# Reservoir constraints in Prodrisk
In Prodrisk, a hydropower module can contain both minimum and maximum reservoir constraints. In the API, both of these constraints can be set in two different ways:
1. The attributes [minVol](https://docs.prodrisk.sintef.energy/objects/module/module.html#minvol) and [maxVol](https://docs.prodrisk.sintef.energy/objects/module/module.html#maxvol) are given with weekly resolution. When using these constraints, the model enforces the reservoir to satisfy the given constraints at the **end** of the week, i.e. the last time step. For the remaining time steps within the week, the following logic is applied:
- If a minvol/maxvol constraint in week $N$ is **stricter** (resp. increased/decreased) than the constraint in week $N-1$, the new constraint will only apply in the **last time step** in week $N$, whereas the earlier constraint applies for the time steps before.
- If a minvol/maxvol constraint in week $N$ is **looser** (resp. decreased/increased) than the constraint in week $N-1$, the new constraint will apply for **all time steps** in week $N$.
In conclusion, when introducing a *stricter* [minVol](https://docs.prodrisk.sintef.energy/objects/module/module.html#minvol) or [maxVol](https://docs.prodrisk.sintef.energy/objects/module/module.html#maxvol) constraint in the model, it should be set starting one week *before* the intended start time, such that it will be satisfied immediately in the intended period.
2. The attributes [min_vol_hourly](https://docs.prodrisk.sintef.energy/objects/module/module.html#min_vol_hourly) and [max_vol_hourly](https://docs.prodrisk.sintef.energy/objects/module/module.html#max_vol_hourly) are given with up to hourly resolution, but are converted to price period resolution within the Prodrisk core. The constraints will apply in the same time steps as given in the input. When these constraints are used to calculate internal values with weekly resolution in the Prodrisk core (i.e. head coefficients, buffer reservoir curves, extra cuts), the average min/max reservoir constraints within the week is used. Note that [min_vol_hourly](https://docs.prodrisk.sintef.energy/objects/module/module.html#min_vol_hourly) is incompatible with a soft reservoir constraint, and will be interpreted as an absolute constraint regardless of the choice of [reservoirMinRestrictionType](https://docs.prodrisk.sintef.energy/objects/module/module.html#reservoirMinRestrictionType).
When running Prodrisk from the CLI, reservoir constraints are only available on weekly resolution ([minVol](https://docs.prodrisk.sintef.energy/objects/module/module.html#minvol), [maxVol](https://docs.prodrisk.sintef.energy/objects/module/module.html#maxvol)). The attributes [min_vol_hourly](https://docs.prodrisk.sintef.energy/objects/module/module.html#min_vol_hourly) and [max_vol_hourly](https://docs.prodrisk.sintef.energy/objects/module/module.html#max_vol_hourly) are only available through the API.
## Soft reservoir constraints in Prodrisk
For the minimum reservoir constraint [minVol](https://docs.prodrisk.sintef.energy/objects/module/module.html#minvol), two choices are available: Absolute or hard reservoir constraints, and soft reservoir constraints. This is set through the attribute [reservoirMinRestrictionType](https://docs.prodrisk.sintef.energy/objects/module/module.html#reservoirMinRestrictionType). This is not the case for [min_vol_hourly](https://docs.prodrisk.sintef.energy/objects/module/module.html#min_vol_hourly) which is always interpreted as an absolute constraint. The intention of a soft constraint is to act as a stop discharge reservoir constraint. The minimum reservoir constraint can be motivated by e.g. public use of nature, or biodiversity.
The hard constraint is easy to understand. For each week when the constraint is active, it limits the reservoir to be within the limit given by the user. If the minimum reservoir constraint is violated, a penalty is added to the objective function.
The soft constraint is slightly more sophisticated. For each week when the constraint is active, a limitation acts on the reservoir, just like for the hard constraint. However, this limitation is not the same as the user gave as input to the system. Instead, an internal, hard constraint is calculated based on the average inflow. This internal, hard constraint works just like the regular hard constraint described above, with a penalty added to the objective function if it is crossed. The user provided, soft constraint on the other hand will not be visible at all in the LP-problem.
Percentiles for the reservoir volume for one reservoir with a soft reservoir constraint of 40% of the maximum reservoir volume from weeks 20 to 35. The brown line shows the soft constraint provided by the user. The pink line shows the internally calculated hard constraint.
If the option *write_penalty_logfiles* in **prodrisk.CPAR** is set to *1*, the constraint will be printed out, showing both the provided soft reservoir constraint and the translated hard reservoir constraint:
Line 1: Weeks. Line 2: User provided soft reservoir constraints. Line 3: Internally calculated hard constraints, used in the LP problem.
### Setting soft reservoir constraints in CLI
To set soft reservoir constriants in the command line interface, this can be done using the *med.exe* program for the relevant module. Selecting the choice **18 Minimalmagasin** and the option **LOK** enables you to add weekly values for the soft reservoir constraint, as shown below:
Minimum reservoir constraints are set using breaking points for weeks from 1 to 52. For longer planning horizons, this is repeated periodically. The picture shows the restriction that is printed in the first figure.