Hydropower generation is computed from the flow passing through the turbine, based on the reservoir release or run-of-river streamflow, and constrained by the turbine's maximum flow capacity. Note that the amount of water that flows through the turbine is calculated differently for local reservoirs, river reservoirs and run-of-river hydropower. For river reservoirs, all water released downstream is sent through the turbines, but water pumped from the reservoir to satisfy direct reservoir withdrawals is not sent through the turbines.

*Release**H** = DownstreamOutflow**H*

For local reservoirs, all linked demand sites are assumed to be downstream of the reservoir, so all reservoir releases are sent through the turbines.

ReleaseH = TransLinkInflowH,DS + ExtraOutflowForHydropowerRequirement

For run-of-river hydropower nodes, the "release" is equal to the downstream outflow from the node.

ReleaseH* = DownstreamOutflow**H*

The volume of water that passes through the turbines is bounded by the maximum turbine flow (entered as data--see Supply and Resources\Reservoir\Hydropower). Note that if there is too much water, extra water is assumed to be released through spillways that do not generate electricity.

*VolumeThroughTurbine**H** = Min( Release**H **,
MaxTurbineFlow**H** )*

The gigajoules (GJ) of energy produced in a month,

*EnergyFullMonthGJ**H**
= VolumeThroughTurbine**H** x HydroGenerationFactor**H*

is a function of the mass of water (1000 kg / m^3) through the turbines multiplied by the drop in elevation, the plant factor (fraction of time on-line), the generating efficiency, and a conversion factor (9.806 kN/m3 is the specific weight of water, and from joules to gigajoules). The plant factor and efficiency are entered as data (see Supply and Resources\Reservoir\Hydropower).

HydroGenerationFactorH = 1000 (kg / m^3) * DropElevationH x PlantFactorH x PlantEfficiencyH * 9.806 / (1,000,000,000 J / GJ)

For reservoirs, the height that the water falls in the turbines is equal to the elevation at the beginning of the month minus the tailwater elevation (entered as data--see Supply and Resources\Reservoir\Hydropower).

DropElevationH = BeginMonthElevationH - TailwaterElevationH

For run-of-river hydropower nodes, the drop in elevation is entered as data (see Supply and Resources\River\Run of River Hydropower).

DropElevationH = FixedHeadH

If a demand priority for hydropower energy has been set for an individual reservoir, WEAP will calculate the supply requirement (volume of water through the turbines) necessary to generate the energy demand.

SupplyRequirementH = EnergyDemandFullMonthGJH / HydroGenerationFactorH