Inflow and Outflow
Reviewed • Standard Hydrology Definition • WaterLakeLevel
# Inflow and Outflow
Reservoir water elevation is governed by the universal conservation of mass equation:
$$\Delta \text{Storage} = \text{Inflow} - \text{Outflow} - \text{Evaporation} - \text{Seepage}$$
- Inflow: Water entering the lake from tributary rivers, direct rainfall, and watershed stormwater runoff (measured in cubic feet per second, CFS).
- Outflow: Water discharged through hydroelectric turbines, dam flood gates, municipal intake pumps, and environmental river releases.
When inflow exceeds outflow, the reservoir rises; when outflow exceeds inflow, the reservoir drops.
The volume rate of water entering a reservoir compared to the volume rate released downstream.
Inflow vs. Outflow Dynamics
Reservoir water elevation is governed by the universal conservation of mass equation:
$$\Delta \text{Storage} = \text{Inflow} - \text{Outflow} - \text{Evaporation} - \text{Seepage}$$
Definitions:
- Inflow: Water entering the lake from tributary rivers, direct rainfall, and watershed stormwater runoff (measured in cubic feet per second, CFS).
- Outflow: Water discharged through hydroelectric turbines, dam flood gates, municipal intake pumps, and environmental river releases.
When inflow exceeds outflow, the reservoir rises; when outflow exceeds inflow, the reservoir drops.
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.