Ground Water, continued
The addition of water to the zone of saturation is known as groundwater recharge. Groundwater recharge in Ford County is derived from precipitation within the county, from influent streams and irrigation ditches, and from subsurface inflow from areas to the west of the county.
Recharge from Local Precipitation
Most of the groundwater recharge in Ford County is derived from precipitation. Of the total annual precipitation of 20.5 inches in Ford County, part runs off through surface channels and is drained from the area by Arkansas River, part is evaporated, part is transpired by plants, and part seeps downward to the zone of saturation and recharges the groundwater reservoir. When the amount of water absorbed in the soil zone is greater than can be held against the pull of gravity, the balance will move downward to the zone of saturation. Usually the belt of soil moisture is largely depleted by the end of the growing season, owing to the removal of much of the available water in this belt by evaporation and transpiration. Consequently, this deficiency must first be satisfied before recharge takes place.
Theis (1937, pp. 564-568) presented evidence to show that the average annual groundwater recharge from rainfall in the southern High Plains is somewhat less than half an inch. The surficial materials in the upland parts of Ford County are comparable to those of the southern High Plains, although the total annual precipitation is somewhat greater. That the recharge by rainfall penetration in Ford County probably averages somewhat less than half an inch a year is suggested by the following considerations: (1) Most of the rainfall occurs during the growing season when the amount of water lost by transpiration and evaporation is the greatest; (2) the downward movement of water is impeded in upland areas north of Arkansas River by cemented beds of caliche in the Ogallala formation and in upland areas south of the river by the relatively impervious Kingsdown silt; and (3) some of the water remains in depression for long periods after heavy rains, showing that very little water percolates downward at these points. No figures are available regarding the annual amount of evaporation in Ford County, but at Hays (about 70 miles northeast of Dodge City) the annual evaporation from a free water surface amounted to about 101 inches for the period April through October, 1939, as recorded by the U.S. Weather Bureau. It seems likely, therefore, that a rather large proportion of the annual precipitation in Ford County is lost through evaporation.
A part of the precipitation that falls is lost through runoff streams, the amount being determined largely by the intensity of the rainfall, the slope of the land, the porosity of the soil, and the type and amount of vegetative cover. Conditions generally a much more favorable for rainfall penetration during a gentle rain of long duration than during a torrential downpour when the rate of runoff is high.
The slope of the land is an important factor in determining the amount of runoff, and, in general, the steeper the slope the greater the runoff. The slope of the land surface in most parts of Ford County is relatively gentle, but steep slopes occur along some of the major streams.
The type of soil also determines in part the proportion of the precipitation that will be lost as runoff and the part that will percolate into the underground reservoir. In general, runoff is greater in places where the soils are tightly compacted and fine-grained than in places where the soils are sandy and loosely compacted.
A vegetative cover decreases the velocity of the runoff thereby offering a better opportunity for part of the water to seep into the ground.
The most favorable areas in Ford County for groundwater recharge seem to be the areas of sand dunes and the shallow-water areas along stream courses (pl. 1). A large percentage of the precipitation that falls on the sand dunes percolates downward rapidly with little loss by evaporation. There is little or no runoff in the areas of sand dunes, as indicated by the almost total lack of drainage channels.
Recharge from Precipitation from Outside the Area
The general slope of the land surface, the dip of the Ogallala formation, the slope of the water table (pl. 1), and the direction of movement of the groundwater are all in an easterly to southeasterly direction; hence, recharge from precipitation that occurs in areas to the west and northwest of Ford County eventually moves into the county and contributes to the available supply of ground water.
Although the Dakota formation is exposed at several different places in Ford County, much of the water that it contains undoubtedly enters the formation from areas of outcrop outside the county. As the regional dip of the Dakota is northeastward, the most logical intake area would be confined to localities southwest of Ford County where the Dakota formation crops out. The Dakota is exposed over wide areas in southeastern Colorado, notably in Las Animas County. Water derived from precipitation falling on the outcrops and from streams flowing across the outcrops is absorbed and flows eastward through the formation into Kansas. Opportunity is afforded for water from the Dakota formation to migrate upward into the Ogallala formation at places where the two formations are in contact and where the Dakota water is under greater head than the Ogallala water. The Dakota is known to be directly overlain by the Ogallala in parts of this area (see log of test hole 4 on p. 208), but in parts of northeastern Ford County where relatively impermeable shale separates the Dakota formation from the Ogallala formation there is no opportunity for movement of water from the Dakota into the Ogallala. In the southern part of the county where the Rexroad member of the Ogallala formation rests directly on the Dakota formation (see logs of test holes 10, 14, 16, 17, 19, 20 and 21, pp. 212-222) the possibilities are very good for movement of water from the Dakota into the overlying Ogallala.
Recharge from Streams and Lakes
Two factors determine whether or not a stream is capable of supplying water to the underground reservoir: (1) the water surface of the stream must be above the water table; and (2) the material above the water table must be sufficiently permeable to permit downward percolation of the water. Streams that satisfy these conditions are called influent streams (fig. 7). Arkansas River is effluent most of the time, as shown by the water-table contours (pl. 1), but as pointed out on page 42, it becomes influent during occasional periods of flood flow.
With the exception of Sawlog creek and its tributaries, most of the smaller streams in Ford County hose water to the underground reservoir during certain periods of the year when they are carrying runoff. The stream channels in several of the streams south of the river are cut in the Kingsdown silt which is relatively impermeable, so that very little water percolates downward to the zone of saturation. The amount of recharge resulting from occasional stream flow in Mulberry creek is not definitely known but is believed to be rather small.
Evidence of recharge from a small ephemeral stream is furnished by the record of water-level fluctuation in well 123, situated about 200 yards east of a small dry tributary to Coon creek (pl. 2). The depth to water level in the well ranges from about 37 to 40 feet. The water level in well 123 rose abruptly starting about the middle of May, 1940, and reached its highest stage of the year in the latter part of June, 1940, the rise amounting to 3.5 feet. This unusual rise seems to be the result of recharge following rather heavy rainfall in April and May. Water was still standing in the bottom of the tributary drainage at the time that the well was measured on June 19, indicating that part of the rise in water level might have occurred as a result of recharge from the stream.
The possibility of recharge from impounded bodies of surface water is provided whenever the level of such, water bodies is above the water table. Ford County Lake and Hain Lake (pl. 1) have been created by damming tributary drainages that have cut into the Ogallala formation. When the lakes are full, therefore, there may be some opportunity for percolation of water through the Ogallala to the underlying water table. The magnitude and rate of recharge from such, sources is not definitely known, but the controlling factors include the height of the lake surface above the water table, the amount of water in storage, and the permeability of the reservoir basin.
Recharge from Irrigation Ditches and Irrigated Lands
In the Arkansas valley rather large quantities of water are pumped from wells or diverted from the river for irrigation. Some of this water seeps into the ground and is added to the zone of saturation. Thus, groundwater recharge may take place in the vicinity of irrigation ditches, and irrigated fields. The effect of seepage from two irrigation ditches upon the water level in a nearby observation well (364) in the Arkansas valley is shown in figure 10 and is discussed on p. 66.
Summary of Recharge
Much of the annual recharge to the groundwater reservoir in Ford County is derived from precipitation that falls on the county and in part from precipitation on outcrop areas outside the county, and from seepage from the Arkansas River at times when it is influent. A part of the local precipitation enters the ground through areas of sand dunes and sandy soil, and small amounts through channels of normally dry streams during periods of flood flow. Some of the water that enters the Dakota formation from precipitation on its outcrops ultimately may recharge the Ogallala formation at places where the two formations are in contact. The amount of average annual recharge cannot be determined on the basis of existing data, but it is probable that only a small percentage of the average annual rainfall of 20.5 inches reaches the zone of saturation.
Groundwater is discharged in Ford County by transpiration and evaporation, seepage into effluent streams, springs, underflow that leaves the county, and by wells. The rate at which it is discharged varies with many factors, but especially with the stage of the water table and with the season of the year. Local differences in conditions cause more groundwater to be discharged in some parts of the county than in others. More groundwater is pumped from irrigation wells in the Arkansas valley than in the upland parts of the county. More water is withdrawn from the zone of saturation by plants by evaporation in areas adjacent to Arkansas River and other perennial streams than in areas where the water table lies at great depth. Natural discharge of groundwater also takes place in the form of water moving slowly out of the county toward the east, as indicated by the water-table contours on the map, plate 1. The water moving out of the county as underflow along the Arkansas valley represents one phase of this type of ground-water discharge. The amount of water that moves out of the county is approximately the amount that enters from the west plus whatever additions to or subtractions from the groundwater reservoir have been made within the county.
It is probable that before any water was pumped from wells in Ford County, the annual discharge of groundwater by natural processes was approximately equal to the annual recharge. Artificial discharge by pumping represents an additional amount of water taken from the underground reservoir without any increase in the amount of replenishment. The development of the groundwater resources of Ford County necessarily will cause some lowering of the water table until the natural discharge through springs and seeps into perennial streams and underground movement of water out of the county is decreased by an amount equal to the withdrawal by pumping. Such adjustments, however, will proceed slowly over a period of many years with only a gradual regional lowering of the water table, as the amount of water in storage is very large. Although quantitative estimates are available for the amount of water discharged from wells in Ford County, the amount of natural discharge is not definitely known.
Transpiration and Evaporation
The roots of plank may draw water directly from the zone of saturation and discharge the water into the atmosphere by the process of transpiration. The rate at which water is withdrawn from the zone of saturation varies with the type of plants, the depth to the water table, the climate and the season of year, the character of the soil, and possibly other factors. The limit of lift by ordinary grasses and field crops is not more than a few feet, but some types of desert plants have been known to send their roots 60 feet or more below the surface to reach the water table (Meinzer, 1923. p. 82). In parts of the county along the valley margins and on the uplands; where the water table is considerably below the reach of the roots of most plants, water is withdrawn from the belt of soil moisture, thereby depleting the supply of soil moisture, but in the Arkansas valley and some of the other stream valleys many of the plants draw water directly from the zone of saturation. Evaporation of water directly from the zone of saturation is confined almost exclusively to the dry bed of Arkansas River and to the land immediately adjoining the stream, where the water table is very shallow. Most of this water is drawn from the zone of saturation and is evaporated at the top of the capillary fringe. In areas where the water table lies at considerable depth no water from the zone of saturation is lost by direct evaporation; in such places only the soil moisture is evaporated. The amount of water discharged by plant transpiration in the Arkansas valley and other parts of Ford County is not definitely known. Wenzel (Lugn and Wenzel, 1938, p. 151) estimated that in the Platte River valley between Chapman and Gothenburg, Nebraska:
"If an average of 12 inches of supplemental water is used annually by the plants whose roots extend to the zone of saturation, the resulting quantity of water discharged by transpiration ... would amount to about 390,000 acre-feet a year, or about 12 times the quantity of water pumped annually from wells."
It is believed that the total quantity of water withdrawn by plant transpiration in the Arkansas valley in Ford County is rather large, but probably is less than in the central Platte valley in Nebraska. The areas bordering Arkansas River in which the water table lies within 10 feet of the surface are much smaller in extent than in the Platte valley.
Seepage into Streams
A stream that stands lower than the water table may receive water from the zone of saturation, and is known as an effluent stream. The principal streams that receive groundwater discharge in Ford County are Arkansas River, during periods of low flow, and Sawlog and Crooked creeks.
As pointed out under the discussion of shape and slope of the water table, groundwater moves in toward the Arkansas valley from both sides as shown by the water-table contours on plate 1. Thus, except at flood stage, the Arkansas River is a gaining stream throughout its course in Ford County; that is, it is effluent with respect to the water table. Where tributary streams have out their channels below the general water table, their flows are also augmented by groundwater during most of the year. Seepage of groundwater occurs along the banks of these streams and in some instances groundwater is discharged through springs along the valley sides. Although water has been observed in the channel of Crooked Creek in that part of its course in Ford County, the stream bed is known to be dry for long periods at points downstream, near Fowler, Meade County. At the county line the creek is entrenched about 15 feet below the general land surface and its channel may or may not be below the water table. Groundwater may be discharged into Crooked Creek during part of the year, but it is probable that stretches of the creek in Ford County would go dry during a part of the year if it were not for several small dams that have been constructed to impound water for stock and for irrigation.
Discharge by Springs
In Ford County some water is discharged through springs. Most of the springs observed are in the northern part of the county along Sawlog creek and its tributaries, and no springs were observed south of the Arkansas valley. The water table under the south uplands lies at considerable depth, and in general the channels of tributary streams lie above the water table; hence, few, if any, opportunities for springs exist. The total quantity of water discharged by springs in Ford County is not definitely known, but it is thought to be small as compared to discharge by other means. The known springs are described below under Recovery.
Discharge from Wells
Discharge of groundwater from wells in Ford County constitutes the principal discharge from the groundwater reservoir. In 1938, approximately 10,435 acre-feet of water was pumped from irrigation wells in the Arkansas valley and from industrial and public supply wells in the county Most of the rural residents of the county derive their domestic and livestock supplies from wells but the total volume of water pumped from these wells is comparatively small.
Kansas Geological Survey, Ford County Geohydrology
Web version April 2002. Original publication date Dec. 1942.
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