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.

Rainfall on impervious surfaces such as paving and roof structures will result in storm water run-off.  Successive site improvements will trigger a requirement for an engineered system to thoughtfully manage storm water run-off.  In improved locations such as new suburban subdivisions, the amount of run-off is significantly increased compared to that of the previously undeveloped acreage.  Storm water management is achieved through storm drainage utilities, which are comprised of a physical network of site grading, curbs, culverts, catch basins, piping, manholes and, sometimes, retention or detention ponds.  Another aspect of storm water management is government regulation, which may impose water quality discharge standards and limits on how much site improvement can be authorized at sensitive project locations.