水利水電工程專業(yè)英語——灌溉與排水篇

1、水利水電工程專業(yè)英語一一灌溉與排水篇1.ConsumptiveUseofWater1 .消耗性用水Consumptiveuseofwaterinvolvesproblemsofwatersupply,bothsurfaceandunderground,aswellasproblemsofmanagementandeconomicsofirrigationprojects.Consumptiveusehasbecomeahighlyimportantfactorinthearbitrationofcontroversiesovermajorstreamsystemswherethepublicw

2、elfareofvalleys,states,andnationsisinvolved.Beforeavailablewaterresourcesofadrainagebasininaridandsemiaridregionscanbesatisfactorilyascertained,carefulconsiderationmustbegiventoconsumptiveuserequirementsforwaterinvarioussubbasins.消耗性用水涉及到地表和地下供水問題，也包括管理和灌溉工程的經(jīng)濟(jì)性問題。消耗性使用已經(jīng)成為一個評判包含河谷、州和國家公共福利的主要河流系統(tǒng)爭議

3、的重要因素。在干旱和半干旱地區(qū)流域的可利用水資源被確定之前，必須仔細(xì)考慮各個子流域內(nèi)對消耗性用水的需求。1.1 DefinitionofConsumptiveUse1.2 消耗性用水的定義Consumptiveuse,orevapotranspiration,isthesumoftwoterms:(1)transpiration,whichiswaterenteringplantrootsandusedtobuildplanttissueorbeingpassedthroughleavesofplantintotheatmosphere,(2)evaporation,whichiswatere

4、vaporationfromadjacentsoil,watersurfaces,orfromthesurfacesofleavesoftheplantWaterdepositedbydew,rainfall,orsprinklerirrigationandsubsequentlyevaporatingwithoutenteringtheplantsystemispartofconsumptiveuse.消耗性用水，或者說騰發(fā)量，是兩項(xiàng)的總和：（1）蒸騰，即水分進(jìn)入植物的根系用來構(gòu)建植物體或者通過植物葉片被傳輸?shù)酱髿庵校唬?）蒸發(fā)，即水分從鄰近的土壤、水面或者從植物葉面蒸發(fā)的過程。通過露水、降

5、雨或者噴灌儲存，隨后未經(jīng)過植物體而蒸發(fā)也是消耗性用水的一部分。Consumptiveusecanapplytowaterrequirementsofacrop,afield,afarm,aproject,oravalley.Whentheconsumptiveuseofthecropisknown,thewateruseoflargerunitscanbecalculated.Hence,theterm"/consumptiveuse“andthefollowingdiscussiongenerallyrefertothecrop.消耗性用水可以應(yīng)用到一棵植物、一塊土地、一片農(nóng)場、一

6、個項(xiàng)目或者一個河谷的用水需求中。當(dāng)作物的消耗性用水為已知時(shí)，就可以計(jì)算更大單元的用水量。因此，術(shù)語“消耗性用水”和接下來的討論主要是指作物。1.3 ConditionsAffectingConsumptiveUseofWater1.2影響消費(fèi)性用水的條件Evaportranspirationisinfluencedbytemperature,irrigationpractices,lengthofgrowingseason,precipitation,andotherfactors.Thevolumeofwatertranspiredbyplantsdependsinpartonthewate

7、rattheirdisposal,andalsoontemperatureandhumidityoftheair,windmovementintensityanddurationofsunlightstageofdevelopmentoftheplant,typeoffoliage,andmatureoftheleaves.騰發(fā)量受到天包、灌溉方式、生長期長度、降水及其他因素影響。植物蒸騰的水量部分取決于其所處位置的水源，還有溫度和空氣的濕度、風(fēng)運(yùn)動、光照的強(qiáng)度和持續(xù)時(shí)間、植物生育階段、葉片類型和葉片成熟度。Consumptiveusebynativevegetationofnoncroppe

8、dlandisfairlyconstantfromyeartoyearwherethewatersupplyisadequate,butsurfaceevaporationincreasesinwetyearsbecauseoftheexpansionofwatersurfaceandmoistsoilareas.Insomeirrigatedvalleysirrigationislavishwhenthewatersupplyisabundant,andincreasedwastewaterandreturnflowareconsequentlyavailablefornonproducti

9、veconsumption.Also,evaporationisincreasedfromthecroppedlandsoils.Plantdiseasesandpestsreduceconsumptiveusebyinhibitingplantgrowth.Sizeofafarmmayaffectwaterusageappreciablysinceborders,sizeofstream,andcroprotationsmaybedifferent.原生植物的消耗性用水在水分供應(yīng)充足地區(qū)每年是相當(dāng)恒定的，但是在豐水年地表蒸發(fā)量卻因?yàn)樗骅缀蜐駶櫷寥烂骅椎臄U(kuò)大而增加。在一些灌溉河谷，當(dāng)供水充足

10、時(shí)灌溉是奢侈的，而且增加了廢水量，因而回水可被用作II生產(chǎn)性耗水。并且，耕作區(qū)土壤的蒸發(fā)量也會增加。植物病蟲害通過抑制植物的生長減少了消耗性用水。農(nóng)場的大小可能會顯著地影響水資源利用，因?yàn)檫吔?、河流大小和植物輪作可能是不同的?)Evapotraspirationfromsoils.Wherethewatertableisnearthegroundsurface,evaporationfromthesoilisalmostequaltoevaporationfromafreewatersurface;whereas,withwaterlevelsatgreaterdepthsbelowthes

11、urface,evaporationfromsoildecreasesuntilitbecomesnegligiblewhenmoisturenolongerreachesthesurfacebycapillaryaction.1) 土壤處的騰發(fā)。在地下水水位接近地表的地方，土壤蒸發(fā)量幾乎與自由水面的蒸發(fā)景相同。而隨著水位降低到距離地表更深處，土壤蒸發(fā)量就會隨之減少，直到當(dāng)水分不再能通過毛細(xì)作用到達(dá)地表時(shí)它便可以忽略不計(jì)。Whereirrigationwaterisappliedbyfloodingmethods/largeamountsofwaterarelostbydirectevapor

12、ationfromsoilsurfaceswithouthavingpassedthroughtheroots,stems,andleavesofplants.Afterlightshowersduringthegrowingseason,considerablerainfall,ifnotall,isretainedontheleaveswhereitsubsequentlyevaporateswithoutenteringtherootsoftheplants.Showersoflessthan5millimetersseldomenterthesoiltoasufficientdepth

13、tobeusedbytheplants.Therefore,waterreceivedfromlightrainshowersorfromlightsprinklerirrigationmaynotbeusedbytheplantsinthetranspirationprocess.Subsequentdiscussionbasedupontheenergyavailableforevaporationshows,however;thatundernormalconditions,waterthatevaporatesdirectlyfromthesoilorleavesisbeneficia

14、llyused,reducingbyalikeamountwaterthatwouldotherwisehavebeentranspiredbytheplants.Ihisistrueprovidedanormalmaturecropexistsoverthesoilsurface.Earlyorlateinthegrowingperiod,whenthecropisnottranspiringmuchwater,evaporationfromawetsoilsurfacemayexceednormalconsumptiveuse.當(dāng)灌溉水是通過漫灌的方式引入的時(shí)候，大量的水就會直接從土壤表面

15、蒸發(fā)，而沒有經(jīng)過植物的根、莖和葉。在生長季節(jié)的小陣雨過后，至少相當(dāng)部分的降雨被截留在葉面上并且隨后從那里蒸發(fā)，并未經(jīng)過植物的根系。小于5mm的降雨幾乎不能進(jìn)入到足夠的深度而被植物利用。因此，從小陣雨或從輕微的噴灌中獲得的水分可能不會被植物用到蒸騰過程中?；谡舭l(fā)使用能量的角度考慮的論述表明，盡管在正常情況卜，直接從土壤和葉面蒸發(fā)的水分得到了充分的利用，這些水若不被蒸發(fā)則會被植物用作蒸騰。這真正地在土壤表面養(yǎng)活了一顆正常成熟的作物。在生長周期的早期或晚期，當(dāng)作物蒸騰最不大時(shí)，從濕潤土壤表面的蒸發(fā)可能會超過正常的消耗性用水。Therearestilldecideddifferencesinopin

16、ionontheeffectofcultivationondirectevaporationofwaterfromsoils.Recentstudiesseemtothrowconsiderabledoubtonadvantagesofsoilmulchesforconservingwaterthroughreductionofevaporationlossesfromsoilsthatarenotexcessivelywetorincontrastwithawatertableatashallowdepth.Therefore,broadgeneralcondusionsconcerning

17、influenceofcultivationondirectevaporationlossesfromsoilsmaybemisleading.Thelargenumberofvariablefactorsinvolved-notablythedifferencesindistancestofreewatersources,inoriginalmoisturecontentofunsaturatedsoils,intexture,structure,andwaterconductivity-makeithazardoustogeneralize.人們對于耕種對水分宜接從土壤蒸發(fā)效果的看法存在著

18、明顯的差異。最近的研究似乎而于減少并非充分濕潤或者地卜水位相對較淺的土壤的蒸發(fā)損失以涵養(yǎng)水分的土壤覆蓋的優(yōu)勢提出了相當(dāng)大的疑問。因此，關(guān)于耕種對于從土壤的直接蒸發(fā)損失影響的結(jié)論可能是有誤的。這其中包含著大量的變量因素使之不能概而論，特別是到自由水源距離的差別、飽和土壤的水分含量、質(zhì)地、結(jié)構(gòu)以及導(dǎo)水性。2) Transpiration.Theprocessbywhichwatervaporescapesfromlivingplants,principallytheleaves,andenterstheatmosphereisknownastranspiration.Duringthegrowin

19、gperiodofacrop,thereisacontinuousmovementofwaterfromthesoilintoroots,upstems,andoutofleavesoftheplants.Velocityofwaterflowingthroughplantsvarieswidelyfrom25to175centimetersperhour;but,underconditionsofunusuallyhightemperature,dryatmosphere,andwind,velocityofthestreammaybegreatlyincreased.Averysmallp

20、roportionofwaterabsorbedbytherootsisretainedintheplants.Iftherateofevaporationattheleavesexceedstherateofabsorptionbytheroots,wiltoccursandgrowthoftheplantisimpeded.Ontheotherhandtheavailablewaterisnotusedefficiently,ifconditionsaresuchastostimulateexcessivetranspiration.2)蒸騰。即水蒸氣通過活體植物溢出，主要是葉子，進(jìn)入到大

21、氣中的過程被稱為蒸騰。在作物生長階段，存在一個連續(xù)不斷的從土壤進(jìn)入植物的根系、上升到莖，然后從葉面溢出的水分運(yùn)動。水分在植物體內(nèi)流動的速度廣泛地分布于每小時(shí)25到175厘米之間：但是，在非正常高溫、干燥的大氣、風(fēng)等條件下，水流的速度可能會明顯增加。根系吸收的非常小的比例的水分被保留在植物中。如果葉面的蒸發(fā)速率超過了根系的吸水速率，那么植物就會枯萎且其生長受到抑制。另一方面，在刺激過度蒸騰的條件卜，可利用水并不能被有效利用。Whenwaterchangesfromliquidtovaporandpassedfromtheplantsintotheatmosphere,about590calori

22、esofheatarerequiredforeverycubiccentimeterbeingconvertedfromliquidtovapor.Hence,ifheatisnotavailable,transpirationceases;andconversely,asavailableheatincreases,transpirationincreases.Heatcancomefromthesoil,theplant,theair;orfromdirectradiation.Themajorityofheatusedfortranspiration,evenovershortperio

23、dsoftime,comesdirectlyfromthesunasradiantenergy.當(dāng)水分從液態(tài)變成氣態(tài)并從植物進(jìn)入到大氣中，每立方厘米的水分從液態(tài)轉(zhuǎn)變成氣態(tài)大約需要590卡路里的熱最。因此，如果沒有熱最可用，蒸騰就會停止；反之，如果可用熱量增加，蒸騰作用增加。熱量可以來自土壤、植物、空氣或者來自直接輻射。用于蒸騰的絕大部分熱量，即使時(shí)間很短，直接來自太陽的輻射能量。Thus,agivenincrementofheatwillvaporizeagivenquantityofwater.Iftheavailableheatisusedforvaporizingwaterfromthe

24、groundsurface,itisnotavailableforvaporizingwaterwithinthecavitiesoftheleaf.Therefore,alightshowerofrainremainingonthesurfaceofleavesoronthegroundcanevaporate,utilizingthebulkofavailableenergy.Whenthisoccurs,thetranspirationofwaterthroughtheplantwillbereducedaccordingly.因此，一定的增加的熱量會汽化一定量的水分。如果可用的熱量被用

25、來汽化地表的水分，它就不會被利用于葉片氣孔處的水分汽化。因此，小陣雨中留存在葉片表面或是地表的水分會利用大最可用的能最。當(dāng)這種情況發(fā)生時(shí)，通過植物體的蒸騰作用就會相應(yīng)地被減少。Forexample,supposearainfallofonly3millimetersoccurredduringthedryseasonandallthemoisturewasinterceptedbyleavesofthegrowingcrop.Assumealsothatdailyconsumptiveusewas6millimetersperday.Experimentsshowthatthebulkofco

26、nsumptiveuseoccursduringtheday.Therefore,ifconsumptiveuseof6millimetersperdayoccursover,say12hours,theaverageuseperhourwouldbe0.5millimeters.Duringmidday,therateislikelytonearlydoubletheaverageor1.0millimetersperhourHence,in3hourstherainfallof3millimeterswouldbeevaporatedandwouldhavebeenfullyutilize

27、d.Oftenoverlookedisthefactthatrainfallof3millimetersmayrepresentonlyone-halfofanormaldailyconsumptiveuse.Observationsoftherecoveryofawiltedaopfollowingalightshowerwillshowthattherainfallisnotwithouteffect.例如，假設(shè)在干旱季節(jié)發(fā)生了一場僅有3mm的降雨，并且所有的水分都被生長中的作物葉片截留。葉舌每天消耗性用水為6mm。試驗(yàn)表明大最的消耗性用水發(fā)生在白天。因此，如果每天的6mm，消耗性用水發(fā)生

28、在，比如說12小時(shí)內(nèi)，那么每小時(shí)平均用水量就是0.5mm。在中午，速率很可能達(dá)到平均水平的兩倍即每小時(shí)1.0mm。因此，在3個小時(shí)內(nèi)3mm的降水就會被蒸發(fā)并被完全利用。經(jīng)常被忽視的享實(shí)是，3亳米的雨量可能只占正?？谙男杂盟囊话?。對于枯萎的植物在一場小陣雨過后恢復(fù)的觀察表明，降雨并非沒有影響。Rainfalloflessthan1centimeterhavefrequentlybeenneglectedinprojectplanningontheerroneousassumptionthatrainmustgetintotherootzonetobefullyeffective.Whenth

29、ecropcoversthegroundsurface,alightrainoralightirrigationwillreducebyalikeamountmoistureremovedfromthesoilbytheroots,providedofcoursethatanexcessivewinddoesnotexist基于雨水必須進(jìn)入到植物根系區(qū)才能完全有效的假設(shè)，小于1厘米的降雨通常在項(xiàng)目規(guī)劃中被忽略。當(dāng)作物覆蓋地表時(shí)，一場小雨或是少最的灌溉會全部通過根系從土壤中吸收走，前提當(dāng)然是沒有太大的風(fēng)。2.Irrigation2.灌溉Irrigationisanancientagricultu

30、ralpracticethatwasused7000yearsagoinMesopotamia,otherancientnotableirrigationsystemswerelocatedinEgypt,China,Mexico,andPeru.Today,approximately11percentoftheworld'scroplandisirrigated.Someofthedensestpopulationsaresupportedbyproducingcropsonirrigatedland,asintheUnitedArabRepublic(Egypt),where100

31、percentofthecroplandisirrigated.ThislandislocatedalongtheNileRiver.Morethan12percentofthecropland,orabout24000000hectares(61000000acres)areirrigatedintheUnitedStates.Irrigationisextremelyimportantinincreasingsoilproductivitythroughouttheworld.濯溉作為一種占老的農(nóng)業(yè)實(shí)踐，最早是用在7000年前的美索不達(dá)米亞，同樣占老的著名的灌溉系統(tǒng)位于埃及、中國、墨西哥和

32、秘魯。今天,全世界大約11%的農(nóng)田可灌溉。一些最密集的人II是由可灌溉徒弟的作物生產(chǎn)所支控，比如阿拉伯聯(lián)合共和國(即埃及)，那里100%的農(nóng)田可灌溉。這片土地位于尼羅河沿岸。在美國有超過12%的農(nóng)田，或者大約24000000公頃(61000000英畝)可以灌溉。灌溉對在全世界范圍內(nèi)提升土壤生產(chǎn)力是極其重要的。2.1 WaterSources2.2 水源Mostirrigationwaterissurfacewaterresultingfromrainandmeltingsnow.Manyriversintheworldhavetheirheadwatersinmountainsandflowt

33、hrougharidandsemiaridregionswherethewaterisdivertedforirrigation.ExamplesincludetheIndusRiver,whichstartsintheHimalayaMountains,andthenumerousriversonthewesternslopetheAndesMountains,whichflowthroughthedesertsofChileandPerutothePacificOcean.Muchofthewaterintheseriverscomesfromthemeltingofsnowinthehi

34、ghmountains.Infact,theextentofthesnowpackismeasuredinordertopredictthestreamflowandamountofwaterthatwillbeavailableforirrigationthenextseason.ManyfarmerswhousewellwaterhavetheirownsourceofirrigationwaterAbout20percentofthewaterusedforirrigationintheUnitedStatescomesfromwells.大部分的灌溉水是來自降雨和融雪的地表水。世界上的

35、很多河流發(fā)源于山區(qū)，流經(jīng)干早和半干旱地區(qū)，在那里河水被引用灌溉。眾多例子中包括印度河，發(fā)源于喜馬拉雅山脈,且在安第斯山脈西施行著眾多的河流，穿過智利沙漠和秘魯注入太平洋。這些河流的很多水來自高山.上的融雪。事實(shí)上，積雪的范圍要被測量以預(yù)測河流流量和卜一季可以用于灌溉的水量。很多用井水的農(nóng)民有他們自己的灌溉水源。在美國大約20%的灌溉水來自井水。2.3 ImportantPropertiesofIrrigatedSoils2.4 灌溉土壤的重要性質(zhì)Soilswellsuitedforirrigationhaveanintermediatetexturetoadepthof1to2metersan

36、dwithoutcompactorwater-impermeablelayers.Loamysoilsthatareunderlainbycoarsesandsandgravelhavegreaterretentionofwaterintheoverlyingsoilthansoilsthatareloamy-texturedthroughoutThetexture,togetherwiththestructure,shouldresultinaninfiltrationrateofabout0.5to8centimetersperhourandamoderatewater-storageca

37、pacity.Thesoilshouldnothaveaninjurioussolublesaltcontent.非常適公灌溉的土壤要有一個中等的質(zhì)地，深度在1到2米，并且沒有密實(shí)或不透水層。卜層為粗砂和礫石的壤質(zhì)土中覆蓋土層比所有土層都是壤土質(zhì)地要有更強(qiáng)的保水性。這種質(zhì)地和結(jié)構(gòu)應(yīng)該能夠得到每小時(shí)0.5到8厘米入滲率保持在和一個適中的儲水能力。土壤中的可溶性鹽含量不應(yīng)達(dá)到有損害性的值。2.5 WaterApplicationMethods2.6 灌水方法Choiceofvariousmethodsofapplyingirrigationwaterisinfluencedbyaconsidera

38、tionof:(1)infiltrationrate,(2)slopeandgeneralnatureofthesoilsurface,(B)supplyofwaterandhowitisdelivered,(4)croprotation,and(5)seasonalrainfall.Themethodsofdistributingwatercanbeclassifiedasflood,furrow,subsurface,sprinkler;anddriportrickle.對于眾多灌溉水施用方法的選擇要受到以下思考的影響：(1)入滲速率；(2)坡度和土壤表面一般性質(zhì)；(3)供水及引水方式；(

39、4)輪作方式；(5)季節(jié)性降雨。配水方法可以分為漫灌、溝灌、地卜.灌、噴灌和滴灌。1) FloodIrrigation.Floodirrigationfloodsthelandbydistributingwaterbygravity.Waterisletintotheupperendofthefield,usuallyfromalargeditch.Sometimesthewaterisdistributedintobasins,whichisacommonpracticefororchards,pastures,andsomegraincrops,includingrice.Floodirr

40、igationcaneasilyover-watersomepartsofafieldwhileleavingsomepartsunderirrigated.1)漫灌。漫灌就是淹過地塊通過重力分配水。通常水從一個大的溝渠中引入田塊的高處。有時(shí)將水分配到低地，這是果園、牧場及一些谷類作物，包括水稻的普遍做法。漫灌的方式很容易讓某些部分灌水過多而另一些部分則沒有灌水。2) FurrowIrrigation.Furrowirrigationissimilartofloodirrigation,thedifferencebeingthatthewaterdistributionisdownfurrow

41、sinsteadofbeingallowedtofloodovertheland.Infurrowirrigation,whichisusedforrowcrops,waterisdistributeddownbetweentherows.Furrowsthathaveaslightgradeorslopearemadeaaossthefield.Theslopeofthefurrowbottomsdependsonsoilinfiltrationrate,whichshouldbesufficienttodistributewateralongtheentirelengthofthefurr

42、ow.Furrowirrigationisunsatisfactoryonsandsoils,becauseoftheveryhighinfiltrationrateandlimiteddistanceofwaterflowdownthefurrows.2)溝灌。溝灌與漫灌相類似，其區(qū)別是水沿著壟溝分配而不是要淹過整塊土地。在用于行栽作物的溝灌中，水順著行間分配。整塊田塊中整理出了較小的等級或坡度的k溝底的坡度取決于土壤入滲速率，它應(yīng)該足夠水分沿著里溝長度的方向分配水。溝灌不適合沙土地，因?yàn)槠淙霛B速率非常高并且水沿著壟溝流動的距離有限。Manymethodsareusedtodistribut

43、ewaterintothefurrows.Amajoradvantageoffloodandfurrowirrigationisthatthewaterisdistributedbygravity,sowaterdistributioncoststenttobelow.有很多方法被用于將水分配到溝中。漫灌和溝灌的一個主要的優(yōu)點(diǎn)是水是通過中.力分配的，所以配水的成本就會低。3) SprinklerIrrigation.Sprinklerarewidelyusedtoirrigatelawns.Sprinklersystemsareversatileandhavesomespecialadvant

44、ages.Onsands,v/herehighinfiltrationratespermitwatertoflowonlyashortdistancedownafurrow,sprinklersareessentialtospreadwateruniformlyoverthefield.Onanunevenlandsurface,expensivelandlevelingandgradingcanbeavoidedbytheuseofsprinklers.Theportablenatureofmanysprinklersystemsmakesthemideallysuitedforusewhe

45、reirrigationisusedtosupplementnaturalrainfall.Whenconnectedtosoilwater-measuringapparati,sprinklersystemscanbemadeautomatic.3)噴灌。噴灌在灌溉草坪中應(yīng)用廣泛。噴灌系統(tǒng)很靈活并且有一些特點(diǎn)。在沙土地，入滲速率只允許水分沿著壟溝在很小的一段距離上流動，噴灌對F將水均勻地在田地上散布是必不可少的。在一個不平整的地表，通過使用噴灌可以避免昂貴的土地平整和分級。噴灌系統(tǒng)的便攜性讓它們非常適用于灌溉被用于補(bǔ)充自然降水的地方。當(dāng)與土壤水分測量儀器連接時(shí)，噴灌系統(tǒng)可以實(shí)現(xiàn)自動化。Lar

46、geself-propelledsprinklersystems,whichpivotinalargecirclearoundawell,havebecomepopularSuchsystemscanirrigatemostofthelandinaquartersection(160acres,or65hectares).InthewesternpartoftheUnitedStates,largegreencircularirrigatedareasproducedbypivotsprinklersareclearlyvisiblefromtheairTheuseonmanyrollinga

47、ndsandysoilshasgreatlyincreasedtheproductionofcornandothergraincropsontheGreatPlains.Insomeareas,however;undergroundwaterreservoirsarebeingdepleted,andsomeirrigatedareasareagainbeingusedasgrazinglandandforfallowedwheatproduction.在一個困繞著一口井的大圈中轉(zhuǎn)動的大型自走式噴灌系統(tǒng)已經(jīng)流行開來。這樣的系統(tǒng)能夠在一刻鐘的時(shí)間內(nèi)灌溉大部分的土地(160英畝，或65公頃)。在美

48、國西部，由支軸噴灌機(jī)制造的大片圓形灌溉區(qū)域在空中都清晰可見。其在人平原地區(qū)很多滾動和沙土地卜.的應(yīng)用已經(jīng)極大地增加了玉米和其他糧食作物的產(chǎn)量。然而在一些地區(qū)，地卜水庫正在枯竭，一些灌溉地區(qū)有被用作牧場和休耕小麥產(chǎn)量。Sprinklerirrigationmodifiestheenvironmentbycompletelywettingplantleavesandthesoilsurfaceandbyaffectingthesurroundingairtemperature.Thehighspecificheatofwatermakessprinklinganeffectivemeansofre

49、ducingfrosthazard.SprinklerirrigationhasbeenusedforfrostprotectioninsuchdiversesituationsasforwintervegetablesinFlorida,strawberriesinMichigan,andcitrusinCalifornia.噴灌通過使植物葉片和土壤地表完全濕潤以及影響周困空氣溫度來改善環(huán)境。水的高比熱讓噴灌成為一個減少霜凍損害的有效手段。噴灌己經(jīng)被用作霜凍防護(hù)，各種不同的情況包括在冬季里弗羅里達(dá)的蔬菜、密歇根州的草莓和加利福尼亞的橘柑。4) SubsurfaceIrrigation.Sub

50、irrigationisirrigatingbywatermovementupwardfromawatertablelocatedsomedistancebelowthesoilsurface.Someareasarenaturallysubirrigatedwherewatertablesarewithinameterortwoofthesoilsurface.ThisoccursinthepoorlydrainedareasoftheSandhillsinnorth-centralNebraska.ArtificialsubirrigationispracticedintheNetherl

51、ands,wheresubsurfacedrainagelinesareusedtodrainsoilsinthewetseasonandthenusedduringdroughtforsubirrigation.Thesubirrigationdependsontheuseofdrainageditchesandpumpstocreateandcontrolawatertable.SubirrigationworkswellonthenearlylevelFloridacoastalplains.Thesandysoilshavehighsaturatedhydraulicconductiv

52、ity,andthenaturalwatertableis1meterorlessfromthesoilsurfacemuchofthetime.4）地下灌溉。地下灌溉是通過水從土壤表面以下某段距離內(nèi)的水位向上運(yùn)動而進(jìn)行灌溉。一些地下水位在土壤表面以卜一兩米內(nèi)的地區(qū)能夠進(jìn)行自然地下灌溉。這種情況發(fā)生在內(nèi)布拉斯加中北部的Sandhills排水不良的地區(qū)。人工地卜灌溉在芬蘭進(jìn)行了實(shí)踐，那里地卜排水管線在雨季被用來排出土壤水分，然后在旱季被用來進(jìn)行地卜灌溉。地卜.灌溉依賴于排水溝和水泵來形成和控制地卜冰位。地卜灌溉在弗多里達(dá)州沿海平原運(yùn)行良好。沙質(zhì)土壤具有較高的飽和水力傳導(dǎo)度，其自然水位大部分時(shí)間距

53、離土壤表面都在1米以下。Subirrigationismoreefficientintheuseofwaterthanothersystems,andisadaptedonlyforspecialsituations.Inaridregions,theupwardmovementofwateranditsevaporationatthesoilsurfaceresultsinsaltaccumulation,subirrigationworksbestwherenaturalrainfallleachesanysaltsthatmayhaveaccumulated.地下灌溉在水的利用方面要比其

54、他系統(tǒng)更加有效，并且只適用于特殊條件。在干旱地區(qū)，水分的向上運(yùn)動及其在土壤表面的蒸發(fā)導(dǎo)致了鹽的聚集，地卜灌溉在自然降水能夠淋溶任何可能累積的鹽類地區(qū)的運(yùn)行效果最好。5) DripIrrigation.Dripirrigationisthefrequentoralmostcontinuousapplicationofaverysmallamountofwatertoalocalizedsoilarea.Plastichosesabout1or2centimetersindiameter;withemitters,areplaceddowntherowsoraroundtrees.Onlyasma

55、llamountofthepotentialrootzoneiswetted.Rootsinthelocalizedareacanabsorbwaterrapidlybecauseahighwaterpotential,orwateravailability,ismaintained.Amajoradvantageofdripirrigationisthelargereductioninthetotalamountofwaterused.Otheradvantagesincludeadaptabilityofsystemtoverysteepland,whereothermethodsareu

56、nsuited,andtheabilitytomaintainamoreuniformsoilwaterpotentialduringthegrowingseason.Potatoesdevelopanirregularshapeandbecomelessdesirableiftubersalternatelygrowrapidlyandslowlybecauseoflargechangesinavailablewaterduringtheperiodoftuberdevelopment5）滴灌。滴灌就是持續(xù)地或者幾乎連續(xù)地將少量的水輸送到一個固定的土壤區(qū)域。帶有滴頭的直徑1或2厘米的塑料管子

57、被放置在行間或者樹的周圍。只有少敢的潛在根系區(qū)域被浸潤。限定區(qū)域內(nèi)的根系能夠很快地吸收水分，因?yàn)槟抢锞S持著較高的水勢且可用水量不變。滴灌的一大優(yōu)勢是用水總量大大減少。其他的優(yōu)點(diǎn)包括系統(tǒng)對陡峭田塊的適應(yīng)能力，在那里其他方法并不適用，以及在生長季節(jié)內(nèi)能夠維持更均勻的土壤水勢的能力。馬鈴薯如果因?yàn)樵倨饓K莖發(fā)育階段可利用水大幅變化，導(dǎo)致其塊莖交替地快速生長和緩慢生長，那么其形狀就會長得不規(guī)則而且不太理想。3. GroundwaterOccurrenceandtheHydrologicalCycle3,地下水賦存和水文循環(huán)AlmostallthestrataintheuppercrustoftheEar

58、thcontainopenspacesasporesorfissureswhichmayholdwater.Wherethesespacesaresufficientlylargeandinterconnected,thatissufficientlypermeable,sothatusefulsuppliesmaybeobtainedfromwellsorsprings,thestratumiscalledanaquifer.Freshwatermayoccuratdepthsof2000mormorebutbelowdepthsofabout600myieldstendstobesmall

59、andgroundwaterqualityusuallydeclines.Anaquicludeisastratumwhichcontainswaterbutduetothesmallsizeoftheporesissoimpermeablethatthemovementofwaterisveryslowandthereforeityieldslittleornowater.Aquifersmaybereplenishedbyinfiltrationfromprecipitation,surfacewaterseepage,orbyseepagefromadjacentaquifers.Surfaceinfiltrationpassesdowninitiallythroughaerationcontainingbothairandwateruntilitreachesthewatertablewhichdefinesthelevelofsaturationinanaquifer.Thezoneofaerationmaybethinandt