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the Colorado Water Conservation Board, conducted a study in 1999 to analyze hydrologic factors that affect ground-water levels in the Arkansas River alluvial aquifer. This report presents the results of that study. More specifically, the report describes water-table conditions in March 1999, historical water levels, and hydrologic factors that affect water levels. The study was limited to the area between the Fort Lyon Canal and the Arkansas River from the Fort Lyon Canal headgate east to the Otero County line (fig. I). The description of March 1999 conditions is based on water level measurements made in 49 wells. The description of earlier conditions (generally 1959 through 1998) is based on time-series records for ground-water levels, ground-water withdrawals, appli- cations of ground water and surface water for irriga- tion, discharge in the Arkansas River, diversions to the Fort Lyon Canal, and channel conditions in the Arkansas River. The study was limited to data that are maintained in a computer system, or archived as digital records. Previous Investigations Several hydrologic investigations have been completed in the study area. The results of these inves- tigations are valuable sources of background informa- tion to interested readers. Konikow and Bredehoeft (1974) characterized the study area as part of a ground-water modeling investigation of ground-water flow and chemistry. Walls and Lunsford (1992) modeled several different water-management alterna- tives designed to affect changes in water-table condi- tions in the study area. Using a digital model that characterized ground-water levels in the area as sensi- tive to leakage from the Arkansas River, leakage from the Fort Lyon Canal, and ground-water withdrawals, they found that most alternatives would have relatively local effects although modifications to the Arkansas River channel could change ground-water levels throughout the area. Goff and others (1998), provided a modeling analysis of the effects of irrigation in the study area and found that salinity of ground water and surface water was related to irrigation activities. The report by Goff and others (1998) is the source of many data used in this study. Dash (1995) reported on the irrigation water use for the Fort Lyon Canal and docu- mented losses from the canal. Acknowledgments The assistance of certain agencies and individ- uals were critical to the completion of this study. In particular, Lloyd Wadleigh and Don Taylor, from Divi- sion II office of the Colorado Division of Water Resources located in La Junta, were extremely helpful with records available from their office that were used for identifying and locating wells for the March 1999 water-level measurements. In addition, many land- owners provided access to wells that were measured; without this access, it would not have been possible to characterize water-table conditions in March 1999. WATER-TABLE CONDITIONS IN MARCH 1999 To document current water-table conditions, the water table was mapped in the spring of 1999 on the basis of measurements from 49 wells. Wells were identified and depths to water from a point where the land surface elevation could be estimated were measured between March 9 and March 19, 1999. the results of those measurements are listed in table I. During the measurement period, flow in the Fort Lyon Canal was steady but relatively low, about 100 to 200 cubic feet per second, and flow in the Arkansas River at La Junta varied from about 50 to 150 cubic feet per second. The depth-to-water measurements were converted to elevations of the water-table surfacE, and the resultant elevations were contoured by hand 1:0 produce a generalized water-table map for the study area that is shown in figure 2. Estimates of land- surface elevations were made using I :24,000-SC2Je topographic maps, and map accuracy and estimation techniques dictated the estimates may depart from actual elevations by as much as 2.5 feet. In places where water-table surfaces are near the land surface, there is often interest in knowing how close the water-table surface is to the land surface. To show this information, digital elevation models of the generalized water-table map (fig. 2) and the land surface were used to create a depth-to-water map. The digital elevation models provide estimates of either ground-water elevation or land surface elevation for individual grid cells in a gridded network of the study area. Ground-water elevations for individual grid cells were estimated using measurements of ground-water WATER-TABLE CONDITIONS IN MARCH 1999 3