If the storage level is at 50% how come the lake isn't half empty?

Water storages are calculated on the amount of water available, which is useful for generating electricity.

The push to turn the turbines relies on the height of the water. The higher the water, the bigger the push.

If the water in the dam drops to a certain level, there's not enough push to make power. We call this point the normal minimal operating level (NMOL).

At this point, even though there might still appear to be plenty of water in the dam, our storage is at 0%

The upper level we call the full supply level (FSL), which would be 100%

Our storages are measured on the difference between these 2 levels Even though it looks like we have plenty of water, it may not be of useful for generating.

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Technical terms for storages and power stations

Just in case you are interested, here are some explanations for the diagram above

Technical terms	Definition & explanation
Full supply level (FSL)	The maximum level at which water can be stored indefinitely, equal to the crest level of the spillway unless otherwise indicated.
Minimum active storage level (MASL)	The highest level of the invert of the lowest outlet used for power generation purposes.
Full supply level volume (FSL volume) = Active Storage Capacity	The volume that can be stored between FSL and MASL.
Normal minimal operating level (NMOL)	Discharge directly to a Power Station The lowest level of storage at which all the machines in the power station can be simultaneously and continuously operated at full gate opening using normal instrumentation and control Discharge not directly to a Power Station The highest level of the invert of the lowest outlet used to release water for power generation purposes.
Normal minimal operating level volume (NMOL volume)	The volume that can be stored between NMOL and MASL.
Planned Minimum Operating Level (PMOL)	The expected level when the total system energy in storage is such that the load can just be met without restriction with a repetition of the nine months historical yields from Nov 1966 to Jul 1967 inclusive
Design flood level (DFL) previously 'Maximum flood level' (MFL)	The highest static water level for which the structures of the reservoir were originally designed.
Spillway Design Flood	The greatest outflow from the spillway during the design storm or design flow sequence.
Dry freeboard	The vertical distance between the DFL and the lowest level of the non-overflow portion of the dam, ie the crest level or wave wall.
Flood Surcharge	The vertical distance between FSL and DFL (Wet Freeboard).
Effective storage capacity	The volume of a reservoir between NMOL and FSL
Economic operating level (EOL) (seasonal)	The monthly or seasonal level above which the reservoir should be maintained to maximise energy potential.
Safety Check Flood Level	The highest static water level for which the dam or embankment is deemed safe.
Safety Check Flood Spillway Capacity	The greatest outflow from the spillway at a reservoir level corresponding to the Safety Check Flood Level.
Danger of Spill Level (DOS) (seasonal)	The seasonal storage level above which the risk of spill is sufficiently high to warrant some operational changes in order to reduce spill risk (eg full gate operation).
Average power conversion factor (APCF)	A multiplier which converts the average flow through the machines at a power station into an average power equivalent.
Storage energy conversion factor (SECF)	A multiplier which converts the measure of the volume of water in the reservoir into an energy equivalent.
Long Term Average Rating (LTA)	For a given thermal generation policy, it is the maximum supplied load for which the annual probability of energy storage falling below the emergency level in any year of an indefinite period, does not exceed 2%.
Firm Load	The maximum average load which can be sustained with 2% reliability over a 15 year horizon with aggregate energy in storage initially 30% full.
Quasi Firm Load	The maximum average load, in excess of firm load, which can be sustained with 2% reliability over a 5 year horizon.

 

Definition of Technical Terms and their Abbreviations

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