20260424 development build

MODFLOW 6 development build, Apr 24, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260422 development build

MODFLOW 6 development build, Apr 22, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260421 development build

MODFLOW 6 development build, Apr 21, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260420 development build

MODFLOW 6 development build, Apr 20, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260419 development build

MODFLOW 6 development build, Apr 19, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260418 development build

MODFLOW 6 development build, Apr 18, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260417 development build

MODFLOW 6 development build, Apr 17, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260416 development build

MODFLOW 6 development build, Apr 16, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).
• When a horizontal flow barrier is placed between two cells, MODFLOW checks to ensure that the cells are adjacent. This check did not function correctly for DISU grids, in some cases, if the IDOMAIN functionality was used to remove cells. The check for two adjacent DISU cells was corrected to work for IDOMAIN functionality.

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Parallel

• The adaptive time stepping algorithm (ATS) was not working correctly in parallel mode. This applies to GWT and GWE models, when the model time step derived from the ATS_PERCEL parameter is not uniform across processes. In such a case, a deadlock occurred and the simulation was stalled. This fix ensures that the actual calculation time steps are synchronized across all processes such that they remain in lockstep.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260415 development build

MODFLOW 6 development build, Apr 15, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.

20260414 development build

MODFLOW 6 development build, Apr 14, 2026.

Bug Fixes And Other Changes To Existing Functionality

Exchanges

• When a MVR connection was included as part of an exchange between two models, it was possible to list a model name in the PACKAGES block of the MVR input file that is not one of the two models included in the exchange. A check was added that reports an error message stating that a model name listed in the exchange MVR input file is not one of the two models connected by the exchange.

Internal Flow Packages

• Resolved mass-balance errors in the Central and UTVD numerical schemes caused by inconsistent interpretation of cell-to-interface distances (cl1/cl2).

Model

• The Flow Model Interface (FMI) package for transport and tracking models conducts TDIS compatibility checks during each stress period to ensure that the model's time discretization is compatible with that of the flow model. In particular, if a flow model stress period has just one time step, the transport or tracking model may have any number of time steps. However, if a flow model stress period has more than one time step, the transport or tracking model must have the same number of time steps. Simulations would previously stop with an error if the transport or tracking model violated this rule for any stress periods up to the simulation's last, but would not raise an error for the last stress period. The compatibility check was updated to include the last stress period. Simulations will now stop with an error if the transport or tracking model violates this rule for any stress period in the simulation.

Stress Packages

• The default values for DENSITY_WATER, HEAT_CAPACITY_WATER, LATENT_HEAT_VAPORIZATION specified in the OPTIONS block were not set as indicated in the MF6IO guide. Defaults for these three variables now set as documented in the MF6IO guide.