MODFLOW 6 development build, Jul 30, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Solution

• Added an optional new COORDINATE_CHECK_METHOD option to the PRT model's Particle Release Package, accepting values 'EAGER' (default) or 'NONE'. 'NONE' disables release-time verification that release point coordinates and release point cell IDs match, which can significantly reduce runtime when releasing many particles.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Parallel

• Fixed a memory access exception that can occur when writing convergence information to a CSV file (through the CSV_OUTER_OUTPUT option in IMS) for a parallel solution.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 21, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 20, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 19, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 17, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 14, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 13, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 12, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 11, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.

MODFLOW 6 development build, Jul 10, 2025.

New Functionality

Basic Functionality

• Add a discretization text input option CRS for a real-world model coordinate reference system, for example, an EPSG integer code, authority string, or Open Geospatial Consortium Well-Known Text (WKT) specification. The entry for CRS does not affect the model simulation, but it is written to a version 2 binary grid file so that postprocessors can locate the grid in space.

Stress Packages

• Add HIGHEST_SATURATED option to the GWT SRC package. When activated, the HIGHEST_SATURATED option will apply a mass source loading rate to the specified CELLID or the highest underlying cell with a non-zero cell saturation. The HIGHEST_SATURATED option has an additional complication for certain types of grids specified using the DISU Package. When the DISU Package is used, a cell may have more than one cell underlying it. If the overlying cell were to become inactive, there is no straightforward method for determining how to apportion the mass source loading rate to the underlying cells. In this case, the approach described by \cite{modflowusg} is used. The mass source loading rate is assigned to the first active cell encountered (determined by searching through the underlying cell numbers from the lowest number to the highest number). In this manner, the total mass source loading rate is conserved; however, the spatial distribution of the applied mass source loading rate may not be maintained as layers become dry or wet during a simulation.

Bug Fixes And Other Changes To Existing Functionality

Internal Flow Packages

• Fixed first-order decay in GWT so that production/decay does not occur when aqueous or sorbed concentration are negative.
• Fixed bugs in the GWF VSC and BUY packages, affecting GWT simulations with multiple species that have different inactive regions. In this uncommon situation, the concentrations used to calculate fluid density could erroneously be set to zero in areas with different inactive regions.

Solution

• With the PRT model's Particle Release Package's EXTEND_TRACKING option enabled, particles beginning the simulation's final timestep in a stationary cell (an active cell with no flow across any boundary) were not terminated gracefully. Particles in these conditions will now terminate properly with status code 10 (timeout).

Stress Packages

• Fixed viscosity ratio assignment in the GWT MAW package when the GWF VSC package is active.