Percent Full
Reviewed • Standard Hydrology Definition • WaterLakeLevel
# Percent Full
Percent full measures the percentage of a reservoir's conservation storage volume currently filled with water:
$$\text{Percent Full} = \left( \frac{\text{Current Storage (ac-ft)}}{\text{Full Conservation Capacity (ac-ft)}} \right) \times 100$$
A common point of confusion is assuming a lake 10 feet below a 100-foot full pool is 90% full. Because reservoirs resemble steep, bowl-shaped valleys with tapering bottoms, the top 10 feet of a lake contains vastly more water volume than the bottom 10 feet. A reservoir that has dropped only 10% in vertical elevation might actually have lost 30% to 40% of its total water storage volume!
The ratio of active water volume currently stored in a reservoir compared to its authorized full conservation capacity.
What Does Percent Full Mean?
Percent full measures the percentage of a reservoir's conservation storage volume currently filled with water:
$$\text{Percent Full} = \left( \frac{\text{Current Storage (ac-ft)}}{\text{Full Conservation Capacity (ac-ft)}} \right) \times 100$$
Why Elevation Percent Is Not Storage Percent
A common point of confusion is assuming a lake 10 feet below a 100-foot full pool is 90% full. Because reservoirs resemble steep, bowl-shaped valleys with tapering bottoms, the top 10 feet of a lake contains vastly more water volume than the bottom 10 feet. A reservoir that has dropped only 10% in vertical elevation might actually have lost 30% to 40% of its total water storage volume!
Explore Other Hydrology & Boating Terms
Acre-Foot
A volumetric water unit representing the amount of water required to cover one acre of land to a depth of one foot.
Conservation Pool
The volume of water stored in a reservoir dedicated to beneficial uses such as drinking water, power, and recreation.
Datum
A reference surface or mathematical baseline from which vertical elevations or geographic coordinates are reckoned.
Dead Pool
The critical low-water elevation below which water cannot flow downstream through gravity outlets.
Drawdown
The deliberate or natural reduction of a lake or reservoir water surface elevation.
Evaporation Loss
The invisible volumetric loss of water from a lake surface into the atmosphere caused by heat, sun, and dry winds.