# PQ curves

## Stations
All producing stations contain a piecewise linear PQ curve that describes the relationship between the discharge and the generation of the station. The PQ curve is normally given with a reference head.

The PQ-curve should be concave during a standard ProdRisk iteration (forward simulation and backward recursion). That is, the first PQ-segment has the highest efficiency, and the subsequent segments have equal or lower efficiencies than the previous one. If a non-concave PQ-curve is defined by the user, ProdRisk establishes a concave approximation for  his purpose by removing non-concave points on the curve. During the final simulation, the default choice is to use the original PQ-curve when finding the total production. In case the original PQ-curve does not have successive lower efficiency with higher discharge, the total production will typically be lower than if we used the concave approximation. In the file Prodrisk.CPAR, there is a parameter called [PQValg](../../settings/cpar_control_parameters/cpar_control_parameters.md). If **PQValg** is set to *0*, ProdRisk will use the concave approximation in the final simulation. If **PQValg** is set to *2*, the final simulation uses a [non-convex PQ curve](../../extra_functionality/nonconvex_pq_curves/nonconvex_pq_curves.md).


The PQ curve is scaled according to the head coefficiernt, as described in {cite:ps}`Gjelsvik_considering_2005`:

![](pq_curve_head.png)
<figure>
    <figcaption>Figure from EVU course 2018, based on theory from "Anders Gjelsvik & Arne Haugstad, Considering Head Variations in a Linear Model for Optimal Hydro Scheduling, 2005".</figcaption>
</figure>

## Pumps
There is no PQ curve for pumps: as the power usage is modelled as a constant, but the flow is dependent on the water head and subject to the same head update as all other Prodrisk head dependencies.