OASIS — A fertile or green spot in a desert or wasteland, made so by the presence of water.

OBSERVATION WELL — A well used to monitor changes in water levels of an aquifer and to obtain samples for water quality analyses.

OBSTRUCTION — Includes, but is not limited to, any dam, wall, wharf, embankment, levee, dike, pile, abutment, protection, excavation, channelization, bridge, conduit, culvert, building, wire, fence, rock, gravel refuse, fill, structure, vegetation or other material in, along, across or projecting into any watercourse which may alter, impede, retard or change the direction and/or velocity of the flow of water, or due to its location, its propensity to snare or collect debris carried by the flow of water, or its likelihood of being carried downstream.

OFF-LINE RESERVOIR — A reservoir constructed to the side of the main canal, usually in a natural drainage channel used to store surplus water runoff during the winter season for use during the irrigation season.

OFFSET — (Irrigation) The difference between the controlled variable and the referenced input, for example, in a canal system, the difference between the actual water level in the canal and the water level at design flow.

OFFSHORE — Situated off the shore but within waters under a country's control, as offshore fisheries.

OFFSTREAM USE — Water withdrawn from a surface water source for uses such as irrigation, municipal and industrial (M&I) water supply, steam electric power generation, etc.

OFFSTREAM USE — Water withdrawn or diverted from a ground or surface-water source for use at another place. Examples of offstream use include public-water supply, industry, irrigation, livestock, thermoelectric power generation, and other uses.

OGEE — A reverse curve shaped like an elongated letter S. The downstream faces of overflow dams are often made in this shape.

OLD FIELD — Cropland that is no longer used to produce an agricultural crop and that has been allowed to revert to natural plant cover.

OLD GROWTH — Forests that either have never been cut or have not been cut for many decades. Forests characterized by a large percentage of mature trees.

ONCE-THROUGH COOLING WATER — Water (fresh or saline) that is withdrawn from a river, stream or other water body (man-made or natural), or a well, that is passed through a steam condenser one time, and then returned to the stream or water body some distance from the intake. Once-through cooling water is used to exchange the heat from the steam condensers commonly used in power production plants to the cooler water. Typically, such waters are required to be cooled in cooling ponds before returning to a stream or other body of water.

ONE HUNDRED-YEAR FLOOD — Having the same meaning as Base Flood, 1 percent Flood, or Hundred-Year Flood.

ONFARM — Activities (especially growing crops and applying irrigation water) that occur within the legal boundaries of private property.

ONSHORE — Coming or moving from the water toward or onto the shore, as a breeze or prevailing wind.

OPEN — An unobstructed area of land or water.

OPEN CHANNEL SYSTEM — A system of conveyance channels where the top flow boundary is a free surface (e.g., canal systems).

OPEN RIVER CHANNEL — A navigation channel in a natural river with improvements limited to removal of obstructions and dredging to obtain adequate depths.

OPERATION AND MAINTENANCE COSTS (O & M) — Average annual costs of project operation and normal maintenance.

OPERATION, MAINTENANCE, AND REPLACEMENT COSTS (O M & R) — The value of goods and services needed to operate a constructed project and make repairs and replacements necessary to maintain the project in a sound operating condition during its economic life.

OPERATIONAL CONCEPT — Mode of operating a canal with respect to location of priorities; usually supply oriented (upstream concept) or demand oriented (downstream concept).

OPERATIONAL LOSSES — Losses of water resulting from evaporation and seepage.

OSMOSIS — The selective passage of liquids through a semipermeable membrane in a direction which tends to make concentrations of all substances on one side of the membrane equal to those on the other side. The semipermeable membrane allows the passage of water but prevents the passage of substances dissolved in the water. The water movement is from the more dilute solution toward the more concentrated solution, and will continue until the two solutions are equal in concentration. If pressure is applied to the more concentrated side, the flow of water will reverse, from the concentrated side to the more dilute side, a condition termed Reverse Osmosis.

OTHER WATER USE — Water used for such purposes as heating, cooling, irrigation (public-supplied only), lake augmentation, and other nonspecific uses. The water can be obtained from a Public Water Supply System, or may be self supplied.

OUTFALL — The place where a sewer, drain, or stream discharges; the outlet or structure through which reclaimed water or treated effluent is finally discharged to a receiving water body.

OUTFLOW, also Outflows — To issue or stream out, in or as if in a flow from a body of water.

OUTFLOW CHANNEL — A natural stream channel that transports reservoir releases.

OUTLET — Point where water exits from a stream, river, lake, reservoir, tidewater, or artificial drain. The mouth of a river where it flows into a larger body of water.

OUTLET CHANNEL — A waterway constructed or altered primarily to carry water from man-made structures, such as terraces, tile lines, and diversions.

OUTLET DISCHARGE STRUCTURE — A structure built to protect the downstream end of a dam's outlet pipe from erosion and is often designed to slow the velocity of released water to prevent erosion of the stream channel.

OVERDRAFT — That quantity of water pumped in excess of the safe yield; the act of overdrawing a water supply or aquifer in amounts greater than replenishment. Also, the sustained extraction of ground water from an aquifer at a rate greater than the recharge rate of the aquifer, resulting in a drop in the level of the water table. Also see Ground Water Overdraft and Ground Water Mining.

OVERFALL — An abrupt change in stream channel elevations. Also, the part of a dam or weir over which the water flows.

OVERFALL DAM — A dam constructed to allow water to overflow the dam's crest.

OVERFLOW — (1) To flow or run over the top, brim, or banks. (2) To be filled beyond capacity, as a container or a waterway.

OVERFLOW RATE — (1) The flow into a basin divided by its total surface area, often expressed in units of gallons per day per square foot. It is used as a design parameter for settling basins. (2) (Water Quality) One of the guidelines for the design of the settling tanks and clarifiers in a water treatment plant; used by plant operators to determine if tanks and clarifiers are over or under used.

OVERFLOW STANDPIPE — A standpipe located in a dam or other structure at an elevation that allows removal of excess water, preventing overflow.

OVERHEAD IRRIGATION — A pressurized irrigation system where water is distributed through pipes to the field and applied through a variety of sprinkler heads or nozzles. Pressure is used to spread water droplets above the crop canopy to simulate rainfall. These systems include portable and traveling guns, solid or permanent fixtures (overhead or pop ups), center pivots, and periodic moving systems. The efficiencies of these sprinkler systems range from 15 to 85 percent; however, the average of 70 percent is commonly used. Also referred to as Sprinkler Irrigation.

OVERLAND FLOW — (1) The flow of rainwater or snowmelt over the land surface toward stream channels. (2) (Water Quality) The discharge of wastewater in such a way that it flows over a defined land area prior to entering a receiving stream. The movement over vegetated land fosters the removal of plant nutrients from the wastewater and constitutes a form of Tertiary Wastewater Treatment. After it enters a stream, it becomes Runoff.

OVERTURN — (1) The sinking of surface water and rise of bottom water in a lake or sea that results from changes in temperature that commonly occur in spring and fall. (2) One complete cycle of top to bottom mixing of previously stratified water masses. This phenomenon may occur in the spring or fall, or after storms, and results in uniformity of chemical and physical properties of water at all depths.

OXBOW — An abandoned meander in a river or stream, caused by neck cutoff. Used to describe the U-shaped bend in the river or the land within such a bend of a river.

OXBOW LAKE — An abandoned meander isolated from the main stream channel by deposition, and filled with water.

OXYGEN SATURATION CAPACITY — The maximum quantity of dissolved oxygen that a liquid exposed to the atmosphere can contain at a given temperature and pressure.