SEASONAL AND LONG-TERM VARIATIONS IN HYDRAULIC HEAD IN A REGIONAL KARST AQUIFER: ROSWELL ARTESIAN BASIN, NEW MEXICO

The Roswell Artesian Basin is located in the lower Pecos Valley of southeastern New Mexico, a semi-arid region with average rainfall of less than 33 cm/yr, yet it is one of the most intensively farmed areas in the state. For more than a century, irrigated agriculture in this area has been sustained by groundwater from a shallow alluvial aquifer and a semi-confined artesian aquifer comprised principally of middle Permian karsted limestones of the San Andres Formation. Both aquifers display significant variations in hydraulic head on a variety of time scales. Large seasonal fluctuations in water levels occur that are directly related to the irrigation cycle in the Artesian Basin, lower in summer months and higher in the winter when less irrigation occurs. Longer-term variations are the result of both human and climatic factors. Since the inception of irrigated agriculture in the Artesian Basin in the early 20th century, water levels in the Artesian Aquifer have fallen by as much as 70 m. The general decline in hydraulic head began to reverse in the mid-1980s due to a variety of conservation measures, including purchase and retirement of irrigated farmland by the state of New Mexico, but this reversal has slowed in recent years because of a period of extended drought.