Glossary of Hydrologic Terms - O
Glossary of Hydrologic Terms - O
These definitions have been compiled from various sources, including the USGS's Water Resources Data, Weather Service Operations Manual, Chapter E-90, and Glossary of Meteorology (AMS, 1959).
- observation well
- A non-pumping well used for observing the elevation of the water table or piezometric surface.
- ogee
- A reverse curve, shaped like an elongated letter "S." The downstream faces of overflow dams are often made to this shape. From the French word ogive.
- one-hundred year flood
- Flood magnitude which has a one chance in one hundred of being exceeded in any future one-year period. The occurrence of floods is assumed to be random in time, or a regularity of occurrence is implied. The exceeding of a one percent chance flood is no guarantee, therefore, that a similar size flood will not occur next week. The risk of epxerienceing a large flood within time periods longer than one year increases in a nonadditive fashion. For example, the risk of exceeding a one percent chance flood (i.e., a one hundred year flood) one or more times during a thirty-year period is 25 percent and during a seventy-year period is 50 percent.
- orifice
- An opening with closed perimeter, usually sharp edged, and of regular form in a plate, wall, or partition through which water may flow, generally used for the purpose of measurement or control of water.
- The end of a small tube, such as a Pitot tube, piezometer, etc.
- orographic precipitation
- Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.
- Organic Act of 1890
- The act that assigned the responsibility of river and floor forecasting for the benefit of the general welfare of the nation's people and economy to the Weather Bureau, and subsequently the National Weather Service.
- orographic precipitation
- Precipitation which is caused by hills or mountain ranges deflecting the moisture-laden air masses upward, causing them to cool and precipitate their moisture.
- outburst flood
- A flood caused by the rapid dumping of a glacier-dammed lake when the ice dam fails.
- outflow channel
- A natural stream channel which transports reservoir releases.
- outlet
- An opening through which water can be freely discharged from a reservoir.
- outlet discharge structure
- Protects the downstream end of the outlet pipe from erosion and is often designed to slow down the speed of released water to prevent erosion of the stream channel.
- overland flow
- The flow of rainwater or snowmelt over the land surface toward stream channels. After it enters a watercourse it becomes runoff.
Return to the WGRFC Home Page
Glossary of Hydrologic Terms - O
http://www.srh.noaa.gov/wgrfc/glossary/o.html
This page maintained by SR-FWR.Webmaster
Updated June 1, 2003
www.srh.noaa.gov
