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Analysis of Hydrologic Factors That Affect Ground-Water Levels in the Arkansas River Alluvial Aquifer Near La Junta, Colorado, 1959-951 By Clifford R. Bossong Abstract The water table is sometimes near the land surface in the vicinity of La Junta, Colorado, and may impair the use of agricultural and personal property. A water-table map prepared for the alluvial aquifer in the vicinity of La Junta, indi- cated that, in March 1999, the Fort Lyon Canal and the Arkansas River provided recharge to the aquifer. A depth-to-water map prepared for the same area and period indicated that the water table was relatively shallow (less than 10 feet) in about 50 percent of the area studied. Available historical water-level records visually indicated that water levels tended to increase throughout the study area during the past nearly 4 decades, and regression analysis quantifies this relation. The available records do not address short-term changes. Several hydrologic factors that affect water levels in the study area were identified, and some simple relations between these factors and changes in water levels also were identified on the basis of coincidence of changes in the time series for the various records. Indications are that flow in the Fort Lyon Canal, surface-water applications for irrigation, and ground-water withdrawals have acted in concert to affect both increases and decreases in ground-water levels. Relatively low levels of ground-water withdrawals in the 1990's may be associated with increases in ground-water levels as well. The elevation for the base of the Arkansas River at the gaging station in La Junta has steadily increased from about 1960 to 1997 and has probably influenced water levels in lowland wells. INTRODUCTION La Junta is located in southeastern Colorado on the Arkansas River (fig. 1). Much of the land near the Arkansas River in the vicinity of La Junta is irrigated for agricultural purposes. Water used for irrigation either is taken from the Fort Lyon Canal, which diverts water from the Arkansas River a few miles west of La Junta, or is withdrawn from the local alluvial aquifer in the flood plain of the Arkansas River, which is highly transmissive and capable of yielding r,~la- tively large amounts of water to wells. The Fort Lyon Canal provides irrigation water for about 90,000 acres of agricultural land between La Junta and Lamar, about 60 miles downstream. The Fort Lyon Canal is the principal source of water used for irrigation in the study area; however, withdrawals from the local allu- vial aquifer can represent as much as 40 to 60 percent of total irrigation when surface-water availabililY is low (Goff and others, 1998). Tracts of agricultural land and many residences in the vicinity of La Junta may be affected by the water table which, in places, is near the land surface. Effects include flooded basements and soggy, or water-logged, conditions in agricultural fields that impair their suitability for agriculture. Conveyance losses from the Fort Lyon Canal, recharge from the Arkansas River, and ground-water withdrawals are potential sources of recharge and discharge to the local aquifer that affecting water-table conditions. In order to better understand hydrologic factors that affect the water table near La Junta. the U.S. Geological Survey (USGS), in cooperation with Abstroct 1