T157 Advances in Understanding at the Groundwater–Surface Water Interface and Challenges for the Future: A Reflection on Tom Winter’s Legacy

Tuesday, 11 October 2011
8:00 AM - 12:00 PM
Room M101AB

SIMULATION OF DRAINAGE-BASIN-SCALE SURFACE-WATER/GROUNDWATER INTERACTION USING THE BASIN CHARACTERIZATION MODEL AND MODFLOW, SAN DIEGO COUNTY, CALIFORNIA

Paper number: T157-14
Program: Technical Sessions
Time: 11:15 AM – 11:30 AM

Authors

Abstract

The San Diego River drainage basin is located in southern California, adjacent to the Pacific Ocean, and covers an area of 1,050 square kilometers. The basin is predominately crystalline rock with a small coastal plain of sediment, includes two large reservoirs and imported water, and is rapidly urbanizing. Local water authorities want a better understanding of the surface-water/groundwater interaction, but even basic hydrogeologic concepts at a basin scale are non-existent. As an aid in creating this understanding, a reconnaissance-level water budget is being developed. Primary budget components are recharge from precipitation, and surface-water/groundwater interaction with the stream network. Surface-water data are limited to four gaged and two estimated streamflow records. Net stream gain/loss is estimated between successive gages. Groundwater data are sparse; there is a deep multiple-completion monitoring well and knowledge of spring locations, but little else. A three-dimensional geologic framework model has been developed for the coastal plain, but groundwater basin boundaries are not clearly understood.

To develop a reconnaissance-level water budget, the Basin Characterization Model (BCM) and a MODFLOW transient simulation are calibrated concurrently. BCM estimates the amount and distribution of runoff and recharge based on surface processes, physical properties, and precipitation. MODFLOW includes initial groundwater boundary conditions, simulates recharge from precipitation, routes runoff through the stream network, and accounts for gains from and losses to the stream network. BCM and MODFLOW parameters are adjusted to minimize differences between measured and simulated average monthly streamflow for 1982–2010. When streamflow residuals are minimized, the remaining differences are reconciled by considering subsurface flow across surface-water divides and across the freshwater/saltwater interface with the Pacific Ocean.