Union County, the northeasternmost county in New Mexico, is rural with an economy based on ranching and agriculture. Surface water resources are limited, thus development of groundwater for stock watering and irrigation is important and extensive. Groundwater studies by the New Mexico Bureau of Geology and Mineral Resources (NMBGMR) in Union County have been conducted in concert with the Northeast Soil and Water Conservation District (NESWCD) and were driven by concerns over recent large groundwater appropriations, the reliability of the groundwater supply for the town of Clayton, and declining water levels that have been observed in many wells over the past few years. The New Mexico Office of the State Engineer (NMOSE) declared the Clayton Underground Water Basin in 2005, ending unrestricted appropriation and development of groundwater in northeast New Mexico. Recently, the NMOSE has started development of a groundwater flow model of the Clayton Basin for administration of water rights. Important input data for a groundwater flow model include accurate delineation of the groundwater surface and an understanding of water level changes over time.