Ground Water, continued
RechargeRecharge is the addition of water to the underground reservoir and may be accomplished in several ways. All ground water within a practicable drilling depth beneath Thomas County--that is, the water in the Tertiary and Quaternary deposits above the Pierre shale--is derived from water that falls as snow or rain either within the area or on nearby areas to the west. Once the water becomes a part of the groundwater body it moves down the slope of the water table, later to be discharged, for the most part at some point beyond the limits of the county.
Recharge from local precipitation--The average annual precipitation in Thomas County is about 18 inches but probably only a small fraction of this amount enters the zone of saturation, thus serving to recharge the groundwater reservoir. The depth to the water table exerts an important influence on the amount and frequency of recharge. The valley of South Sappa Creek receives above average recharge, for the water table stands only a few feet below the surface and the material above the water table consists dominantly of sand and gravel. The gently rolling divide areas underlain by thick silt deposits are areas of below average recharge because of good surface drainage and low permeability of surficial materials.
Rodent burrows and sod cracks probably are important avenues of access for rainwater entering the ground. During dry seasons extensive sod cracks are reported to have developed in various parts of Thomas County. Although I have not observed these cracks they are reported to attain a width of several inches and to extend along the surface for several hundred feet. Cracks or openings not visible at the surface probably occur within the body of the massive silt. This was demonstrated when the test drill lost circulation while drilling several test holes through this material.
Recharge to the groundwater reservoir is indicated by a rise in the water level in wells not influenced by pumping. Selected wells throughout the county have been measured periodically and the records of six of these are given in figure 8 with the monthly rainfall data at Colby. Three of these wells (25, 43, 71) are less than a half mile from the city pumping plants of Brewster, Colby, and Rexford and therefore may be influenced by the pumping of these city wells. In any case they show no recognizable correlation with rainfall. Well 72 is the only one of the three wells unaffected by pumping having a water level of less than 50 feet. Although the correlation between the water-level fluctuations in this well and the precipitation at Colby is not too apparent, the water level nevertheless seems to exhibit some response to rainfall. Well 37, which has a water level of more than 70 feet, had a pronounced rise in water level starting in July, 1944, in response to the unusually heavy rainfall from April to July, 1944. In well 60, which has a depth to water level of more than 110 feet, there was a slight rise in water level during the period of record.
Recharge from streams and ponds--During periods of maximum flow in the major streams of the county some water probably leaves the channels and moves into the adjacent alluvium and underlying Ogallala formation. Most of the water from surface streams that recharges the groundwater reservoir represents water that fell as rain within the county, for North and South Sappa creeks are the only two streams that have any appreciable drainage area outside of Thomas County.
The massive silt of the Sanborn formation which underlies much of the upland surface of the county is relatively impervious, thus retarding or preventing the downward percolation of water. Many shallow undrained depressions on this surface catch and hold rainwater and prevent surface runoff. The fact that these shallow depressions, some of which are more than 150 feet above the water table, are occupied by shallow ponds for weeks after a heavy rain seems to indicate that even in these areas the water percolates downward very slowly and most of it evaporates (pls. 5A and 6A). Recent studies of recharge from shallow depressions in the High Plains of Texas (White, Broadhurst, and Lang, 1940, pp. 6-8) showed that recharge from some ponds was quite rapid, whereas in others it was slow or nonexistent. The problems of the two areas are not entirely similar, for in the Texas area beds of caliche locally prevent downward movement of water, whereas in Thomas County caliche does not seem to be effective in this respect but nearly all the ponds are underlain by homogeneous deposits of silt. In advocating his hypothesis for the origin of High Plains depressions, Johnson (1901) assumed that significant quantities of water entered the ground from these depression ponds.
Recharge from subsurface inflow--As indicated by the slope of the water table (pl. 1), the movement of ground water in this area is in an easterly direction; hence recharge from precipitation or stream flow that occurs in areas immediately adjacent to the west eventually moves into this area and contributes to the available supply of ground water. As shown by the cross sections in figures 3 and 4, the water-bearing materials are relatively thin along the western edge of the county in the area south of Brewster; therefore, the quantity of water entering the county as subsurface inflow is probably small.
Kansas Geological Survey, Thomas County Geohydrology|
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Web version Nov. 2001. Original publication date Dec. 1945.