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001371 GLOSSARY OF WATER MANAGEMENT TERMS l-H HEADwATER BElfEFITS--The benefits brought abollt by the storage or release of water by a reservoir project I1pstream. Application of the term is l.\Sually in reference to benefits to a downstream 1:>;ytIroe.ilec'b:tlic powerplant. HISTORICAL FLOOD DAMAGE--Flood damage meas\U'ed by the prices and CClDditio,ns existing at the t:lme Of' a :PBrticuJ..ar flood. HYDROGBN ION CONCEtlTRATION--See pH. HYPOLIMNICIf--'l'he lOWer level, in a body of water having the:rmaJ. !Itratifica- tion, that extends fran the thermocline to the bottan and which has nearly unifom temperature and is virtually removed fran surface influences. HYDROLOGIC BENCHMARK--A hydrologic unit sllch as a basin or a ground water bod;y that has been designated, because of its expected :freedom fran the effects of man, as a benchmark. Data:from such basins may provide a standard with which data from less independent basins can be can- ' pared so that changes wrought by man's interference can be distingllished :from changes cal.\Sed by variations in the natural regimen. HYDROLOGIC BUDGET--An accounting of a:l.l inflow to, outflow fran, and changes in storage within, a hydrologic unit such as a drainage basin, soil zone, aquifer, lake, or project area. HYDROLOGIC CYCLE--The circuit of water movement fran the atmosphere to the earth and return to the atmosphere through variol.\S stages or pro- cesses as precipitation, interception, runOff, infiltration, perCOlation, storase, evapo:ration, and transpiration. HYDROLOGIC SOIL COVER COMPLEX--A canbination of a hydrologic soil group and a type of cover. , HYDROLOGIC SOIL GROUP--A grol1p of soils having the same runoff potential under s:lm:1lAr storm and cover conditions. HYDROPHYTE--A plant which grows in water, or in saturated soils.