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Fluctuations in Water Levels The hydrographs in figure 4 are developed from relatively long records in which the wells were measured systematically, usually in the spring and fall of each year, as a minimum. In general, the deepest depths to water are measured in the fall and the shal- lowest in the spring. For a period of time such as used in this study, about 40 years, measurements of ground- water levels in the spring and fall probably do a reasonable job of characterizing the extremes of ground-water level fluctuations. The range in water-level measurements at indi- vidual wells is included in table 3. The mean of the ranges is 6.24 feet. The mean range reported here is nearly twice the mean range reported by Konikow and Bredehoeft in 1974. The difference between these two reported ranges reflects the relatively robust nature of the data used for the study described in this report, which covers a period of decades, compared to the Konikow and Bredehoeft (1974) study for which only I year of data was available. To determine if the measured ranges have a distinctive spatial pattern, they were plotted on a map. The mapped ranges seemed to visually indicate a tendency for ranges to be small near the Arkansas River and to increase away from the river. However, closer examination indicated that although fluctuations greater than 10 feet and sometimes as much as 30 feet commonly occur in areas near the Fort Lyon Canal, relatively large fluctuations of about 10 feet also occur near the Arkansas River and there were no statistically significant relations between geographic setting and the range of water level fluctuation. The magnitude of fluctuations near the river are probably due to occa- sional high flows related to flooding or local effects of ground-water withdrawals, or both. Short-Term Variation in Water Levels One question that may arise, especially when a temporal aspect to patterns in water levels has been identified, is "How quickly do water levels change?" Many of the historical water-level records based on spring and fall measurements do not describe potential short-term changes in water levels. The historical records do include some wells that have been measured as frequently as weekly; however, these wells are not located in areas where relatively large fluctuations in water levels have been measured. Some of the remaining records are in areas where water levels have had relatively large flucma- tions and have been measured monthly or bimonthly for at least some part of the record. These records indi- cate that water levels, in some areas, can change at least as much as I foot per month-a rate that would produce the mean fluctuation in 6 months (table 3). However, water levels could change at rates greater than the rates reported for this study. The historical records also were evaluated for adequacy in their ability to represent changes in cross- sectional gradients between the Fort Lyon Canal and the Arkansas River. Five roughly north-south cross sections of three to seven wells were identified, and the GWSI data base was accessed to obtain historical water levels along the cross sections that were measured within a reasonably narrow window of time (2 weeks). The results of this evaluation principally indicated that the historical records are not ideal for preparation of cross sections. In almost all cases, water levels were not available for all points in a given cross section. In the few cases when data were sufficient, indications are that gradients along some parts of some cross sections became reversed. That is, for a given set of wells, sometimes the gradient might be towards the river and sometimes the gradient might be away from the river; however, it was not possible to determine if such changes occurred routinely or even if they occurred in the short term. HYDROLOGIC FACTORS THAT AFFECT GROUND-WATER LEVELS Ground-water levels are controlled by recharge to and discharge from the aquifer. In relatively simple systems, recharge may consist only of precipitation, and discharge may consist only of evapotranspiration and ground-water withdrawals made from wells completed in the aquifer. However, there are additional sources of recharge and discharge in the study area that complicate the system. Some of these additional sources, particularly those related to irrigation, repre- sent major changes from a natural setting. The addi- tional sources of recharge and discharge that can be related to hydrologic processes in the study area are referred to as "hydrologic factors" and are discussed in this section. Precipitation and evapotranspiration are not discussed. HYDROLOGIC FACTORS THAT AFFECT GROUND.WATER LEVEL.5 15