# The boundary conditions

The offtakes at nodes must have a boundary condition. The available boundary conditions are:

• Flow rate (Pump): Q(t)
• Fixed elevation: Z(t)
• Rating curve function (tabulated): Q(Z) (or rather Z(Q))
• Function Q power alpha: $Q(t)=Q_{ref} [(Z(t)-Z_{0})/(Z_{ref}-Z_{0})]^{\alpha}$

The upstream and downstream nodes are also considered as locations where "offtakes" must be defined in order to provide the boundary conditions for the hydraulic calculation algorithms. The terminology may seem abusive but it is necessary to choose a generic term. This is defensible in the sense that it is the points that allow injecting or withdrawing from the flow in the hydraulic network, at the upstream and downstream nodes as well as at the intermediate nodes.

The conditions at the upstream and downstream nodes can be chosen from this list. The details given for downstream nodes are also valid for upstream and intermediate nodes.

All combinations are possible in transient mode.

Some combinations are impossible in steady state: Q(t) upstream and downstream. Indeed in this case either there is no solution, or there is an infinity of solutions. The most traditional configuration is Q(t) upstream and Q(Z) downstream. Other options that do not explicitly specify upstream flow will involve iterations of the Fluvia calculation program to find the flow that satisfies the upstream conditions chosen.