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National Engineering Handbook United States Department of Agriculture Part 630 Hydrology Natural Resources Conservation Service Chapter 22 Glossary Rain clouds Cloud formation Precipitation n o i Evaporation t a n r n o i a i p t l e a i s t c e o n o s a g e r v T m m o m o n r o r f r o f f i s t n a o r m ti i a a p e r s r o n t p a s a r v T m Surface runoff E o r f Infiltration Soil Percolation Rock Ocean Ground water Deep percolation (210–VI–NEH, Amend. 49, February 2012) Chapter 22 Glossary National Engineering Handbook Part 630 Hydrology Issued February 2012 The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familial status, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, or because all or a part of an individual’s income is derived from any public assistance program. (Not all prohibited bases apply to all pro- grams.) Persons with disabilities who require alternative means for commu- nication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination, write to USDA, Director, Office of Civil Rights, 1400 Independence Avenue, SW., Washington, DC 20250–9410, or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal opportunity provider and employer. (210–VI–NEH, Amend. 49, February 2012) Acknowledgments Chapter 22, Glossary, was originally prepared by Victor Mockus (retired) in 1956. It was reprinted with minor revisions in 1971. This version of chapter 22 was updated under the guidance of Claudia C. Hoeft, national hydraulic engineer, Natural Resources Conservation Service (NRCS), Washington, DC, with input from Helen Fox Moody, hydraulic engineer, NRCS, Beltsville, Maryland. (210–VI–NEH, Amend. 49, February 2012) 22–i Chapter 22 Glossary National Engineering Handbook Part 630 Hydrology 22–ii (210–VI–NEH, Amend. 49, February 2012) Chapter 22 Glossary Contents 630.2200 Introduction 22–1 630.2201 Definitions 22–1 630.2202 Abbreviations and symbols 22–14 630.2203 Conversion factors 22–15 (210–VI–NEH, Amend. 49, February 2012) 22–iii Chapter 22 Glossary National Engineering Handbook Part 630 Hydrology 22–iv (210–VI–NEH, Amend. 49, February 2012) Chapter 22 Glossary 630.2200 Introduction 630.2201 Definitions A selected list of definitions of terms used in hydrolog- This section contains definitions for hydrologic ic evaluations is given in this chapter. This list is not terms commonly used within the U.S. Department of intended to be exhaustive or to cover every hydrologic Agriculture (USDA), Natural Resources Conservation term. Other useful definitions are given in: Service (NRCS). Definitions for underlined terms can be found elsewhere in the list of definitions. • Nomenclature for Hydraulics (1962) American Abbreviations and symbols representing many of these Society of Civil Engineers, Manual No. 43 terms are found in section 630.2202. • Soil and Water Terminology (2007) American Society of Agricultural and Biological Engineers, Standard S526.3 • General Introduction and Hydrologic Definitions, (1960) Manual of Hydrology: Part 1. General Surface-Water Techniques, Geological Survey Water-Supply Paper 1541–A. Methods and prac- tices of the Geological Survey. HTML version (1995) available at: http://water.usgs.gov/wsc/glossary.html • Water Words Dictionary (2011) Nevada Department of Conservation and Natural Resources and Nevada Division of Water Planning, available at: http://water.nv.gov/ programs/planning/dictionary/ • Environmental Management Glossary, Fourth Edition (2006) Soil and Water Conservation Society of American • Glossary of Geology, Fifth Edition (2005) American Geological Institute This chapter also contains a list of useful conversion factors. (210–VI–NEH, Amend. 49, February 2012) Chapter 22 Glossary National Engineering Handbook Part 630 Hydrology Acre-foot The amount of water that will cover one acre to a depth of 1 foot. Equals 43,560 cubic feet. Abbreviated AF. Actual retention Actual retention after runoff begins, in inches. Abbreviated F. Albedo The percentage of incoming radiation that is reflected by a natural surface such as the ground, ice, snow, or water. The term is often used for the reflectivity of snow in particular. Annual flood The maximum instantaneous peak discharge in a water year. Annual flood series A list of the maximum flood peak discharges occurring in each year for the period of record. Annual runoff The total natural discharge of a stream for a year, usually expressed in inches depth or acre-feet. See water yield. Annual yield The total amount of water obtained in a year from a stream, spring, well, etc. Usually expressed in inches depth, acre-feet, millions of gallons, or cubic feet. Antecedent moisture condition See antecedent runoff condition. Abbreviated AMC. Antecedent runoff condition The average condition of a watershed when flooding occurs. Abbreviated ARC. Formerly called antecedent moisture condition. Area-depth curve A graph showing the change in average rainfall depth as the size of the area receiving the rainfall changes. Areal rainfall The average rainfall over an area, usually derived from or discussed in contrast with point rainfall. Atmospheric vapor The gaseous form of water that occurs in the atmosphere. Auxiliary spillway A spillway designed to convey excess water safely through, over, or around a dam. May also be referred to as an emergency spillway. Auxiliary spillway hydrograph The hydrograph used to evaluate the stability of the auxiliary spillway to pass flow at a safe velocity. May also be referred to as an “emergency spill- way hydrograph.” Auxiliary spillway system A single auxiliary spillway or a combination of auxiliary spillways designed to work together. Base flow The sustained or fair-weather discharge that persists after storm runoff and associated quick return flow are depleted. It is usually derived from groundwater discharge or gradual snow or ice melt over extended periods of time, but need not be continuous flow. It can be based on annual or sea- sonal periods depending upon when major floods usually occur. It may also be defined as the stream discharge derived from groundwater sources. It is sometimes considered to include flow from regulated lakes or reservoirs. Channel flow Water flow in a defined channel, either natural or humanmade. Connected impervious area Describes a situation where runoff from an impervious area flows directly to storm drains, street gutters, or steam channels. Conservation storage Water impounded for consumptive uses, such as municipal, industrial, and irrigation water supply, and nonconsumptive uses such as recreation, and fish and wildlife. Conservation tillage A tillage practice that leaves residues on the soil surface for erosion con- trol and water conservation. It includes specific residue management practices, such as no-till, mulch-till, or ridge-till. 22–2 (210–VI–NEH, Amend. 49, February 2012) Chapter 22 Glossary National Engineering Handbook Part 630 Hydrology Consumptive use A term used mainly by irrigation engineers to describe the amount of water used in crop growth plus evaporation from the soil. See also evapotranspi- ration. Correlation A statistical index that measures linear variation between variables. Cover The vegetation and vegetational debris, such as mulch and residue, that ex- ist on the soil surface. In some classification schemes, such as table 9–1 in NEH630.09, fallow or bare soil is considered the minimum cover class. Cover class See land use and treatment class. Crest The elevation of the uppermost surface of a dam or an earth spillway. Crest staff gage A gage used to make a quick visual observation of water surface levels in reservoirs, rivers, streams, irrigation channels, weirs, and flumes. Criterion variable The dependant variable in a statistical regression analysis. Cross section The shape of a channel, stream, or valley determined by a line approxi- mately perpendicular to the main path of water flow, along which measure- ments of distance and elevations are determined. Cubic feet per second A volumetric unit of water flow. Sometimes called second-feet. Abbreviated cfs. Cubic feet per second per square Measures the volume of water flowing per second per square mile of drain- mile age area. Assumes uniformly distributed runoff. Abbreviated csm. Curve number A dimensionless number of 98 or less that relates runoff to the soil-cover complex of a watershed. The curve number indicates the runoff potential of a soil-cover complex during periods when the soil is not frozen. Higher numbers mean greater runoff. Abbreviated CN. Also called a runoff curve number (RCN). Dam An artificial barrier, together with any associated spillways and appurte- nant works, across a watercourse or natural drainage area, that does or may impound or divert water. Damage reach A length of floodplain or valley selected for damage evaluation. Degree-day A day with an average temperature one degree above a defined basis or threshold. The average is usually obtained by averaging the maximum and minimum temperature for the day. Depending on usage, the threshold temperature may vary. For example, in snowmelt studies, a degree-day is defined as a day with an average daily temperature above 32 °F, so a day with an average temperature of 40° F gives (40°– 32°) = 8 degree-days. Degrees of freedom The number of independent pieces of information, or parameters, required to form a statistical estimate. Depression storage The volume of water stored on the soil surface. Depth-area curve A graph showing the change in average rainfall depth as size of area chang- es. Design storm A specified rainfall depth and rainfall distribution used to estimate runoff in design of hydraulic structures.
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