1、Designation: D3404 91 (Reapproved 2013)Standard Guide forMeasuring Matric Potential in Vadose Zone UsingTensiometers1This standard is issued under the fixed designation D3404; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the yea
2、r of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () 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 andpr
3、actical 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 as t
4、hestandard. 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 deter
5、mine 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 professional
6、judgment. 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 projects
7、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 and t
8、he 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-merou
9、s 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 satu
10、ratedhydraulic 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 pored
11、iameters. 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,extent o
12、f 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 dataalone c
13、an 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 thes
14、esituations.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 used1This guide is under the jurisdiction ofASTM Committee D18 on Soil and Rockand is the direct responsi
15、bility of Subcommittee D18.21 on Groundwater andVadose Zone Investigations.Current edition approved June 15, 2013. Published June 2013. Originallyapproved in 1991. Last previous edition approved in 2004 as D3404 91 (2004).DOI: 10.1520/D3404-91R13.2The boldface numbers in parentheses refer to a list
16、of references at the end ofthis standard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1to measure matric potential between the values of 0 and -867cm of water; this range includes most values of saturation formany soils (12).4.3 Ten
17、siometers directly and effectively measure soil-watertension, but they require care and attention to detail. Inparticular, installation needs to establish a continuous hydraulicconnection between the porous material and soil, and minimaldisturbance of the natural infiltration pattern are necessary f
18、orsuccessful installation. 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 orr
19、emote areas, measurement 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 effortan
20、d money.4.5 Pressure sensors 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-tran
21、sducerspecifications, 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 The
22、ory5.1 In the absence 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 52K1Z! (1)where:q =the specific
23、flow,FLTG,K =the unsaturated hydraulic conductivity,FLTG, = the matric potential of the soil water at a point, L,Z = the elevation at the same point, relative to some datum,L, and = the gradient operator, L1.The sum of + Z commonly is referred to as the hydraulichead.5.2 Unsaturated hydraulic conduc
24、tivity, K, can be expressedas a function of either matric potential, , or water content, L3 of water/L3of soil, although both functions are affected byhysteresis (5). If the wetting and drying limbs of the K()function are known for a soil, time series of onsite matric-potential profiles can be used
25、to determine which limb is moreappropriate to describe the onsite K(), the correspondingvalues of the hydraulic-head gradient, and an estimate of fluxusing Darcys Law. If, instead, K is known as a function of ,onsite moisture-content profiles (obtained, for example, fromneutron-scattering methods) c
26、an be used to 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,dKdanddKdare large, withincertain ranges of or , making estimates of K particularlysensitive to onsite-measurement
27、 errors of or . (Onsite-measurement errors of also have direct effect on ( + Z)inDarcys Law). Other sources of error in flux estimates canresult from inaccurate data used to establish the K()orK()functions (accurate measurement of very small permeabilityvalues is particularly difficult) (16); use of
28、 an analyticalexpression for K()orK() that facilitates computersimulation, but only approximates the measured data; aninsufficient density of onsite measurements to define ad-equately the or profile, which can be markedly nonlinear;onsite soil parameters that are different from those used toestablis
29、h K()orK(); and invalid assumptions about the stateof 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 refinement of the
30、theory 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=the tension of
31、an elemental volume of fluid,FMLT2G,PAT= the absolute pressure of the standard atmosphere,FMLT2G, andPF= the absolute pressure of the same elemental volume orfluidFMLT2G.Soil-water tension (or soil-moisture tension) similarly isequal to the difference between soil-gas pressure and soil-waterpressure
32、. Thus:TW1PG5 PW(3)where:TW= the tension of an elemental volume of soil water,FMLT2G,PG= the absolute pressure of the surrounding soil gas,FMLT2G, andD3404 91 (2013)2PW= the absolute pressure of the same elemental volume ofsoil water,FMLT2G.In this guide, for simplicity, soil-gas pressure is assumed
33、 tobe 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:1.000 bar 5 100.0 kPa 5 0.9869 atm5 (4)1020 cm of water at 4C51020 g per cm2in a standardgravitational field.A standard gravitational field is assum
34、ed 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 unsatu-rated soil arises from the attraction of the soil-particle surfacesfor wa
35、ter 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 a thoroughdiscus
36、sion 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 surface (usually a ce
37、ramic 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 normaloperation, the saturated pores o
38、f the cup prevent bulk move-ment of soil 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 pressure is trans
39、mittedthrough 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 PW5Hg2 H2O!r 2 H2Oh1d! (5)wh
40、ere:TW= the soil-water tension relative to atmosphericpressure, in centimetres of water at 4C,PA= the atmospheric pressure, in centimetres of water at4C,PW= the average pressure in the porous cup and soil, incentimetres of water at 4C,Hg= the average density of the mercury column, in gramsper cubic
41、centimetre,H2O= the average density of the water column, in gramsper cubic centimetre,r = the reading, or height of mercury column above themercury-reservoir surface, in centimetres,h = the height of the mercury-reservoir surface aboveland surface, in centimetres, andd = the depth of the center of t
42、he cup below land surface,in centimetres.5.7 Although the density of mercury and water both varyabout 1 % between 0 and 45C, Eq 8 commonly is used withHgand H2Oconstant.5.7.1 Using Hg= 13.54 and H2O= 0.995 (the median val-ues for this temperature range) yields about a 0.25 % error (1.5cm H2O) at 45C
43、, for Tw 520 cm H2O. This small, butneedless, error can be removed by using the following densityfunctions:Hg5 13.595 2 2.458 31023T! (6)andFIG. 1 Enlarged Cross Section of Porous Cup-Porous MediumInterfaceD3404 91 (2013)3H2O5 0.999714.879 31025T! 2 5.909 31026T!2(7)where: Hgand H2Oare as defined ab
44、ove, 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 thermistor incontact with the tubing, buried at about 45 % of the depth ofthe porous cup. Air temperature is an adequate estimate forexposed segm
45、ents.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 offset the effects ofaltitude, the height of the gage above the porous cup (see Fig.3(b), and changes in the internal characteristics of the ga
46、gewith 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 container of water. This settingis done at the altitude at which the tensiometer will be used andit needs to be repeated periodically after ins
47、tallation either byremoving the tensiometer from the soil or by unscrewing thegage and measuring a tension equal to that used in the originalcalibration. The gage then reads directly the tension in theporous cup. Use of a vacuum gage without an adjustable zeroreading could result in inaccurate measu
48、rements because thezero reading could become negative and, therefore, would beindeterminate.5.9 Pressure transducers convert pressure, or pressuredifference, into a voltage (or current) signal. The pressuretransducer can be connected remotely to the porous cup withtubing (22, 24), attached directly
49、to the cup (19, 32),ortransported between sites (24).An absolute pressure transducermeasures the absolute pressure (PP) in its port.Agage pressuretransducer measures the difference between ambient-atmospheric pressure (PA) and the pressure in its port (PP),FIG. 2 Three Common Types of Tensiometers: (a) Manometer;(b) Vacuum Gage; and (c) Pressure TransducerFIG. 3 Porous-Cup and Tube DesignsD3404 91 (2013)4known as gage pressure. When PPresponse time2.Nonetheless, as defined here can be used compara
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