Reservoirs are used for storing a supply of usable fresh water. They are most commonly created by damming a natural surface water tributary in an existing basin, but they can be artificially constructed as well. Water stored in reservoirs can have various functions, including serving as a source of potable water, irrigation, recreation and hydroelectric power. Reservoirs are designed and evaluated based on their storage capacity, which is commonly measured in acre-feet. Changes in reservoir storage capacity over a given time period are defined by an assessment of inputs and outputs. Inputs include tributary inflow, precipitation, and water from groundwater flow nets. Outputs are made up of draw-downs for irrigation and potable water, regulated flow to release excess storage or provide turbine power, and losses due to evaporation and seepage. Reservoir (and pond) covers and liners are sometimes used on a smaller scale project to prevent losses from surface evaporation and seepage into the containment basin. Subsurface water stored in soils as groundwater or in aquifers is a variation of the reservoir concept.
Ponds can be naturally occurring or man-made. Generally, they differ from reservoirs in their size, although no exact definition exists. Some hydrologists define ponds as lacking wave action, which allows for an undisturbed water column that permits settling to occur. They also may lack a defined point-source of tributary surface water flow, which most reservoirs have. Ponds are also used for storing water supplies, such as for remote community fire-fighting, but have other engineering uses when slow flow rates and the settling of particles in the water is required. Retention ponds are used in storm water run-off management to settle fine soil particles, and are continually filled with water. Retention ponds are constructed along a storm sewer utility system to contain most of the fine soil particles carried by storm water flows. They reduce the turbidity of downstream flow and lessen environmental impacts at the final discharge point. Detention ponds manage excess flows. They contain storm water for briefer periods during peak run-off flows and then drain out, returning to a dry condition. Many smaller waste-water treatment plants rely on stabilization ponds as a critical part of their process. Stabilization ponds are only several feet deep and rely on naturally-occurring biological processes (photosynthesis and decomposition) to break down organic wastes.