New Mexico Geological Society Annual Spring Meeting — Abstracts

In response to increasing needs to understand the water budget of New Mexico, we have constructed estimates of historical groundwater storage change in the shallow (less than 1000 ft), unconfined alluvial aquifers of the state. Shallow, unconfined alluvial aquifers in New Mexico form the major groundwater reservoirs in many of the urbanized and agricultural areas of the state. This collocation means that alluvial aquifers have been commonly used as a major water source over the last 70 years, possibly leading to declines in storage through time. We present preliminary estimates of historical groundwater storage changes in the alluvial aquifers of New Mexico, separated by USGS HUC-8 (hydrologic unit code, level 8) boundaries. These aquifers are mostly in Rio Grande and Basin-and-Range physiographic provinces—they are either in basins with the Rio Grande flowing through them or they are in the closed basins to the east and west of the Rio Grande basins. Our estimates are based on depth-to-water measurements available from the USGS online database, and datasets that conform to USGS measurement standards. Water level measurements affected by pumping or those that were taken during irrigation season were removed, except for locations in rangeland areas with poor data coverage. The measured depths-to-water are then used to determine a correlation length of water levels for each basin for every decade from the 1950s to present. The water levels are then interpolated using both ordinary kriging and inverse-distance-weighting methods. The interpolated fields are restricted to the radius of correlation lengths, and to Quaternary sediments mapped at the 1:500,000 scale.

While our results are preliminary, we see a few patterns beginning to emerge. Groundwater withdrawals in closed basins appear to vary by the proportions of the water balance—pumping rates and locations compared with recharge rates and locations. In open basins, such as the El Paso-Las Cruces HUC or the Albuquerque HUC, the storage changes appear to be linked to the degree of connection of groundwater with the surface water, pumping demands and the presence of mountain-block recharge. Open basins are less sensitive than closed basins to pumping demands as long as (a) there is an initial connection with surface water, and (b) pumping does not lower the water to the point that the groundwater does not respond strongly to annual river flows. The Albuquerque HUC is a case where both of these conditions are met. The Santa Fe HUC and El Paso-Las Cruces HUCs, respectively, are examples of behaviors when (a) and (b) are not met, and groundwater is currently lowering due to greater demand independent of the river. Our results provide a coarse resolution view of groundwater storage in New Mexican alluvial aquifers. They do provide direct examples of how geology, hydrology and society are reflected in our groundwater resources at the basin scale.