New Mexico Geological Society Annual Spring Meeting — Abstracts

Estancia Basin in central New Mexico is a closed basin comprising an area of about 5000 km² and has been the focus of many studies of changes in Late Pleistocene pluvial Lake Estancia. It is generally believed that groundwater discharge from the Manzano Mountains west of the basin was mostly responsible for maintaining the lake during lowstands, with relatively little contribution from the surface water. However, an unknown volume of surface water runoff contributed to the hydrologic balance during lake highstands. Surface water modeling of Estancia basin is in progress to estimate past surface water runoff and to understand the effects of climate change.

Estancia Basin does not have a single discharge point and a number of subwatersheds drain to the center of the basin. Today, no surface runoff from the Manzano Mountains reaches the center of the basin, which contains saline lakes (playas) where groundwater escapes by evaporation. For surface runoff modeling, the drainage area of the entire watershed was subdivided into 22 subwatersheds ranging in size from about 20 km² to 1300 km² using USGS 30 m DEM and topographic parameterization software TOPAZ (Garbrecht and Martz, 1995). A smiace runoff model was developed for each subwatershed using the modeling system SWAT (Arnold et ai., 1996). SWAT is a physically based distributed parameter continuous time hydrologic model which simulates the processes of surface runoff, river routing, evapotranspiration, infiltration and subsurface flow. Hydrologic properties of soil were obtained from USDA SCS soil surveys and were confirmed by field determination at selected points in the basin.

The model was calibrated on a relatively small watershed (drained by Tajique creek) comprising an area of about 50 km². Due to relatively high relief of the basin, the precipitation and temperature interpolation algorithms reflected elevation differences. Sensitivity analyses indicate that precipitation and temperature are the most important variables in model simulations, followed by vegetation type and soil roughness parameters. Least important are channel and subbasin geometric characteristics. Results indicate that some calibration is required to match model results to the observed values and several model runs using estimates of climate change are presented to illustrate the importance of atmospheric forcing on the hydrologic balance in Estancia Basin. Future work includes coupling of SWAT with a groundwater model (MODFLOW) and an energy balance (lake) model, and comparing surface runoff and hydrologic balance with scenarios of global climate change.