INTERMITTENT SATURATION OF SHALLOW AQUIFER SEDIMENTS ALONG THE MIDDLE RIO GRANDE, NEW MEXICO

Ecological problems along the middle Rio Grande (MRG) in New Mexico, a regulated river, have brought about the need for improved understanding of the hydrology of the river and its floodplain. Water table fluctuations along the river have been monitored as part of ongoing research to improve the riparian evapotranspiration (ET) term in the MRG water budget. Pressure transducers were installed in the shallow alluvial aquifer at eight sites along a 160-km reach. Water table readings were taken every 30 minutes (with 2 mm resolution) during the 1999 and 2000 growing seasons.

Comparison between Rio Grande discharge data and our well hydrographs provided information on how climatic and anthropogenic changes affect the riparian aquifer. The data indicate three major influences on the water table stage at different time scales: 1.) Riparian vegetation draws down the water table each day during the growing season, creating a diel (24-hour) signal. 2.) Climate provides a seasonal pulse to the MRG system. Spring snowmelt generates high river flows, which caused overbank flooding before dams were constructed. Most rainfall in the MRG basin occurs during the late summer, producing short-term water table highs after runoff from showers. 3.) Human activity influences water levels at irregular times throughout the year to meet the needs of irrigation, water supply, endangered species, and flood control.

Diel water table fluctuations were compared to ET estimates produced from 3-D eddy covariance (micrometeorological) methods at four sites. Significant correlation between the two data sets suggests the possibility of estimating ET using low-cost, low-maintenance well hydrographs. Additionally, the high-resolution well hydrograph was used to define a zone of intermittent saturation (ZIS), in which the subsurface environment fluctuates between oxidizing and reducing conditions on varying time scales. Ongoing research at UNM seeks to characterize the biogeochemistry of ZIS sediments and waters.