Ground Water, continued
Ground-water RechargeThe 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 PrecipitationMost 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 AreaThe 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 LakesTwo 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 LandsIn 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 RechargeMuch 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.
Kansas Geological Survey, Ford Geohydrology|
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Web version April 2002. Original publication date Dec. 1942.