Cheyenne sandstone.
Northeastern area--The northeastern area includes about 12 sections along North Fork of Cimarron River in the northeastern corner of the county, in which the water table lies 80 to 100 feet below the land surface and the depth of the wells ranges from 100 to 125 feet. The water in this area is obtained from the sands and gravels of the Ogallala formation.
Cimarron valley area--The Cimarron valley area comprises a belt about 2 miles wide that parallels the river across the county. The water in part of the Cimarron valley lies at very shallow depths. Stock wells in the alluvium of Cimarron River range in depth from 15 to 30 feet and the water level is 10 to 15 feet below the land surface. South of the river the alluvium probably extends for a considerable distance under the cover of dune sand, and the depth to water is also moderately shallow here. North of the river some wells penetrate redbeds and obtain water at depths of 75 to 100 feet. Other wells north of the river get water from the Ogallala formation at depths of 65 to 100 feet.
The Cimarron valley area is bordered on both sides by much larger areas in which the depth to water level ranges from 50 to 100 feet. The large area north of the river is described below as the "Richfield area," and the eastern part of the smaller area south of the river is described below as the "area south of Cimarron River."
Richfield area--The Richfield area comprises a large area in the central, west-central, and north-central parts of the county, extending north to the Stanton county line, in which the depth to water level ranges from 50 to 100 feet. It also includes a small area in which the depth to water level is less than 50 feet. In the northern part of this area water is obtained from the Cockrum sandstone and in the southern part the Ogallala formation supplies the wells. In the northwestern corner of the area the Cockrum sandstone yields very little water to wells but farther south it is more permeable and yields large supplies.
In the southern part of the area the Cockrum sandstone is absent and the Ogallala is the principal water-bearing formation. The Ogallala lies on redbeds and consists of 75 to 125 feet of silt, sand, and gravel. The lowermost 30 to 60 feet of these beds is saturated with water.
Area south of Cimarron River--This area includes about 60 sections of land just south of and parallel, to Cimarron River. The depth to water level ranges from 50 to 115 feet and the depth of wells ranges from 75 to 400 feet. The Ogallala is the principal water-bearing formation in this area.
Southern area--The southern area is in the southern part of the county and it includes all of the county south of the 150-foot isobath line, which roughly parallels Kansas highway 45. The depth to water level ranges from 150 to about 225 feet and the depth of wells ranges from about 160 to nearly 250 feet. The Ogallala formation supplies water to the wells in this area.
The water table does not remain static but fluctuates similar to the surface of a lake. Whether the water table rises or declines depends upon the ratio of the amount of inflow into the ground-water reservoir to the amount of withdrawal. If the inflow exceeds the draft, the water table will rise; conversely, if the draft exceeds the inflow into the ground-water reservoir the water table will decline.
The ground-water reservoir may be replenished by rainfall that moves through the soil and descends to the zone of saturation, by recharge from flood waters in streams such as North Fork of the Cimarron, and by subsurface inflow from areas to the north and west. The factors that cause decline in the water table in Morton County are loss of water by subsurface flow into areas to the east and south, loss by evaporation and transpiration, loss by seeps and springs, and loss by pumpage from wells. The factors that cause a rise in the water table in a given area are all directly related to the precipitation on that area or on adjacent areas; therefore, the water table might be expected to fluctuate in direct response to precipitation. This is true in many areas, but in Morton County there are many factors which alter the effect that precipitation has upon the water table. After long dry periods the soil moisture may be depleted and must be completely replenished before any appreciable amount of moisture can percolate downward to the water table. North of Cimarron River where the soil is very thick several moderate rains might be necessary to replenish the soil moisture. Such rains, therefore, would have little or no effect on the water table. Similarly, it would require a great deal of time for moisture that falls as precipitation upon adjacent areas to move downward to the water table and then to move laterally into Morton County. Precipitation in areas to the west that produces flood waters in North Fork of the Cimarron have a more immediate local effect upon the water table in Morton County.
The ground-water supply in Morton County is depleted principally by pumpage and by underground movement into areas to the east and south. The loss of water through seeps and springs is relatively small and takes place only along Cimarron River. The loss by evaporation and transpiration is also small because in most parts of the county the water table lies too deep below the land surface to be affected by these processes.
The fluctuations of the water table may be determined by observing and recording the changes in the water levels in wells [Table 4]. In July, 1939, periodic water-level measurements were begun on 19 wells located at strategic points in Morton County. The depth to water level in each well is measured once a month in order to determine the fluctuations of the ground-water table. The measurements made to date are listed in the following table. All measurements before November 1, 1939, were made by me; those after that date were made by Richard B. Christy.
The water levels in 6 of the 19 wells declined during the period between July, 1939, and June, 1940. The water level in the other 13 wells showed a net rise during the same period. The net variation ranged from 0.01 foot to 1.92 feet and averaged about 0.25 foot. In some wells the water level rose steadily, in others it declined steadily, and in 4 wells it remained almost stationary. The water levels in many wells rose rapidly until October or November and then declined steadily during the winter and spring. Not enough measurements have been made to show any definite relation of the fluctuation of the ground-water table in this area to the precipitation. As the observation-well program continues more data will become available and a more definite relation to the precipitation may develop.
The water-level measurements made in 1939 are being published in the 1939 annual water-level report of the U. S. Geological Survey, Water-Supply Paper 886, and future measurements will be published in ensuing reports of this series. The well numbers used in this re-port and in Water-Supply Paper 886 are given in table 5.
Table 5--Observation wells in Morton county.
|Well number in this report||Well number in water-supply paper 886|
Kansas Geological Survey, Morton County Geohydrology
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Web version Sept. 2004. Original publication date March 1942.