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1、Irrigation Water QualityStandardsandSalinity ManagementStrategiesB-16674-03Nearly all waters containdissolved salts and traceelements,many of whichresult from the naturalweathering of the earthssurface.In addition,drainage waters from irri-gated lands and effluentfrom city sewage andindustrial waste
2、 water canimpact water quality.Inmost irrigation situations,the primary water qualityconcern is salinity levels,since salts can affect boththe soil structure and cropyield.However,a number oftrace elements are found inwater which can limit itsuse for irrigation.Generally,“salt”is thoughtof as ordina
3、ry table salt(sodium chloride).How-ever,many types of saltsexist and are commonlyfound in Texas waters(Table 1).Most salinityproblems in agricultureresult directly from thesalts carried in the irriga-tion water.The process atwork is illustrated inFigure 1,which shows abeaker of water containinga sal
4、t concentration of 1percent.As water evapo-rates,the dissolved saltsremain,resulting in a solu-tion with a higher concen-tration of salt.The sameprocess occurs in soils.Salts as well as other dis-solved substances begin toaccumulate as water evapo-rates from the surface andas crops withdraw water.Wa
5、ter Analysis:Units,Terms andSamplingNumerous parameters areused to define irrigationwater quality,to assesssalinity hazards,and todetermine appropriate man-agement strategies.A com-plete water quality analysiswill include the determina-tion of:1)the total concentration ofsoluble salts,2)the relative
6、 proportion ofsodium to the othercations,3)the bicarbonate concen-tration as related to theconcentration of calciumand magnesium,and 4)the concentrations ofspecific elements andcompounds.3Irrigation Water Quality Standardsand Salinity ManagementGuy Fipps*Table 1.Kinds of salts normally found in irri
7、gation waters,with chemical symbols and approxi-mate proportions of each salt.1(Longenecker and Lyerly,1994)Chemical nameChemical symbolApproximate proportionof total salt contentSodium chlorideNaClModerate to largeSodium sulfateNa2SO4Moderate to largeCalcium chlorideCaCl2ModerateCalcium sulfate(gyp
8、sum)CaSO42H2OModerate to smallMagnesium chlorideMgCl2ModerateMagnesium sulfateMgS04Moderate to smallPotassium chlorideKClSmallPotassium sulfateK2SO4SmallSodium bicarbonateNaHCO3SmallCalcium carbonateCaCO3Very SmallSodium carbonateNa2CO3Trace to noneBoratesBO-3Trace to noneNitratesNO-3Small to none1W
9、aters vary greatly in amounts and kinds of dissolved salts.This water typifies many used for irrigation in Texas.*Associate Professor and ExtensionAgricultural Engineer,Department ofAgricultural Engineering,The TexasA&M University System,CollegeStation,Texas 77843-2117.The amounts and combina-tions
10、of these substancesdefine the suitability ofwater for irrigation and thepotential for plant toxicity.Table 2 defines commonparameters for analyzingthe suitability of water forirrigation and providessome useful conversions.When taking water samplesfor laboratory analysis,keep in mind that waterfrom t
11、he same source canvary in quality with time.Therefore,samples shouldbe tested at intervalsthroughout the year,partic-ularly during the potentialirrigation period.The Soiland Water Testing Lab atTexas A&M University cando a complete salinityanalysis of irrigation waterand soil samples,and willprovide
12、 a detailed computerprintout on the interpreta-tion of the results.Contactyour county Extensionagent for forms and infor-mation or contact the Labat(979)845-4816.Two Types of SaltProblemsTwo types of salt problemsexist which are very differ-ent:those associated withthe total salinity and thoseassoci
13、ated with sodium.Soils may be affected onlyby salinity or by a combi-nation of both salinity andsodium.Salinity HazardWater with high salinity istoxic to plants and poses asalinity hazard.Soils withhigh levels of total salinityare callsaline soils.Highconcentrations of salt inthe soil can result in
14、a“physiological”droughtcondition.That is,eventhough the field appears tohave plenty of moisture,the plants wilt because theroots are unable to absorbthe water.Water salinity isusually measured by theTDS(total dissolved solids)or the EC(electric conduc-tivity).TDS is sometimesreferred to as the total
15、salinity and is measured orexpressed in parts per mil-lion(ppm)or in the equiva-lent units of milligrams perliter(mg/L).EC is actually a measure-ment of electric current andis reported in one of threepossible units as given inTable 2.Subscripts are usedwith the symbol EC to iden-tify the source of t
16、he sam-ple.ECiwis the electric con-ductivity of the irrigationwater.ECeis the electricconductivity of the soil asmeasured in a soil sample(saturated extract)taken4Figure 1.Effect of water evaporation on the concentration of salts in solution.A liter is 1.057 quarts.Ten grams is.035 ounces or about 1
17、 teaspoonful.Types of Salinity Problemsaffectscan lead tosalinityplantssaline soilhazardconditionaffectscan lead tosodiumsoilssodic soilconditioncalculated from the ratio ofsodium to calcium andmagnesium.The latter twoions are important sincethey tend to counter theeffects of sodium.Forwaters contai
18、ning signifi-cant amounts of bicarbon-ate,the adjusted sodiumadsorption ratio(SARadj)issometimes used.Continued use of water hav-ing a high SAR leads to abreakdown in the physicalstructure of the soil.Sodium is adsorbed andbecomes attached to soilparticles.The soil thenbecomes hard and compactwhen d
19、ry and increasinglyimpervious to water pene-tration.Fine textured soils,especially those high inclay,are most subject tothis action.Certain amend-ments may be required tomaintain soils under highSARs.Calcium and magne-sium,if present in the soilin large enough quantities,will counter the effects oft
20、he sodium and help main-tain good soil properties.Soluble sodium per cent(SSP)is also used to evalu-ate sodium hazard.SSP isdefined as the ration ofsodium in epm(equivalentsper million)to the totalcation epm multiplied by100.A water with a SSPgreater than 60 per centmay result in sodium accu-mulatio
21、ns that will cause abreakdown in the soilsphysical properties.Ions,Trace Elements andOther ProblemsA number of other sub-stances may be found inirrigation water and cancause toxic reactions inplants(Table 3).After sodi-um,chloride and boron are5Table 2.Terms,units,and useful conversions for understa
22、nding water quality analysis reports.SymbolMeaningUnitsTotal Salinitya.ECelectric conductivitymmhos/cmmhos/cmdS/mb.TDStotal dissolved solidsmg/LppmSodium Hazarda.SARsodium adsorption ratiob.ESPexchangeable sodium percentageDeterminationSymbolUnit of measureAtomic weightConstituents(1)cationscalciumC
23、amol/m340.1magnesiumMgmol/m324.3sodiumNamol/m323.0potassiumKmol/m339.1(2)anionsbicarbonateHCO3mol/m361.0sulphateSO4mol/m396.1chlorideClmol/m335.5carbonateCO3mol/m360.0nitrateNO3mg/L62.0Trace ElementsboronBmg/L10.8Conversions1 dS/m=1 mmhos/cm=1000 mhos/cm1 mg/L=1 ppmTDS(mg/L)EC(dS/m)x 640 for EC 5 dS
24、/mTDS(lbs/ac-ft)TDS(mg/L)x 2.72Concentration(ppm)=Concentration(mol/m3)times the atomic weightSum of cations/anions(meq/L)EC(dS/m)x 10Keymg/L=milligrams per literppm=parts per milliondS/m=deci Siemens per meter at 25 Cfrom the root zone.ECdisthe soil salinity of the satu-rated extract taken frombelo
25、w the root zone.ECdisused to determine the salin-ity of the drainage waterwhich leaches below theroot zone.Sodium HazardIrrigation water containinglarge amounts of sodium isof special concern due tosodiums effects on the soiland poses a sodiumhazard.Sodium hazard isusually expressed in termsof SAR o
26、r the sodiumadsorption ratio.SAR isof most concern.In certainareas of Texas,boron con-centrations are excessivelyhigh and render waterunsuitable for irrigations.Boron can also accumulatein the soil.Crops grown on soils hav-ing an imbalance of calci-um and magnesium mayalso exhibit toxic symp-toms.Su
27、lfate salts affectsensitive crops by limitingthe uptake of calcium andincreasing the adsorptionof sodium and potassium,resulting in a disturbancein the cationic balancewithin the plant.The bicar-bonate ion in soil solutionharms the mineral nutri-tion of the plant throughits effects on the uptakeand
28、metabolism of nutri-ents.High concentrationsof potassium may introducea magnesium deficiencyand iron chlorosis.Animbalance of magnesiumand potassium may betoxic,but the effects of bothcan be reduced by high cal-cium levels.6Table 3.Recommended limits for constituents in reclaimed water for irrigatio
29、n.(Adapted from Rowe and Abdel-Magid,1995)ConstituentLong-termShort-termRemarksuse(mg/L)use(mg/L)Aluminum(Al)5.020Can cause nonproductivity in acid soils,but soils at pH 5.5 to 8.0 will precipitate the ion and eliminate toxicity.Arsenic(As)0.102.0Toxicity to plants varies widely,ranging from 12 mg/L
30、 for Sudan grass to less than 0.05 mg/L for rice.Beryllium(Be)0.100.5Toxicity to plants varies widely,ranging from 5 mg/L for kale to 0.5 mg/L for bush beans.Boron(B)0.752.0Essential to plant growth,with optimum yields for many obtained at a few-tenths mg/L in nutrient solutions.Toxic to many sensit
31、ive plants(e.g.,citrus)at 1 mg/L.Most grasses relatively tolerant at 2.0 to 10 mg/L.Cadmium(Cd)0.010.05Toxic to beans,beets,and turnips at concentrations as low as 0.1 mg/L in nutrient solution.Conservative limits recommended.Chromium(Cr)0.11.0Not generally recognized as essential growth element.Con
32、servative limits recommended due to lack of knowledge on toxicity to plants.Cobalt(Co)0.055.0Toxic to tomato plants at 0.1 mg/L in nutrient solution.Tends to be inactivated by neutral and alkaline soils.Copper(Cu)0.25.0Toxic to a number of plants at 0.1 to 1.0 mg/L in nutrient solution.Fluoride(F)1.
33、015.0Inactivated by neutral and alkaline soils.Iron(Fe)5.020.0Not toxic to plants in aerated soils,but can contribute to soil acidifi-cation and loss of essential phosphorus and molybdenum.Lead(Pb)5.010.0Can inhibit plant cell growth at very high concentrations.Lithium(Li)2.52.5Tolerated by most cro
34、ps at up to 5 mg/L;mobile in soil.Toxic to citrus at low doses recommended limit is 0.075 mg/L.Manganese(Mg)0.210.0Toxic to a number of crops at a few-tenths to a few mg/L in acid soils.Molybdenum(Mo)0.010.05Nontoxic to plants at normal concentrations in soil and water.Can be toxic to livestock if f
35、orage is grown in soils with high levels of available molybdenum.Nickel(Ni)0.22.0Toxic to a number of plants at 0.5 to 1.0 mg/L;reduced toxicity at neutral or alkaline pH.Selenium(Se)0.020.02Toxic to plants at low concentrations and to livestock if forage is grown in soils with low levels of added s
36、elenium.Vanadium(V)0.11.0Toxic to many plants at relatively low concentrations.Zinc(Zn)2.010.0Toxic to many plants at widely varying concentrations;reduced toxicity at increased pH(6 or above)and in fine-textured or organic soils.Classification ofIrrigation WaterSeveral different measure-ments are u
37、sed to classifythe suitability of water forirrigation,including ECiw,the total dissolved solids,and SAR.Some permissiblelimits for classes of irriga-tion water are given inTable 4.In Table 5,the sodi-um hazard of water isranked from low to veryhigh based on SAR values.Classification of Salt-Affected
38、 SoilsBoth ECeand SAR are com-monly used to classify salt-affected soils(Table 6).Saline soils(resulting fromsalinity hazard)normallyhave a pH value below 8.5,are relatively low in sodiumand contain principallysodium,calcium and mag-nesium chlorides and sul-fates.These compoundscause the white crust
39、which forms on the surfaceand the salt streaks alongthe furrows.The com-pounds which cause salinesoils are very soluble inwater;therefore,leachingis usually quite effective inreclaiming these soils.Sodic soils(resulting fromsodium hazard)generallyhave a pH value between8.5 and 10.These soils arecall
40、ed“black alkali soils”due to their darkenedappearance and smooth,slick looking areas causedby the dispersed condition.In sodic soils,sodium hasdestroyed the permanentstructure which tends tomake the soil impervious towater.Thus,leachingalone will not be effectiveunless the high salt dilu-tion method
41、 or amend-ments are used.Water QualityEffects on Plantsand Crop Yield Table 7 gives the expectedyield reduction of somecrops for various levels ofsoil salinityas measuredby EC under normal grow-ing conditions,and Table 8gives potential yield reduc-tion due to water salinitylevels.Generally foragecro
42、ps are the most resistantto salinity,followed by fieldcrops,vegetable crops,andfruit crops which are gen-erally the most sensitive.Table 9 lists the chloridetoleranceof a number ofagricultural crops.Boronis a major concern in someareas.While a necessarynutrient,high boron levelscause plant toxicity,
43、andconcentrations should notexceed those given in Table10.Some information isavailable on the susceptibil-ity of crops to foliar injuryfrom spray irrigation withwater containing sodium7Table 4.Permissible limits for classes of irrigation water.Concentration,total dissolved solidsClasses of waterElec
44、tricalGravimetric ppmconductivity mhos*Class 1,Excellent250175Class 2,Good250-750175-525Class 3,Permissible1750-2,000525-1,400Class 4,Doubtful22,000-3,0001,400-2,100Class 5,Unsuitable23,0002,100*Micromhos/cm at 25 degrees C.1Leaching needed if used2Good drainage needed and sensitive plants will have
45、 difficulty obtainingstandsTable 5.The sodium hazard of water based on SAR Values.SAR valuesSodium hazard of waterComments1-10LowUse on sodium sensitive crops such as avocados must be cautioned.10-18MediumAmendments(such as Gypsum)and leaching needed.18-26HighGenerally unsuitable for continuous use.
46、26Very HighGenerally unsuitable for use.Table 6.Classification of salt-affected soils based on analysis of saturation extracts.(Adapted from James et al.,1982)CriteriaNormalSalineSodic Saline-SodicECe(mmhos/cm)44SAR131313and chloride(Table 11).Thetolerance of crops to sodi-um as measured by theexcha
47、ngeable sodium per-centage(ESP)is given inTable 12.Salinity and Growth StageMany crops have little toler-ance for salinity duringseed germination,but sig-nificant tolerance duringlater growth stages.Somecrops such as barley,wheatand corn are known to bemore sensitive to salinityduring the early grow
48、thperiod than during germi-nation and later growthperiods.Sugar beet and saf-flower are relatively moresensitive during germina-tion,while the tolerance ofsoybeans may increase ordecrease during differentgrowth periods dependingon the variety.Leaching for SalinityManagement Soluble salts that accumu
49、-late in soils must be leachedbelow the crop root zone tomaintain productivity.Leaching is the basic man-agement tool for control-ling salinity.Water isapplied in excess of thetotal amount used by thecrop and lost to evapora-tion.The strategy is to keepthe salts in solution andflush them below the r
50、ootzone.The amount of waterneeded is referred to as theleaching requirementor theleaching fraction.Excess water may beapplied with every irriga-tion to provide the waterneeded for leaching.How-ever,the time intervalbetween leachings does notappear to be critical provid-ed that crop tolerances are8Ta