HYDROGEOLOGY AND WATER BUDGET OF SUNSHINE VALLEY, TAOS COUNTY, NEW MEXICO

Sunshine Valley in northern Taos County, New Mexico has regional hydrologic significance due to the large accretion to the adjacent Rio Grande and Red River and the recent transfer of 1752 acre-feet/year of water rights downstream from the valley as part of the Aamodt Settlement Agreement. The Sunshine Valley aquifer is laterally and vertically heterogeneous, with sand and gravel layers overlying and interbedded with fractured and highly transmissive basalt flows. Recharge is dominated by high-elevation winter precipitation in the mountains to the east of the valley. Little recharge occurs across the valley floor. Recharge arrives in the valley aquifer as infiltrating streamflow and irrigation water. More than half of the recharge occurs as groundwater underflow into the aquifer directly from the mountain block. The steep topography and geologic features of the Questa caldera probably influence the large amount of recharge by underflow.

An estimated 1000 to 2000 acre-feet per year of groundwater has been lost from storage since the 1980s, corresponding to water-level declines of a few feet per year across the region. Water budget calculations are generally consistent with the storage change results, with estimated discharges falling between the estimated upper and lower bounds of recharge inputs. However, the water budget analysis is constrained by fundamental data limitations and is neither accurate nor precise enough to independently confirm or refute the independent estimates of groundwater storage losses.

Reduction of groundwater pumping due to the water-rights transfer can ultimately result in additional water flowing though the Sunshine Valley aquifer to discharge in the Rio Grande and Red River on a time scale of a few to several tens of years. Current levels of groundwater withdrawals for irrigation are not likely to be the main cause of the storage changes since the 1980s. Trends in decreasing regional precipitation, and increasing temperature and surface water-use are more likely factors affecting the amount of water in storage in the aquifer and the amount discharging. Regardless of the amount of groundwater pumping, continued declines in annual precipitation and streamflow and increases in mean annual temperature will decrease the amount of recharge to and discharge from Sunshine Valley.