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Ultraviolet radiation (UV)

A portion of the electromagnetic spectrum with wavelengths shorted than visible light. The sun produces UV, which is commonly split into three bands of decreasing wavelength. Shorter wavelength radiation has a greater potential to cause biological damage on living organisms. The longer wavelength ultraviolet band, UVA, is not absorbed by ozone in the atmosphere. UVB is mostly absorbed by ozone, although some reaches the Earth. The shortest wavelength band, UVC, is completely absorbed by ozone and normal oxygen in the atmosphere.

Underflow

The downstream flow of water through the permeable deposits that underlie a stream and that are more or less limited by rocks of low permeability.

Underground storage tank

For purposes of Subtitle I of the Resource Conservation and Recovery Act, this is any tank used to store petroleum or hazardous substances regulated under the Comprehensive Environmental Response, Compensation, and Liability Act, the volume of which is 10% or more beneath the surface of the ground. This does not include, among other things, any farm or residential tank of 1,100 gallons or less capacity used for storing motor fuel for noncommercial purposes, tanks used for storing heating oil for consumptive use on the premises, or septic tanks.

Unfinished oils

All oils requiring further refinery processing, except those requiring only mechanical blending. Includes naphtha and lighter oils, kerosene and light gas oils, heavy gas oils, and residuum.

Unit hydrograph

The hydrograph of direct runoff from a storm uniformly distributed over the drainage basin during a specified unit of time; the hydrograph is reduced in vertical scale to correspond to a volume of runoff of 1 inch from the drainage basin.

United Nations Framework Convention on Climate Change

The international treaty unveiled at the United Nations Conference on Environment and Development (UNCED) in June 1992. The UNFCCC commits signatory countries to stabilize anthropogenic (i.e. human-induced) greenhouse gas emissions to "levels that would prevent dangerous anthropogenic interference with the climate system." The UNFCCC also requires that all signatory parties develop and update national inventories of anthropogenic emissions of all greenhouse gases not otherwise controlled by the Montreal Protocol. Out of 155 countries that have ratified this accord, the United States was the first industrialized nation to do so. See Conference of the Parties, Berlin Mandate, Kyoto Protocol, Montreal Protocol.

Universal soil loss equation

A formula used to estimate erosion rates by considering climate, soils, and topographic conditions at a site, as well as any degree to which the use and management of the soil reduce erosion. It is being replaced by a revised universal soil loss equation.

Uplands

Land at higher elevations than the alluvial plain or low stream terrace; all lands outside the riparian-wetland and aquatic zones. Used often in discussion of federal land management practices.

Upwelling

The rising of cold water from the deeper areas of the ocean to the surface. This phenomena often occurs along the California coast during the summer.

Usable storage

The volume normally available for release from a reservoir below the stage of the maximum controllable level.

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