GSFLOW: Coupled Groundwater and Surface-Water Flow Model, version 2.2.1

GSFLOW is a coupled Groundwater and Surface-Water Flow model based on the integration of the U.S. Geological Survey Precipitation-Runoff Modeling System (PRMS; Markstrom and others, 2015) and the U.S. Geological Survey Modular Groundwater Flow Model (MODFLOW-2005, Harbaugh, 2005; MODFLOW-NWT, Niswonger and others, 2011). In addition to the basic PRMS and MODFLOW simulation methods, several additional simulation methods were developed and existing PRMS modules and MODFLOW packages were modified to facilitate integration of the models. Methods were developed to route flow between the PRMS Hydrologic Response Units (HRUs) and the MODFLOW finite-difference cells, and between HRUs and streams and lakes. The modular programming structure of PRMS and MODFLOW support integration while retaining independence.

GSFLOW simulates coupled groundwater/surface-water flow in one or more watersheds. Simulations integrate flow across the land surface, within subsurface saturated and unsaturated materials, and within streams and lakes. Required inputs include: 1) climate data (such as measured or estimated precipitation, maximum and minimum air temperature, and optionally solar radiation, potential evapotranspiration, humidity, and wind speed), 2) groundwater stresses (such as withdrawals) and 3) boundary conditions to account for inflows to and outflows from the modeled region (such as streamflow and heads). GSFLOW simulations can be used to evaluate the effects of land-use change, climate variability, and groundwater withdrawals on surface and subsurface flow. The model incorporates well documented methods for simulating runoff and infiltration from precipitation; balancing energy and mass budgets of the plant canopy, snowpack, and soil zone; and simulating the interaction of surface-water and groundwater in watersheds. The spatial and temporal domain of GSFLOW simulations can range from a few square kilometers to several thousand square kilometers, over time periods that range from months to several decades. An important aspect of GSFLOW is its ability to conserve water mass and to provide comprehensive water budgets.

GSFLOW allows three simulation modes—coupled (GSFLOW and GSFLOW5), PRMS-only (PRMS and PRMS5), and MODFLOW-only (MODFLOW). The capability of having PRMS-only and MODFLOW-only simulations in GSFLOW allows incremental model setup that provides flexibility in model calibration.

GSFLOW operates on a daily time step. The MODFLOW variable-length stress period is retained so that changes in stress or boundary conditions can be specified on any time step. Only the first stress period can be designated as steady for integrated simulations. No computations pertaining to PRMS are executed for an initial steady-state stress period. While PRMS computations always operate on a daily time step, MODFLOW-only simulations support time steps other than daily, and the user can specify the time step length within the MODFLOW discretization input file. There have been several applications of GSFLOW to real-world systems since the initial release of the software in 2008. Many of these applications are referenced on the USGS GSFLOW webpage (https://www.usgs.gov/software/coupled-ground-water-and-surface-water-fl…).