1、Designation: D 3404 91 (Reapproved 2004)Standard Guide forMeasuring Matric Potential in Vadose Zone UsingTensiometers1This standard is issued under the fixed designation D 3404; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the y
2、ear of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide covers the measurement of matric potentialin the vadose zone using tensiometers. The theoretical an
3、dpractical considerations pertaining to successful onsite use ofcommercial and fabricated tensiometers are described. Mea-surement theory and onsite objectives are used to developguidelines for tensiometer selection, installation, and opera-tion.1.2 The values stated in SI units are to be regarded a
4、s thestandard. The inch-pound units given in parentheses are forinformation only.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and de
5、termine the applica-bility of regulatory limitations prior to use.1.4 This guide offers an organized collection of informationor a series of options and does not recommend a specificcourse of action. This document cannot replace education orexperience and should be used in conjunction with professio
6、naljudgment. Not all aspects of this guide may be applicable in allcircumstances. This ASTM standard is not intended to repre-sent or replace the standard of care by which the adequacy ofa given professional service must be judged, nor should thisdocument be applied without consideration of a projec
7、ts manyunique aspects. The word“ Standard” in the title of thisdocument means only that the document has been approvedthrough the ASTM consensus process.2. Terminology2.1 Definitions of Terms Specific to This Standard:2.1.1 accuracy of measurementthe difference between thevalue of the measurement an
8、d the true value.2.1.2 hysteresisthat part of inaccuracy attributable to thetendency of a measurement device to lag in its response toenvironmental changes. Parameters affecting pressure-sensorhysteresis are temperature and measured pressure.2.1.3 precision (repeatability)the variability among nu-me
9、rous measurements of the same quantity.2.1.4 resolutionthe smallest division of the scale used fora measurement, and it is a factor in determining precision andaccuracy.3. Summary of Guide3.1 The measurement of matric potential in the vadose zonecan be accomplished using tensiometers that create a s
10、aturatedhydraulic link between the soil water and a pressure sensor. Avariety of commercial and fabricated tensiometers are com-monly used. A saturated porous ceramic material that forms aninterface between the soil water and bulk water inside theinstrument is available in many shapes, sizes, and po
11、rediameters. A gage, manometer, or electronic pressure trans-ducer is connected to the porous material with small- orlarge-diameter tubing. Selection of these components allowsthe user to optimize one or more characteristics, such asaccuracy, versatility, response time, durability, maintenance,exten
12、t of data collection, and cost.4. Significance and Use4.1 Movement of water in the unsaturated zone is ofconsiderable interest in studies of hazardous-waste sites (1, 2,3, 4)2; recharge studies (5, 6); irrigation management (7, 8, 9);and civil-engineering projects (10, 11). Matric-potential dataalon
13、e can be used to determine direction of flow (11) and, insome cases, quantity of water flux can be determined usingmultiple tensiometer installations. In theory, this technique canbe applied to almost any unsaturated-flow situation whether itis recharge, discharge, lateral flow, or combinations of t
14、hesesituations.4.2 If the moisture-characteristic curve is known for a soil,matric-potential data can be used to determine the approximatewater content of the soil (10). The standard tensiometer is usedto measure matric potential between the values of 0 and -867cm of water; this range includes most
15、values of saturation formany soils (12).4.3 Tensiometers directly and effectively measure soil-watertension, but they require care and attention to detail. In1This guide is under the jurisdiction of ASTM Committee D18 on Soil and Rockand is the direct responsibility of Subcommittee D18.21 on Ground
16、Water andVadose Zone Investigations.Current edition approved July 1, 2004. Published July 2004. Originally approvedin 1991. Last previous edition approved in 1998 as D 3404 - 91 (1998).2The boldface numbers in parentheses refer to a list of references at the end ofthe text.1Copyright ASTM Internatio
17、nal, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.particular, installation needs to establish a continuous hydraulicconnection between the porous material and soil, and minimaldisturbance of the natural infiltration pattern are necessary forsuccessful installat
18、ion. Avoidance of errors caused by airinvasion, nonequilibrium of the instrument, or pressure-sensorinaccuracy will produce reliable values of matric potential.4.4 Special tensiometer designs have extended the normalcapabilities of tensiometers, allowing measurement in cold orremote areas, measureme
19、nt of matric potential as low as -153m of water (-15 bars), measurement at depths as deep as 6 m(recorded at land surface), and automatic measurement usingas many as 22 tensiometers connected to a single pressuretransducer, but these require a substantial investment of effortand money.4.5 Pressure s
20、ensors commonly used in tensiometers in-clude vacuum gages, mercury manometers, and pressure trans-ducers. Only tensiometers equipped with pressure transducersallow for the automated collection of large quantities of data.However, the user needs to be aware of the pressure-transducerspecifications,
21、particularly temperature sensitivity and long-term drift. Onsite measurement of known zero and “full-scale”readings probably is the best calibration procedure; however,onsite temperature measurement or periodic recalibration in thelaboratory may be sufficient.5. Measurement Theory5.1 In the absence
22、of osmotic effects, unsaturated flowobeys the same laws that govern saturated flow: Darcys Lawand the Equation of Continuity, that were combined as theRichards Equation (13). Baver et al. (14) presents DarcysLaw for unsaturated flow as follows:q 52Kc1Z! (1)where:q =the specific flow,FLTG,K =the unsa
23、turated hydraulic conductivity,FLTG,c = the matric potential of the soil water at a point, L,Z = the elevation at the same point, relative to somedatum, L, and = the gradient operator, L1.The sum of c + Z commonly is referred to as the hydraulichead.5.2 Unsaturated hydraulic conductivity, K, can be
24、expressedas a function of either matric potential, c, or water content,uL3of water/L3of soil, although both functions are affectedby hysteresis (5). If the wetting and drying limbs of the K (c)function are known for a soil, time series of onsite matric-potential profiles can be used to determine whi
25、ch limb is moreappropriate to describe the onsite K (c), the correspondingvalues of the hydraulic-head gradient, and an estimate of fluxusing Darcys Law. If, instead, K is known as a function of u,onsite moisture-content profiles (obtained, for example, fromneutron-scattering methods) can be used to
26、 estimated K, andcombined with matric-potential data to estimate flux. In eithercase, the accuracy of the flux estimate needs to be assessedcarefully. For many porous media,dKdcanddKduare large, withincertain ranges of c or u, making estimates of K particularlysensitive to onsite-measurement errors
27、of c or u. (Onsite-measurement errors of c also have direct effect on (c + Z)inDarcys Law). Other sources of error in flux estimates canresult from inaccurate data used to establish the K (c)orK (u)functions (accurate measurement of very small permeabilityvalues is particularly difficult) (16); use
28、of an analyticalexpression for K (c)orK (u) that facilitates computersimulation, but only approximates the measured data; aninsufficient density of onsite measurements to define ad-equately the u or c profile, which can be markedly nonlinear;onsite soil parameters that are different from those used
29、toestablish K (c)orK (u); and invalid assumptions about thestate of onsite hysteresis. Despite the possibility of large errors,certain flow situations occur where these errors are minimizedand fairly accurate estimates of flux can be obtained (6, 17).The method has a sound theoretical basis and refi
30、nement of thetheory to match measured data markedly would improvereliability of the estimates.5.3 The concept of fluid tension refers to the differencebetween standard atmospheric pressure and the absolute fluidpressure. Values of tension and pressure are related as follows:TF5 PAT2 PF(2)where:TF=th
31、e tension of an elemental volume of fluid,FMLT2G,PAT= the absolute pressure of the standard atmosphere,FMLT2G, andPF= the absolute pressure of the same elemental volumeor fluidFMLT2G.Soil-water tension (or soil-moisture tension) similarly isequal to the difference between soil-gas pressure and soil-
32、waterpressure. Thus:TW1 PG5 PW(3)where:TW= the tension of an elemental volume of soil water,FMLT2G,PG= the absolute pressure of the surrounding soil gas,FMLT2G, andPW= the absolute pressure of the same elemental volumeof soil water,FMLT2G.In this guide, for simplicity, soil-gas pressure is assumed t
33、obe equal to 1 atm, except as noted. Various units are used toexpress tension or pressure of soil water, and are related to eachother by the equation:D 3404 91 (2004)21.000 bar 5 100.0 kPa 5 0.9869 atm 51020 cm of water at 4C 51020 g per cm2in a standardgravitational field. (4)A standard gravitation
34、al field is assumed in this guide; thus,centimetres of water at 4C are used interchangeably withgrams per square centimetre.5.4 The negative of soil-water tension is known formally asmatric potential. The matric potential of water in an unsatur-ated soil arises from the attraction of the soil-partic
35、le surfacesfor water molecules (adhesion), the attraction of water mol-ecules for each other (cohesion), and the unbalanced forcesacross the air-water interface. The unbalanced forces result inthe concave water films typically found in the intersticesbetween soil particles. Baver et al. (14) present
36、 a thoroughdiscussion of matric potential and the forces involved.5.5 The tensiometer, formally named by Richards andGardner (18), has undergone many modifications for use inspecific problems (1, 11, 19-31). However, the basic compo-nents have remained unchanged. A tensiometer comprises aporous surf
37、ace (usually a ceramic cup) connected to a pressuresensor by a water-filled conduit. The porous cup, buried in asoil, transmits the soil-water pressure to a manometer, avacuum gage, or an electronic-pressure transducer (referred toin this guide as a pressure transducer). During normal opera-tion, th
38、e saturated pores of the cup prevent bulk movement ofsoil gas into the cup.5.6 An expanded cross-sectional view of the interface be-tween a porous cup and soil is shown in Fig. 1. Water held bythe soil particles is under tension; absolute pressure of the soilwater, PW, is less than atmospheric. This
39、 pressure is transmittedthrough the saturated pores of the cup to the water inside thecup. Conventional fluid statics relates the pressure in the cup tothe reading obtained at the manometer, vacuum gage, orpressure transducer.5.6.1 In the case of a mercury manometer (see Fig. 2(a):TW5 PA2 PW5 rHg2rH
40、2O!r 2rH2Oh 1 d! (5)where:TW= the soil-water tension relative to atmospheric pres-sure, in centimetres of water at 4C,PA= the atmospheric pressure, in centimetres of waterat 4C,PW= the average pressure in the porous cup and soil, incentimetres of water at 4C,rHg= the average density of the mercury c
41、olumn, ingrams per cubic centimetre,rH2O= the average density of the water column, in gramsper cubic centimetre,FIG. 1 Enlarged Cross Section of Porous Cup-Porous MediumInterfaceFIG. 2 Three Common Types of Tensiometers: (a) Manometer; (b)Vacuum Gage; and (c) Pressure TransducerD 3404 91 (2004)3r =
42、the reading, or height of mercury column abovethe mercury-reservoir surface, in centimetres,h = the height of the mercury-reservoir surface aboveland surface, in centimetres, andd = the depth of the center of the cup below landsurface, in centimetres.5.7 Although the density of mercury and water bot
43、h varyabout 1 % between 0 and 45C, Eq 5 commonly is used withrHgand rH2Oconstant.5.7.1 Using rHg= 13.54 and rH2O= 0.995 (the median val-ues for this temperature range) yields about a 0.25 % error (1.5cm H2O) at 45C, for Tw 520 cm H2O. This small, butneedless, error can be removed by using the follow
44、ing densityfunctions:rHg5 13.595 2 2.458 3 1023T! (6)andrH2O5 0.9997 1 4.879 3 1025T! 2 5.909 3 1026T!2(7)where: rHgand rH2Oare as defined above, andT = average temperature of the column, in C.5.7.2 Average temperature of the buried segment of watercolumn can be estimated with a thermocouple or ther
45、mistor incontact with the tubing, buried at about 45 % of the depth ofthe porous cup. Air temperature is an adequate estimate forexposed segments.5.8 Most vacuum gages used with tensiometers are gradu-ated in bars (and centibars) and have an adjustable zero-reading. The zero adjustment is used to of
46、fset the effects ofaltitude, the height of the gage above the porous cup (see Fig.3(b), and changes in the internal characteristics of the gagewith time. The adjustment is set by filling the tensiometer withwater and then setting the gage to zero while immersing theporous cup to its midpoint in a co
47、ntainer of water. This settingis done at the altitude at which the tensiometer will be used andit needs to be repeated periodically after installation either byremoving the tensiometer from the soil or by unscrewing thegage and measuring a tension equal to that used in the originalcalibration. The g
48、age then reads directly the tension in theporous cup. Use of a vacuum gage without an adjustable zeroreading could result in inaccurate measurements because thezero reading could become negative and, therefore, would beindeterminate.5.9 Pressure transducers convert pressure, or pressure dif-ference,
49、 into a voltage (or current) signal. The pressuretransducer can be connected remotely to the porous cup withtubing (22, 24), attached directly to the cup (19, 32),ortransported between sites (24). An absolute pressure transducermeasures the absolute pressure (PP) in its port. A gage pressuretransducer measures the difference between ambient-atmospheric pressure (PA) and the pressure in its port (PP),known as gage pressure. When PPresponse time2. None-theless, t as defined here can be used comparatively to helpevaluate tensiometer design. Greater
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