Water resources during drought conditions and postfire water quality in the upper Rio Hondo Basin, Lincoln County, New Mexico, 2010-13

Abstract

Stakeholders and water-resource managers in Lincoln County, New Mexico, have had long-standing concerns over the impact of population growth and groundwater withdrawals. These concerns have been exacerbated in recent years by extreme drought conditions and two major wildfires in the upper Rio Hondo Basin, located in south-central New Mexico. The U.S. Geological Survey (USGS), in cooperation with Lincoln County, initiated a study in 2006 to assess and characterize water resources in the upper Rio Hondo Basin. Data collected during water years 2010–13 are presented and interpreted in this report. All data presented in this report are described in water years unless stated otherwise. Annual mean streamflow at the Rio Ruidoso at Hollywood, N. Mex., streamflow-gaging station was less than 50 percent of the average streamflow during 2011–13 and was of similar magnitude to annual mean streamflow values measured during the drought of the 1950s. The first zero-streamflow values for the period of record (1954–2013) were recorded at the Rio Ruidoso at Hollywood, N. Mex., streamflow-gaging station on June 27–29, 2013. The lowest annual mean streamflow on record (1969–80; 1988–2013) occurred in 2011 at the Eagle Creek below South Fork near Alto, N. Mex., streamflow-gaging station, with the station recording zero streamflow for approximately 50 percent of the year. Discrete and continuous groundwater-level measurements indicated basinwide water-level declines during drought conditions in 2011–13. The average water-level change among 37 wells in which discrete groundwater-level measurements were collected was -7.6 ft from 2010 to 2013. The largest water-level declines were observed in the upper reaches of the Rio Bonito and Rio Ruidoso watersheds, and smaller declines were observed in the lower reaches of the watersheds. In general, water-level changes observed during 2010–13 were on the order of decadal-scale changes that previously have been observed in the upper Rio Hondo Basin. Stable-isotope data indicate that high-elevation winter precipitation generally contributes more to groundwater recharge than summer rains, except when there are large summer recharge events. This implies that little recharge is occurring at the lower elevations in the upper Rio Hondo Basin because these areas receive a smaller amount of total precipitation, receive a smaller proportion of the annual total falling as winter precipitation, and have higher average temperatures that result in more evaporative losses. Groundwater in the upper Rio Hondo Basin is a mix of younger and older water, and recharge likely is occurring primarily at higher elevations but there may be some areas where localized recharge is occurring at lower elevations. Surface-water- and groundwater-quality results from samples collected in 2012–13 were examined to characterize overall chemistry and were compared to historical waterquality data from streams in the upper Rio Hondo Basin collected during 1926–57. In general, specific conductance showed an increasing trend moving eastward (downstream) through the upper Rio Hondo Basin in surface-water and groundwater samples. Surface-water and groundwater samples appear to have similar overall major-ion chemical characteristics when compared to historical water-quality data. Geology was found to influence the chemical characteristics of surface-water and groundwater samples, with relatively higher concentrations of sulfate occurring in samples collected at lower elevations in the Permian regional aquifer system. Surface-water sample results also were analyzed to determine differences in unfiltered and filtered water-quality samples of streams in burned and unburned watersheds after the occurrence of the Little Bear Fire in June 2012. Samples were collected after postfire monsoon rain events and during periods of stable hydrologic conditions. The first postfire monsoon rain event in July 2012 generally produced the highest measured concentrations of selected fire-related constituents in unfiltered samples collected in the burned watersheds relative to later samples collected in burned watersheds and all samples collected in the unburned watershed. Monsoon rain events have impacted water quality by delivering larger sediment loads and fire-related constituents into streams in the upper Rio Hondo Basin. Changes in climate and increased groundwater and surface-water use are likely to affect the availability of water in the upper Rio Hondo Basin. Increased drought probably will increase the potential for wildfires, which can affect downstream water quality and increase flood potential. Climate-research predicted decreases in winter precipitation may have an adverse effect on the amount of groundwater recharge that occurs in the upper Rio Hondo Basin, given the predominance of winter precipitation recharge as indicated by the stable isotope results. Decreases in surface-water supplies because of persistent drought conditions and reductions in the quality of water because of the effects of wildfire may lead to a larger reliance on groundwater reserves in the upper Rio Hondo Basin. Decreasing water levels because of increasing groundwater withdrawal could reduce base flows in the Rio Bonito and Rio Ruidoso. Well organized and scientifically supported regional water-resources management will be necessary for dealing with the likely scenario of increases in demand coupled with decreases in supply in the upper Rio Hondo Basin.