liminfoMODFLOW Reference
Free reference guide: MODFLOW Reference
About MODFLOW Reference
This MODFLOW Reference is a searchable guide to the USGS groundwater flow modeling software, covering MODFLOW 6 simulation structure, spatial and temporal discretization packages (DIS, DISV, DISU, TDIS), and hydraulic property packages (NPF, STO, IC) with typical parameter values.
The reference includes detailed boundary condition packages such as CHD (Constant Head), WEL (Well), RCH (Recharge), RIV (River), DRN (Drain), GHB (General Head), and EVT (Evapotranspiration), each with formula explanations and practical stress period data examples.
It also covers FloPy, the Python interface for MODFLOW, with code examples for building models, setting hydraulic properties, configuring boundary conditions, and visualizing results as map views and cross-sections using matplotlib.
Key Features
- MODFLOW 6 simulation structure overview with GWF model, TDIS, and package hierarchy
- Spatial discretization packages: DIS (structured), DISV (vertically layered unstructured), DISU (fully unstructured)
- Hydraulic property references for NPF (conductivity), STO (storage), and IC (initial conditions) with typical values
- Seven boundary condition packages (CHD, WEL, RCH, RIV, DRN, GHB, EVT) with formulas and stress period examples
- FloPy Python code examples for model creation, simulation execution, and result extraction
- FloPy visualization examples including PlotMapView contour maps and PlotCrossSection profiles
- MODPATH particle tracking and MT3D-USGS solute transport model summaries
- Hydraulic conductivity reference table with typical values for gravel, sand, silt, and clay
Frequently Asked Questions
What is MODFLOW and what is it used for?
MODFLOW is a free, open-source groundwater flow modeling software developed by the U.S. Geological Survey (USGS). It uses the 3D finite-difference method to simulate saturated groundwater flow. MODFLOW 6, the latest version released since 2017, supports structured and unstructured grids and is widely used in hydrogeology, environmental engineering, and water resource management.
What is the difference between DIS, DISV, and DISU discretization packages?
DIS defines a structured rectangular grid using NLAY, NROW, and NCOL. DISV allows vertically layered unstructured grids such as Voronoi or triangular meshes, defining cells per layer. DISU is fully unstructured, where cell-to-cell connections are defined explicitly, offering maximum geometric flexibility.
How do I set up a pumping well in MODFLOW?
Use the WEL (Well) package. Specify the well location as (layer, row, col) and the pumping rate Q in m^3/d. Negative values represent extraction (pumping), and positive values represent injection. Different pumping rates can be specified for each stress period.
What is the NPF package and what parameters does it require?
The NPF (Node Property Flow) package defines hydraulic conductivity and flow properties for each cell. Key parameters include ICELLTYPE (0 for confined, >0 for unconfined), K (horizontal hydraulic conductivity in m/d), K22 (Y-direction conductivity), and K33 (vertical conductivity). It also supports anisotropy and wetting/drying options.
How do I use FloPy to build and run a MODFLOW model?
Import flopy, create a MFSimulation object, add a ModflowGwf model, define the grid with ModflowGwfdis, set hydraulic properties with ModflowGwfnpf, add boundary conditions, then call sim.write_simulation() and sim.run_simulation(). Results can be read using gwf.output.head().get_data() for head values.
What boundary condition should I use for a river in the model?
Use the RIV (River) package. It requires three parameters per cell: STAGE (river water level in m), COND (riverbed conductance in m^2/d, calculated as K*W*L/M), and RBOT (riverbed bottom elevation). Flow direction depends on whether the groundwater head is above or below the riverbed.
What is the difference between steady-state and transient simulations?
Steady-state simulations compute equilibrium conditions where heads do not change over time, so the STO (Storage) package is not needed. Transient simulations model time-varying conditions and require the STO package with specific storage (Ss) for confined layers and specific yield (Sy) for unconfined layers.
What are typical hydraulic conductivity values for different soil types?
Typical horizontal hydraulic conductivity values (K in m/d): gravel 100-1000, sand 1-100, silt 0.001-1, clay less than 0.001. Specific storage (Ss) typically ranges from 1e-4 to 1e-6 (1/m), and specific yield (Sy) ranges from 0.01 to 0.30.