REGIONAL HYDROGEOLOGY AND LONG-TERM WATER BUDGET OF THE UPPER RIO HONDO BASIN, LINCOLN COUNTY, NEW MEXICO

The Upper Rio Hondo Basin occupies 585 square miles in south-central New Mexico, and comprises three general hydrogeologic terranes: the high elevation “Mountain Block,” the “Central Basin” piedmont area, and the lower elevation “Hondo Slope”. As many as a dozen bedrock units serve as aquifers, exhibiting locally complex flow conditions, but forming a continuous aquifer on the regional scale. Streams and aquifers in the basin are closely interconnected, with numerous gaining and losing stream reaches across the study area. In general, the aquifers are characterized by low storage capacity and respond to long- and short-term variations in recharge with marked water-level fluctuations on short (monthly) and long (decadal) time scales. Droughts and local groundwater withdrawals have caused rapid water-table declines in some areas, but periodically heavy monsoons and snowmelt events have rapidly recharged aquifer storage in areas with favorable geologic structure, including recently mapped faults and paleokarst surfaces. A long-term, regional-scale water budget was prepared to provide quantitative estimates of precipitation, evapotranspiration, streamflow, groundwater outflow, and water use. The chief inflow is watershed yield from the Mountain Block terrane, supplying about 38,000 to 42,000 acre-feet per year to the basin, as estimated by comparing the residual of precipitation and evapotranspiration with local streamgage data. There is a high degree of interannual variability in precipitation, and consequently, in the water supply. The chief outflows are about 21,000 acre-feet per year of runoff to the Rio Hondo and almost 14,000 acre-feet per year of depletions from domestic and agricultural uses. Uncertainties in groundwater outflow (from 2,000 to 6,000 acre-feet per year) and ranges in the estimates of inflow and outflow terms account for imbalances in the budget and highlight areas for future study. Changing water use patterns, concentrated areas of groundwater withdrawal, and variations in precipitation have created localized areas where water table declines and diminished surface flow are of concern; however, over the past six decades no net long-term loss of aquifer storage is apparent.