Graph of simulated flows in San Benito River
Simulated San Benito River flow profile

Key Issues

  • Creation of an adaptable, all-purpose model
  • Deviation from calibrated conditions when new capabilities added
  • Accuracy of local applications of a regional model
  • Model documentation not up to date


  • The modular structure of the MODFLOW code easily allows new model capabilities to be added
  • Calibration was checked and refined locally as needed for specific applications
  • Each new model enhancement documented in a technical appendix of the project report
Map showing simulated changes in groundwater levels across the model area
Simulated change in groundwater level
Map showing simulated changes in total dissolved solid concentrations across the model area
Simulated change in groundwater TDS

Groundwater Model as a Living Tool with Long-Term Value for Groundwater Management

San Benito County, California

A MODFLOW model of the northern part of the Gilroy-Hollister groundwater basin was originally developed in 2001. More than 12 years later it is still evolving and serving to evaluate groundwater management issues.
The San Benito model grid superimposed on a map of the area
Model grid and recharge zones

The groundwater basin underlying northern San Benito County has had a dynamic history. Land use is primarily agricultural, but the City of Hollister and rural residential development have been expanding. Historical groundwater overdraft led to the importation of Central Valley Project water beginning in the early 1990s. Aided by several wet years and active recharge activities, groundwater levels recovered. In some areas the recovery was too successful: artesian wells began flowing for the first time in 80 years, and the water table began encroaching into the root zone.

San Benito County Water District is the agency responsible for managing groundwater and imported water. Recognizing the need for a quantitative tool, it hired Gus Yates — now a senior hydrologist at Todd Groundwater — to develop a regional groundwater flow model in 2002. The initial investment in model development has paid for itself many times over as the model has been applied to a host of specific projects and investigations:

  • Time of travel analysis for percolated municipal wastewater
  • Comparison of potential recycled water use sites (impacts on water levels and water quality)
  • Impacts of North County pumping on creek flow and groundwater levels
  • New wells and wastewater re-operation in the Sunnyslope area
  • Water balance calculations for annual groundwater reports
  • Salt balance calculations for a salt and nutrient management plan
  • Optimization of North County groundwater development

The model has been incrementally improved and expanded over the years to meet the needs of specific projects. These enhancements have included expansion of the active flow domain to include hills underlain by Purisima Formation, addition of the MT3DMS solute transport module, use of MODPATH and ZoneBudget post-processors to quantify flow vectors and local water balances, extraction of additional output for plotting stream flow profiles, and local re-layering and recalibration to simulate flowing wells. An update of the simulation period was completed in 2014.

Graph of observed and model-simulated groundwater levels
Measured and simulated groundwater levels