90TH PERCENTILE — (Water Quality) Term used in conjunction with water sampling standards as required under the Safe Drinking Water Act (SDWA) and amendments thereto. The 90th percentile value is calculated by first placing all sample results in order from the lowest concentration to the highest concentration (i.e., concentration of specific contaminants). Next, assign each sample result a number, starting with the number 1 for the lowest (concentration) result up to the highest concentration being given the number equal to the total number of samples collected from a particular water supply system. Then multiply the total number of samples collected by 0.9. The sample result with the number corresponding to this calculated value is the 90th percentile.

NADIR — Refers to a low or the lowest point, as the lowest point of a lake or other body of water attained of a certain period of time (period of record).

NARROW — (1) A body of water with little width that connects two larger bodies of water. (2) A part of a river or an ocean current that is not wide. Often used in the plural, i.e., narrows.

NATANT — Floating or swimming win water.

NATURAL CONTROL — A stream-gaging control which is natural to the stream channel, in contrast to an artificial control constructed by man.

NATURAL EROSION — Wearing away of the earth's surface by water, ice, or other natural agents under natural environmental conditions of climates and vegetation.

NATURAL FLOW — The rate of water movement past a specified point on a natural stream from a drainage area for which there have been no effects caused by stream diversion, storage, import, export, return flow, or change in Consumptive Use caused by man-controlled modification to land use. Natural flow rarely occurs in a developed county.

NATURAL PRECIPITATION — Represents the average annual precipitation (rainfall, snow, and sleet) measured at a number of different weather stations.

NATURAL RECHARGE — The replenishment of groundwater storage from naturally-occurring surface water supplies such as precipitation and stream flows.

NATURAL RESOURCE — A material source of wealth, such as timber, fresh water, or a mineral deposit, that occurs in a natural state and has economic value. Natural resources are considered Nonrenewable when they do not naturally replenish themselves within the limits of human time or Renewable when they are more or less continuously replenished in the course of natural events within the limits of human time.

NAVIGATE — (Nautical) To voyage over water in a boat or ship; to travel by water; sail.

NAVIGATIONAL WATER USE — Water utilized as a means of commercial (and sometimes recreational) transportation. Includes water used to lift a vessel in a lock, or maintain a navigable channel level. Navigational water use is considered a non-consumptive instream use of water and is generally not measured or accounted for.

NET CONSUMPTIVE USE — The Consumptive Use decreased by the estimated contribution by rainfall toward the production of irrigated crops. Net consumptive use is sometimes called the Crop Irrigation Requirement.

NET DUTY OF WATER — The amount of water delivered to the land to produce a crop, measured at the point of delivery to the field.

NET ECONOMIC BENEFITS — Economic benefits minus economic costs.

NET PRECIPITATION — The potential for Leachate generation from a waste disposal site. It is computed for a specific location by subtracting the annual evaporation from lakes in the region from the normal annual rainfall.

NET (STORM) RAIN — The portion of rainfall during a storm which reaches a stream as direct surface flow.

NET RESERVOIR EVAPORATION — The difference between the total evaporation from the reservoir water surface and the Evapotranspiration from the reservoir area under pre-reservoir conditions, with identical precipitation considered for both conditions.

NET WATER DEMAND — The amount of water needed in an irrigation or water service area to meet all requirements. It is the sum of Evapotranspiration of Applied Water (ETAW) in an area, the Irrecoverable Losses from the distribution system, and the outflow leaving the irrigation area. It excludes, however, the water reused in the area. Sometimes used interchangeably with Net Water Use.

NET WATER USE — Refers to water withdrawals plus or minus water transfers. In most areas, the net water use and water withdrawals are equal. However, in areas involved in water transfers (imports and exports), the net water use represents the actual amount of water used regardless of the amount of water actually withdrawn. Sometimes used interchangeably with Net Water Demand.

NET WATER YIELD — The available water runoff at a given location, both surface and subsurface, after the upstream uses by man's activities, use by Phreatophytes, and evaporation from upstream free water surfaces.

NEUTRAL SOIL — A soil in which the surface layer, at least to normal plow depth, is neither acid nor alkaline in reaction, approximately 7.0 pH.

NON-CONSUMPTIVE WATER USE — Nonconsumptive water use includes water withdrawn for use that is not consumed, for example, water withdrawn for purposes such as hydropower generation. This also includes uses such as boating or fishing where the water is still available for other uses at the same site. The terms Consumptive Use and Nonconsumptive Use are traditionally associated with water rights and water use studies, but they are not completely definitive. No typical consumptive use is 100 percent efficient; there is always some return flow associated with such use either in the form of a return to surface flows or as a ground water recharge. Nor are typically nonconsumptive uses of water entirely nonconsumptive. There are evaporation losses, for instance, associated with maintaining a reservoir at a specified elevation to support fish, recreation, or hydro-power, and there are conveyance losses associated with maintaining a minimum streamflow in a river, canal, or ditch.

NON-DEGRADATION — An environmental policy which disallows any lowering of naturally occurring quality regardless of preestablished health standards.

NONDISCHARGING TREATMENT PLANT — A treatment plant that does not discharge treated wastewater into any stream or river. Most are pond systems that dispose of the total flow they receive by means of evaporation or percolation to groundwater, or facilities that dispose of their effluent by recycling or reuse, for example spray irrigation or groundwater discharge.

NON-IRRIGATED CROPLAND — Those non-irrigated cultivated lands that are used for the production of grain crops (harvested and/or grazed), orchard, and field crops.

NONREIMBURSABLE COSTS — Water project costs allocated to general statewide or national beneficial purposes and funded from general fund revenues.

NONRENEWABLE RESOURCES — Natural resources that do not naturally replenish themselves within the limits of human time. Also see Natural Resources and Renewable Resources.

NONSTRUCTURAL MEASURES — Measures for managing, utilizing, or controlling water and related lands without structural development to achieve the desired objective. Such measures include best management practices, flood plain zoning, flood warning systems, education and legal restraints, and preservation, as well as the more common land management measures.

NONSTRUCTURAL FLOOD CONTROL MEASURES — Measures such as zoning ordinances and codes, flood forecasting, flood proofing, evacuation and channel clearing, flood fight activities, and upstream land treatment or management to control flood damages without physically restraining flood waters.

NON-TRANSIENT NON-COMMUNITY WATER SYSTEM — A public water system that regularly serves at least 25 of the same non-resident persons per day for more than six months per year.

NONUNIFORM FLOW — (Hydraulics) Flow in which the mean velocity or cross-sectional area vary at successive channel cross-sections. If the velocity at a given cross-section is constant with time, it is referred to as Steady Nonuniform Flow. If the velocity changes with time at each cross-section, it is known as Unsteady Nonuniform Flow.

NONWITHDRAWAL USE — Use which does not require diversion. Navigation, recreation, waste disposal and conservation of fish and wildlife are examples of nonwithdrawal uses. Such uses are typically nonconsumptive uses of water. Also referred to as Instream Use and In-Channel Use.

NORIA — A water wheel with buckets attached to its rim, used to raise water from a stream, especially for transfer to an irrigation channel.

NORMAL (HYDROLOGIC) — A central value (such as an arithmetic average or median) of annual quantities for a 30-year period ending with the first year of a decade, e.g., 1931-1960, 1961-1990.

NORMAL ANNUAL PRECIPITATION — Average annual precipitation during a base period.

NORMAL DAILY TEMPERATURE — The average daily mean temperature for a given date, computed for a specific 30-year period.

NORMAL WATER LEVEL — For a reservoir with a fixed overflow, the lowest crest level of that overflow. For a reservoir whose outflow is controlled wholly or partly by movable gates, siphons or other means, it is the maximum level to which water may rise under normal operating conditions, exclusive of any provision for flood surcharge.

NORMAL YEAR — A year during which the precipitation or streamflow approximates the average for a long period of record.

NORMALIZED DEMAND — The process of adjusting actual water use in a given year to account for unusual events such as dry weather conditions, government interventions for agriculture, rationing programs, or other anomalies.

NOTCH — The opening in a dam or spillway for the passage of water.

NO TILL FARMING — Planting crops without prior seedbed preparation, into an existing cover crop, sod, or crop residues, and eliminating subsequent tillage operations.

NUISANCE FLOODING — Flooding which causes public inconvenience, but little or no property damage.