# HydroGeoSphere/Nodal Flux Head Constraints

The old instructions Specified volumetric flowrate and Specified volumetric flowrate, head constrained are no longer used, as they are covered by the boundary condition type Nodal flux described below. If you want to force the nodal flux to operate within a specified hydraulic head range, you can do so using the following instruction:

## Nodal flux head constraints

edit**bc_constraint_1, bc_constraint_2**Minimum and maximum constraining head values for positive nodal flux (i.e. inflow).**bc_constraint_3, bc_constraint_4**Minimum and maximum constraining head values for negative nodal flux (i.e. outflow).

If the nodal flux is positive, and the head at the node is less than **bc_constraint_1**, then the flux will be applied until such time as the head at the node exceeds **bc_constraint_2**. Once the head at the node exceeds **bc_constraint_2**, then the flowrate will be set to zero until such time as the head at the node drops below **bc_constraint_1**.

If the flowrate is negative, and the head at the node is greater than **bc_constraint_3**, then the flowrate will be applied until such time as the head at the node drops below **bc_constraint_4**. Once the head at the node drops below **bc_constraint_4**, then the flowrate will be set to zero until such time as the head at the node exceeds **bc_constraint_3**.

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