SUMMARY OF GSFLOW NOTE: Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. ABSTRACT GSFLOW is a coupled Ground-water and Surface-water FLOW model based on the integration of the U.S. Geological Survey Precipitation-Runoff Modeling System (PRMS, Leavesley and others, 1983) and the U.S. Geological Survey Modular Ground-Water Flow Model (MODFLOW-2005, Harbaugh, 2005). 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 among the PRMS Hydrologic Response Units (HRUs), between HRUs and the MODFLOW finite-difference cells, and between HRUs and streams and lakes. PRMS and MODFLOW have similar modular programming methods, which allow for their integration while retaining independence that permits substitution of and extension with additional PRMS modules and MODFLOW packages. PRMS is implemented in the U.S. Geological Survey Modular Modeling System (Leavesley and others, 1996), which provides input and output and integration functions used by PRMS and GSFLOW modules. GSFLOW was developed to simulate coupled ground-water/surface- water flow in one or more watersheds by simultaneously simulating flow across the land surface, within subsurface saturated and unsaturated materials, and within streams and lakes. Climate data consisting of measured or estimated precipitation, air temperature, and solar radiation, as well as ground-water stresses (such as withdrawals) and boundary conditions are the driving factors for a GSFLOW simulation. GSFLOW can be used to evaluate the effects of such factors as land-use change, climate variability, and ground-water 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 with ground water, in watersheds that range from a few square kilometers to several thousand square kilometers, and for 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 for three simulation modes--integrated, PRMS-only, and MODFLOW-only. 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. In addition to the MODFLOW variable-length stress period used to specify changes in stress or boundary conditions, GSFLOW uses internal daily stress periods for adding recharge to the water table and calculating flows to streams and lakes. Only the first stress period specified in the MODFLOW input files can be designated as steady state for integrated simulations. No computations pertaining to PRMS are executed for an initial steady-state stress period. HISTORY GSFLOW Version 1.0.00 03/05/2008 - Initial release. SYSTEM REQUIREMENTS GSFLOW is written in Fortran 90 and C programming languages. The code has been used on personal computers running various forms of the Microsoft Windows operating system and LINUX based computers. FUNTIONALITY This version of GSFLOW includes the following PRMS and GSFLOW modules, MODFLOW-2005 packages, and Modular Modeling System routines: PRMS modules: basin_prms -- Basin Module cascade_prms -- Cascading-Flow Module obs_prms -- Observed-Data Module soltab_hru_prms -- Potential Solar-Radiation Module temp_1sta_prms -- One-Station Temperature-Distribution Module temp_laps_prms -- Lapse-Station Temperature-Distribution Module temp_dist2_prms -- Inverse-Distance Temperature-Distribution Module xyz_dist -- Multiple Linear Regression Precipitation- and Temperature-Distribution Module precip_prms -- One-Station Precipitation-Distribution Module precip_laps_prms -- Lapse-Station Precipitation-Distribution Module precip_dist2_prms -- Inverse-Distance Precipitation-Distribution Module ddsolrad_hru_prms -- Degree-Day Solar-Radiation Distribution Module ccsolrad_hru_prms -- Cloud-Cover Solar-Radiation Distribution Module potet_jh_prms -- Jensen-Haise Potential-Evapotranspiration Module potet_hamon_hru_ -- Hamon Potential-Evapotranspiration Module prms potet_epan_prms -- Pan-Evaporation Potential-Evapotranspiration Module intcp_prms -- Precipitation-Interception Module snowcomp_prms -- Snow Module srunoff_smidx_ -- Nonlinear source Area Surface-Runoff and Infiltration casc Module srunoff_carea_ -- Linear Source Area Surface-Runoff and Infiltration casc Module soilzone_gsflow -- Soil-Zone Module gwflow_casc_prms -- Ground-Water Reservoir Module (for PRMS-only simulations) strmflow_prms -- Streamflow Module (for PRMS-only simulations) hru_sum_prms -- HRUs Flow-Summary Module basin_sum_prms -- Watershed Flow-Summary Module MODFLOW-2005 packages: BAS -- Basic Package BCF -- Block-Centered Flow Package LPF -- Layer-Property Flow Package HUF -- Hydrogeologic-Unit Flow Package HFB -- Horizontal Flow Barrier Package CHD -- Time-Variant Specified-Head Option WEL -- Well Package MNW -- Multi-Node Well Package GHB -- General Head Boundary Package FHB -- Flow and Head Boundary Package SFR -- Streamflow-Routing Package LAK -- Lake Package UZF -- Unsaturated-Zone Package GAG -- Gage Package SIP -- Strongly Implicit Procedure Package PCG -- Preconditioned Conjugate Gradient Package DE4 -- Direct solver GMG -- Geometric MultiGrid Solver Package GSFLOW modules: gsflow_prms -- Computational-Sequence Control for PRMS and GSFLOW Module gsflow_modflow -- Computational-Sequence Control for MODFLOW Module gsflow_setconv -- Unit-Conversions Module gsflow_prms2mf -- PRMS to MODFLOW Integration Module gsflow_mf2prms -- MODFLOW to PRMS Integration Module gsflow_budget -- Watershed-Budget Summary Module gsflow_sum -- Flow-Components Summary Module Modular Modeling system (MMS) routines: gsflow_main -- Main Program to Manage PRMS Modules and MODFLOW-2005 Packages mms_util -- MMS Utility Routines DOCUMENTATION Markstrom, S.L., Niswonger, R.G., Regan, R.S., Prudic, D.E., and Barlow, P.M., 2008, GSFLOW--Coupled Ground-water and Surface-water FLOW model based on the integration of the Precipitation-Runoff Modeling System (PRMS) and the Modular Ground-Water Flow Model (MODFLOW-2005): U.S. Geological Survey Techniques and Methods 6-D1, 240 p. REFERENCES Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological Survey modular ground-water model--the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6-A16, variously paginated. Leavesley, G.H., Lichty, R.W, Troutman, B.M., and Saindon, L.G., 1983, Precipitation-runoff modeling system--User's manual: U.S. Geological Survey Water-Resources Investigations Report 83-4238, 207 p. Leavesley, G.H., Restrepo, P.J., Markstrom, S.L., Dixon, M., and Stannard, L.G., 1996, The Modular Modeling System (MMS): User's manual: U.S. Geological Survey Open-File Report 96-151, 142 p. CONTACTS U.S. Geological Survey Office of Ground Water 411 National Center Reston, VA 20192 (703) 648-5001 ogw_webmaster@usgs.gov