Ground Water, continued
Ground-water DischargeWater is discharged from the underground reservoir that lies beneath Finney and Gray counties by transpiration and evaporation, seepage into streams, springs, underflow that leaves the area, and by wells. Other things being equal, the stage of the water table and the season of the year are the principal factors controlling the rate at which ground water is discharged. Transpiration and evaporation of water from the zone of saturation are confined exclusively to those areas where the water table is very shallow, such as the Arkansas valley. The greatest amount of water withdrawn from the underground reservoir by wells is in the vicinity of Garden City where the greatest amount of irrigation is carried on. Seepage of ground water into streams occurs only along Arkansas and Pawnee rivers, and discharge by springs is confined to the area occupied by Pawnee River and its tributaries. In areas where the water table lies at great depth there is no natural discharge except for the water that is moving laterally out of the area beneath the surface. A few domestic and stock wells pump water from the ground-water reservoir in such areas, but the amount of water withdrawn in this way is relatively small.
Water-table contours on the map (pl. 1) indicate that water is slowly moving out of the County beneath the surface toward the east. The amount of water leaving Finney and Gray counties in this way is approximately equal to the amount of water that enters the area from the west plus additions to, or subtractions from, the ground-water reservoir that occur within the two counties.
Before the development of wells in Finney and Gray counties, the annual natural discharge of ground water probably was approximately equal to the annual recharge. The artificial discharge of water by wells has interrupted this balance between natural discharge and recharge. Through a period of many years, however, there will be a readjustment of natural discharge to take into account the artificial discharge of water by wells, and after this readjustment takes place there will again be an approximate balance between annual discharge and annual recharge. Such a readjustment of discharge, however, may result in a gradual regional lowering of the water table: This regional lowering of the water table probably will be very slight, for the amount of water in underground storage is very large.
Transpiration and evaporation
Water may be taken into the roots of plants directly from the zone of saturation or from the capillary fringe, and discharged from the plants by the process known as transpiration (Meinzer, 1923a, p. 48). The depth from which plants will lift ground water varies with different plant species and different types of soil. 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 sand their roots 60 feet or more below the surface to reach the water table (Meinzer, 1923, p. 82). Most ordinary grasses and field crops obtain water only from the upper few feet of soil. Plants such as corn, wheat, and barley probably obtain water from depths not exceeding 7 feet. Alfalfa is unusual in that it will send its roots to a depth of 20 or 30 feet in order to obtain water.
Discharge of ground water by transpiration from the zone of saturation or from the capillary fringe in Finney and Gray counties is limited to the stream valleys and to the deeper depressions in the Finney basin where the water table is within 10 or 15 feet of the land surface. Water-loving trees, particularly cottonwoods, are found along most of the stream valleys and around some of the depressions in the Finney basin. Where cottonwood trees occur, the depth to water generally does not exceed 20 feet (Meinzer, 1927, p. 58). In the Arkansas valley the discharge of ground water from the zone of saturation by transpiration is limited to areas adjacent to Arkansas River where the water table is comparatively close to the surface.
The amount of water discharged annually by plant transpiration is probably negligible in those areas where the water table lies more than 20 feet below the surface; however, in the Arkansas valley and parts of the Finney basin, where the water table is shallow, the amount of ground water discharged by plants is large. Wenzel (Lugn and Wenzel, 1938, p. 151) estimated that in the Platte River valley between Chapman and Gothenburg, Nebraska, an average of 12 inches of supplemental water is used annually by plants whose roots extend to the zone of saturation, or about 12 times the quantity of water that was at that time pumped annually from wells.
In areas where the water table is shallow, some ground water from the zone of saturation evaporates directly into the atmosphere. Water discharged in this manner is drawn from the zone of saturation and is evaporated from the capillary fringe. If the capillary fringe does not extend to within a few feet of the land surface, no water is lost from the zone of saturation by evaporation. The depth to ground water over most of Finney and Gray counties is great enough so that in most areas no water is lost by evaporation. The greatest loss of ground water by evaporation occurs in the bed of Arkansas River when it is dry. Some water probably is discharged by evaporation from the underground reservoir in those depressions in the Finney basin where the water table is very shallow. The rate of evaporation of ground water is determined principally by the type of soil, the depth to the water table, and weather conditions, all of which have great variations in Finney and Gray counties.
A stream that stands lower than the water table may receive water from the zone of saturation, and is known as an effluent stream (fig. 14). Arkansas River is the only stream in this area that gains water from the main ground-water reservoir. Part of Pawnee River receives effluent seepage, but the water comes from the alluvium in the Pawnee valley which is isolated from the main underground reservoir by an outcrop area of a relatively impermeable Cretaceous shale.
Except at flood stages, Arkansas River is a gaining stream throughout its course in Gray County and eastern Finney County. During the 18-year period from October 1, 1922, to September 30, 1940, (table 7) the river gained in flow between Garden City and Lamed each year except four (1922-1923, 1923-1924, 1924-1925, 1935-1936). Part of this gain in flow was supplied by runoff from precipitation, and a part came from the ground-water reservoir as effluent seepage.
There is a delicate balance between the level of Arkansas River and the adjacent water table throughout most of Finney County. Fluctuations of the water table or changes in discharge of the river may cause the Arkansas to become either a losing or gaining stream. During certain periods the bed of Arkansas River in Finney County is dry and lies above the water table (p. 75). During these periods a large part of any water flowing in the river after heavy rains sinks into the alluvium and joins the underground reservoir. At other times the Arkansas in Finney County is a gaining stream and receives water from the underground reservoir.
Discharge from springs
Some water is discharged through springs along Pawnee River and its tributaries as described on pages 87, 88, but springs are not known to occur in any other area in Finney and Gray counties. All of the springs are along the escarpment of the High Plains and appear at or near the contact between the water-bearing sands and gravels of the Ogallala formation and the underlying Cretaceous shales and limestones. Although all of the springs occur in valleys none of them has sufficient flow to give rise to streams. It is probable that there is a large amount of seepage along the escarpment that is not noticeable, the water probably being lost by evaporation and transpiration faster than it seeps out of the ground. The water-table contours (pl. 1) show that much more water is being discharged along the escarpment than is indicated by the few small springs that were observed.
Discharge from wells
In 1939, more than 38,000 acre-feet of water was pumped from irrigation, industrial, and public supply wells in the two counties. Additional water is withdrawn by domestic and stock wells, but the total amount of water pumped from these wells is comparatively small.
When a well is pumped, the water table in the vicinity of the well declines and takes a form similar to an inverted cone, called the cone of depression. When pumping stops, the cone of depression is gradually filled with water from adjacent areas. As a result, the regional water table declines slightly until the surface of the water table again becomes practically smooth. After the end of the pumping season in Finney and Gray counties, the regional water table tends gradually to assume a form similar to the form it had before pumping began. The regional water level in the reservoir, however, generally is lower than it would have been had there been no pumping.
Pumping a well lowers the water level not only in the pumped well but also in all wells that are within the area of influence (area affected by the cone of depression). The influence of a pumping well on the water level of a nearby well is shown by the hydrograph of well 138 in figure 17. Well 138 is about 300 yards from a large irrigation well (139). The hydrograph is based upon monthly Water-level measurements; therefore, it does not show the changes in water level that occur between measurements.
The time and length of the pumping season in this area vary from Year to year, depending mainly on the amount and distribution of precipitation. The main pumping season usually extends from May or June through September or October, although some wells are pumped at other times during the year. The low water levels in wells in the pumping area usually are reached at or near the end of the pumping season. After pumping has stopped, there is a period of recovery during which time the water table gradually rises.
After a season of heavy pumping, the water table is usually lower than it was at the beginning of the pumping season. This lowering of the water table during the pumping season is not permanent, for the water table generally rises as a result of the recharge caused by precipitation that falls during non-pumping seasons. If heavy pumping and a deficiency in annual precipitation continue year after year, there will be a cumulative decline in the regional water table (pp. 71, 72, fig. 13). If the precipitation is great enough during the growing season to supply the necessary moisture to the crops, there will be little or no heavy pumping and the water table will not decline appreciably; if the precipitation is greater than the needs of the crops, the water table may actually rise.
Kansas Geological Survey, Finney and Gray County Geohydrology|
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Web version April 2002. Original publication date Dec. 1944.