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<br />by pumping. (2) The ;He'd from which \I'<.lter occurring at a
<br />gi\'cn point or location on a stream arginates. In such C;.l5e the
<br />tenn is s~'nunymrJUs with drainage .Hca and watershed. (3) The
<br />term is ::llso llSed in a general sense, 10 apply [0 the flow of ,all
<br />liquids under the force of gravity. l. 4) The wilter features ot a
<br />map such as SC.H. lakes. ronds. streams, and c,:ln31s.
<br />DRAINAGE. WELL The remo\',11 of surplus or excess surface
<br />or ground water b\' sinking wells Ie> a pOFJUS formation in \..:hich
<br />the h\'dro<;r~tlc head IS lower thilll thdt of the water on the
<br />surface to be dramed. and carrying off the surface \\"l[er through
<br />these welb. \\'hen filled with :,lOnes, the well is called a ciry
<br />well.
<br />DRA \VDO\VN - (J) The magnitude of the ch3nge in surface
<br />elc\'ation of a bodv of water as a result of the withdrawal of
<br />water therefrom. (2) The ITldgnitude of the lowering of the
<br />water surface in a well, <lnJ (Jf the water table or piezometric
<br />surface adjilcf'nt to the well. resulrim: from the \.vithdrat..:al of
<br />water from the well by pumpin~. (3) The difference in eleva-
<br />tion between the water surface ele\'ation at a constriction in a.
<br />stream or conduit and the ele\'ation that would exist if the con
<br />stricti(ln were absent.
<br />EVAPORATION, TOTAL The sum of Ihe water !c'St (It'm a
<br />gl\ en land area durmg an\.' specific time by transpiration from
<br />\.egeratlon and budding ot plant tissue: b~' e\'aporation from
<br />water ~urfaces. mOIst sod grams. and snow: and by mtercepl1on.
<br />The term is applied prim.ni\y l') stre<l.m drainage basins for the
<br />period of the annual climatIC cycle or subdivision of th:ll period.
<br />Total e\'aporatlOn is es~entJ:.llly precipitdtlon upon the drainage
<br />basin mInus runoff. corrected f(Jr change in storage \"Olume with-
<br />In the basin ::lnd for subsurface le!1kage. It has been vauously
<br />called e\'aporallon from land areas, elJapormiOlI, eWlpo.trcltlspira-
<br />lion, 10'>5, U'llter lo~ses. and fir off.
<br />FLOOD. PROBABLE, l\IAXIl\lU!\I - The maximum flood for
<br />which there i~ reasonable chance that it .....ill occur on a gi\'en
<br />stream at a selected site. It is often assumed to bc equal to the
<br />mJ.Y.lmum fl,wd (,bser....ed in area'.; having Ihe same or similar
<br />physiographic .lnd meteorologic;:.] characteristics. Such a tlood
<br />would \'ery !J~ely be le~s than the nl<lximum possible flood.
<br />FLO\V, RETURN - Any f1o\\' which rcturns to a stream chan.
<br />nel after diversion for beneficial usc (lr other purposes. In irri-
<br />gation, water applied (0 an area which IS not consumed in
<br />eVilpofdtion or transpir,Hion, 'lnd return~ to a surface stream or
<br />ground-w:Jter ;Jqulfcr.
<br />FLO\N, SUBSUnrACE - (J) That punii)ll of the water which
<br />infiltrates the soil surLlCe and moves laterally Ihrough the ul?pe'r
<br />soil horizons unlll its course is intercepted by the channel of
<br />the stream or unul it returns 10 the surtdce at some J?oint down-
<br />stream of its point of inflltrali'Jn. (2) The rate of flow or dis-
<br />charge of ground water or subsurbce wati;;t.
<br />FLUl\lE, RATING - (I) An open conduit built in a channel
<br />to mainram a consistent r"'gimen for the purpose of mea~uIlng
<br />the flow and del'eloping the 5tagc-discharge relation. (2) A
<br />flume containing still water and used for the purpose of rating
<br />ClIrrellt meters, Pitot tuhes, etc.
<br />GAGE. ElVEr. . A device for ml:'ISIHing river stage. i.e.. t,) in-
<br />dicate the heIght of the w.ller sudace abol'e a specific point.
<br />Types in common use include staff gage, \VJterst:.lge recorder,
<br />and wire weight gJge.
<br />GAGING. STREAi\! The operatiQn of meJsuring the velocity
<br />of a stream of water in :3 channel or an open conduit, and the
<br />area of cross-section of the W<lter, for !he purpose of determinmg
<br />the discharge. Also called gagillg
<br />GALLERY. (1) An un~erground structure designed and in-
<br />st3l1ed for the purpose ot coUeeting perCo)lating water. (2) A
<br />passageway in a structure, such <lS a dam, \\',Her treatment plant.
<br />erc., used for obtaining access to interior parts, or to can) pipes.
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<br />or to house ffi:3chinery. (3) An underground conduit, or reser-
<br />....oir Of passa~e.
<br />GATE, HEAD. (I) A gate at the entrance \(I a cond';Jit, such
<br />as a piye line, penstock, or canal. (2) The gate at the h.lg~ le~el
<br />end rA a lock. (3) The gate at the entrance to 3n lITIgatIon
<br />ditch.
<br />GRAVITY, SPECIFIC The ratIo of the wt:'ight of a gh'en
<br />volume of a substance (Q the weight of an equal volume of
<br />water.
<br />HYDROLOGY - The applied sCience concerned \.\lit~ th~ waters
<br />of the earth in all ItS states - their occurrences. distribution,. a.nd
<br />circulation through the unending hrdro)og.ic c~:cle of: preclplta:
<br />tion: consequent runoff. stream flo\\', mhltratlon. and stora~e,
<br />e\'entual. e\'aporati~n; and reprec'rita.tion. It .is concer,ned w~th
<br />the physlc8]. chemIcal. and phYS1~10g1Cal reae.l1ons of \\ater with
<br />the test ()f the earth, and its [elatl<:1n to the ll~e of the earth.
<br />HYDROLOGY, GROUND,WATER ' The b<aoch of h"drol,
<br />ogy that treats of ground wat~r. its occurrence. and. movements:
<br />its Tepleni~hmenrs :mJ depletlon. the properties of rocks that
<br />control ground-w<ltcr movement <lnd ~lOrage: and the methods of
<br />in\'cstig3.tion and utilization of grl)und water.
<br />IJ\IBIBITION - The proce~s b.v which pbnts absurb w:.lter from
<br />the soil.
<br />INFLUENCE, CONE OF - The depre~sion, [('ughly conical in
<br />shape, produced in a \-vater table, or other piezometric surface,
<br />bv the exrraction of WJter from a well at a gl\'en rate. The
<br />volume of the cone will \'ary with the rate of withdrawal of
<br />water. Also called COliC of depressioll.
<br />IRRIGATION. OVER The application of more water than is
<br />necessarv for the needs of \'e~etation. resulting m loss of water
<br />through - seepage and ledching - wllh the resu!tclnt loss of humus,
<br />nitrogen, and other mineral elements of the ~oil.
<br />IRRIGATION, SUPPLEr..IENTAL - The wCltenng (-,f crops III
<br />reglOnS where nonnal ramfal\ ordmanl\' supphe~ mo~t of tht;
<br />mOisture It IS used during dry periods to prevent rt:tLlrdarion of
<br />growth.
<br />IRRIGATION. WINTER The irngdtion or hnd~ Juring the
<br />non-growing season in 0Cder to store water )n the suil for sub-
<br />sequent use by Jllant~.
<br />LE:\CHING - (I) The remo\'LlI uf soluble cc>ostituems from
<br />soils or other mdteria} bv percolating liquid. ("2) The relllo\'<ll of
<br />salts and albli from soils by abundant irrigatioJl combined with
<br />draInage. (3) Thl:: dis\lm:al of a liquid through a non watertight
<br />artifiCl<l1 strUCture. condull, or Jlorous materi<ll by d(l\vnward f)t
<br />lateral drainage. or both. into thc surrounding permeable SOIL
<br />LEVEE - (I) A dd<e or eml'lwhment, generally constructed on
<br />or parallel to the banks of a st~edm, lake. or ?tlH'f body uf wa~er.
<br />for (H) the purpose of protecting the land Side. fwm lOundallon
<br />bv flood waters. or (b) t(l contine the stream fJo\v to ItS regulJr
<br />channel. (2) The construction of il levee.
<br />LEVEL, HYDnOSTATIC - The level or ele\'atlOn to wlllch
<br />the top of a column of water would me, .it afforded opportunity
<br />t() do so. from an artesIan aquifer. ("Ir bdsm. in <1n open condull,
<br />or from a conduit under pressure.
<br />LOG, \VELL A chrnllologlCal record of the soil :mJ rock
<br />form~tions which wcre encountered In the operation at sinkrng
<br />a well. with either their thickness. or the elevation of the tflP
<br />and bottom of c,)ch fOITn8tl0l1 gll'en. It ,d~o usu;llly lI1clude...
<br />stJIC1l1CntS ,b tn the lirll'.dl'gic composition and 11',Jter.bcdring
<br />characteri~ric~ of cdch form<ltion.
<br />LOSS, CONVEY ANCE - The loss of Wdler t'WIll ;l conduit due
<br />ICi leak<lge, seepage, C','3p0r,I\I\JI1. or e\rapo'\r<l.n"pitatirm.
<br />MAP, GEOLOGIC A mJp which shows the boundaries and
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