Using NI LabVIEW to automate analyses of water flow to a subsurface drain in a layered soil

Transcription

1 Using NI LabVIEW to autoate analyses of water flow to a subsurface drain in a layered soil CONSTANTIN BUTA*, CARMEN MAFTEI*, CORNEL CIUREA, *COSMIN BUCUR**, LUCIAN BALASA** *Civil Engineering Faculty and, **Mecanical, Industrial and Maritie Engineering Faculty Ovidius University of Constanta 24, Maaia Av., Constanta ROMANIA costi_buta@yaoo.co Abstract: - A drainage experient wit a tank odel was conducted in laboratory to exaine water flow to a subsurface drain and to acquire experiental data about te water flow caracteristics of layered soils to serve as a base for nuerical analyses. In order to reduce analysis tie and iprove data consistency, an autoatic tool (National Instruents LabVIEW software) to acquisition data and teir analyses ave been developed. Key-Words: - subsurface drainage, soil ydraulic properties, pysical odel, LabVIEW software Introduction Te purpose of installing subsurface drains in soils is to reove water in order to iprove soil pysical conditions for environental protection, for agricultural production and for engineering operations. Te proble facing te drainage engineering is terefore one of te pysics of water flow troug bot saturated and unsaturated soils, caused by pysical conditions at te soil surface and at te drain periperies. Knowledge te soil ydraulic properties in order to understanding te beavior of water oveent in soil are iportant for any application of drainage design. Te soil ydraulic properties can be caracterized by te soil water retention curve, θ(), and te ydraulic conductivity curve, K() or K(θ). Tese, can be deterined in situ or by laboratory etods [7]. Te traditionally laboratory etods to estiate te ydraulic conductivity including infiltration or pereaeter tests using constant-ead or falling-ead configurations (Klute, 986) on core saples taken fro te field site. Te pressure plate outflow etod (Gardner, 956) is utilized to estiate te soil water potential during drainage of te soil core. Recent tecnological developents are deonstrated tat te ost effective and efficient eans of easuring soil water potentials are te tensioeters (Hoelscer, 993, Jonson et al., 25). In order to reduce analysis tie and iprove data consistency, an autoatic tool to acquisition tensioeter data and teir analyses ave been developed. We also present a pysical odel to exaine water flow to a subsurface drain and to acquire experiental data. Te ain purpose of te study was to validate a fast and accurate etod for easuring retention curves in wic te LabVIEW progra is used for te coputer anageent of tensioeters fro a pysical odel. 2 Proble Forulation 2. Teory To copute soil water fluxes in unsaturated soils, a coon approac is to nuerically solve te Ricards equation, wic usually requires te paraeterization of te soil water retention curve (Capbell, 985; Jarvis, 99). θ ( ) = K( ) K( ) () t z z were: is te soil water pressure ead (te negative of atric potential, ), t is tie, z is soil dept, K is te unsaturated ydraulic conductivity and θ is te voluetric water content. Inverse tecniques of paraeter estiation are one of ore kinds of soil water penoena observations, followed by repeated solutions of governing equations. Hydraulic soil properties are described by functions. A paraeter optiization process estiates te paraeters. Te unsaturated soil ydraulic properties, θ() and K(), are in ISBN:

2 van Genucten odel Coputational Engineering in Systes Applications (Volue II) generally igly nonlinear functions of te pressure ead. Data fro several tensioeters give sufficient inforation to estiate soil ydraulic paraeters. Different analytical odels are used for te ydraulic properties (Brooks and Corey, 964, van Genucten, 98, Vogel and Cislerova, 988, Durner, 994, Kosugi, 996), te ost popular ones being tose by Brooks and Corey (BC odel), van Genucten (VG odel) and lognoral pore size distribution odel of Kosugi (LN odel). Te Brooks and Corey odel [] is given by: λ S =, >, and S =, (2) e b e b b θ θ were S = r is te effective saturation, e θ θ s r θ s and θ r are te saturated and residual water contents, respectively; α (L - ) is an epirical paraeter wose inverse is often referred to as te air entry value or bubbling pressure, and λ is a pore-size distribution paraeter affecting te slope of te retention function. In tis study, soil water retention was described by te van Genucten odel [3], one of te widely used odels in te specific literature: S = (3) e ( ) n + α were θ θ S = r is te effective saturation e θ θ s r θ - is te voluetric water content (L3L-3); - is te soil water pressure ead (L) and θ, θ, α, n, - are te paraeters to be r s estiated. To reduce te nuber of paraeters and to facilitate te calculation of ydraulic conductivity, te restriction = was adopted. n Te Kosugi odel [5] is given by: ln S = Q (4) e σ were Q(x) is te copleentary cuulative noral distribution function, defined by Q(x)=-Φ(x), in wic Φ(x) is a noralized for of te cuulative noral distribution function. Deterination of unsaturated ydraulic conductivity by laboratory experient is tie consuing, and terefore researcers ave attepted to relate te soil water retention function wit te unsaturated ydraulic conductivity (Fig.). θ z θ ( z) θ Fig. Algorit to relate te soil water retention function wit te unsaturated ydraulic conductivity k k s Darcy's law ( θ ) θr k( θ ) Hydraulic conductivity can be described by te Muale - van Genucten equation (van Genucten, 98)[2]: 2 ( ) K S = K S L S (5) e s e e were θ θ S = r is te effective saturation; e θ θ s r K is te saturated ydraulic conductivity(lt - ) and s L is te paraeter of tortuosity. 2.2 Experiental setup Te drainage pysical odel [4], [7] consists of a etal box wit te frontal wall fro glass, ounted on a etal substrate and aving te following diensions: 3 (L) x (H) x2 (L) (Fig. 2). θs θ ISBN:

3 Pressure control Alientation Acquisition systes for watereter Acquisition systes for tensioeter Watereter Rain siulator (drip syste) PC Drain Piezoetric table Layered soil profile Fig. 2 Laboratory drainage box (pysical odel) Te front side of te odel is transparent to facilitate visual observations and potograpy. Te rain siulator is designed as a drip syste (Gardena icro irrigation syste) wic consists of two lines wit dropping on te line (one dropper as a capacity of 2l/). Tus, precipitation ay be given an intensity of 7/, wic creates water excess. At te sae tie it can be given different volues of water, eaning tat can anage an intensity of 35/. At te botto of te tank odel tere are two drains screwed in te front wall of te tank. One drain is fro corrugated PVC wit diaeter about 5, te oter looks like a rigid spiral tube wit diaeter of 5. Drains are set at 5 fro te tank botto and fro te side wall to iniize te influence of te boundary conditions (infiltration besides te tank walls). Tese two drains will produce te pattern of distance between te drains of 6 and 3 respective, and will function as a drain or two drains. Drains are provided wit valves to control te operating ode of te odel. To prevent clogging, te drains are wrapped wit geotextile. Te tank botto is provided fro to c wit piezoetric outlets protected by a geotextile. Piezoeter capillary tubes ade of PVC are caugt on a piezoetric table. Tis resulted in a total of 54 piezoetric tubes. Te odel is equipped wit a set of eigt tensioeters SKT 85T - SDEC France (Fig. 3) wit pressure transducers (-V), introduced by te back side of te tank and arranged in tree vertical sections of te odel, at equal distances and at various depts (2 c and 4 c). Fig. 3 Tensioeter SKT 85T (SDEC France) Tese eigt tensioeters fitted wit pressure transducers will be attaced to a central box of data acquisition (data acquisition syste wit two SCXI odules fro National Instruents) wic is connected to a coputer (Fig. 4). Fig. 4 Data acquisition syste wit two SCXI odules- 2 - DAQ For te coputer anageent of tensioeters we ave used te LabVIEW progra [6], wose principle scee is presented in Fig. 5 Output voltage of te tensioeters is autoatically converted into units of pressure ead (Pa) using te calibration equations, experientally deterined, based on te certificate issued by te anufacturer. Te easureent range of tis sensor is Pa (bar) to -95 Pa. 3 Proble Solution Using LabVIEW grapical syste design software, we built an application tat consists of a custoized user interface, a display interface for te follow-up, and a recording. Te data we ad to acquire includes a variety of soil ydraulic paraeters. National Instruents LabVIEW software is used for instruent control and autoation. A single prograing code, or virtual instruent in LabVIEW, includes a block diagra and a front panel. Te block diagra contains grapical representations of functions to read fro, or write to an instruent (e.g., pressure transducer or valves). ISBN:

4 Soil water retention curve on pysical odel Horizon I (5-3c) Suction (c) Voluetric water content (c 3 /c 3 ) Tensioeter Fig. 5 LabVIEW progra scee Te front panel (Fig 6) is te user interface, wic as controls for selecting test paraeters tat depend on te test etod selected. Four coonly used MSO testing regies are supported by te LabVIEW control code; for eac, te following data are displayed on te coputer screen and written to a spreadseet file: tensioeter reading, water volue in te burette, elapsed tie, and laboratory roo teperature. Suction (c) Fig. 7 Soil water retention curve Water retention curve - ysteresis effect Voluetric water content (c 3 /c 3 ) wetting drainage Fig. 8 Hysteresis effect on te water retention curve Fig. 6 Front panel for Labview platfor progra - Pressure ead easureent First te syste will autoatically read soil tensioeters. Eac tensioeter uses a vacuu transducer wose output voltage varies linearly wit te aount of tension in te tensioeter. A data acquisition syste (DAQ) is prograed to scan te transducers at selected intervals and record te inforation. Te inforation is ten transferred to a coputer for calculations and plotting of soil water potential wit tie (see Fig. 6). 3. Caracterization and paraeter estiation Wen te soil water potential easureent is cobined wit a soil water content easureent, a soil water retention curve is obtained. To copute soil water fluxes in unsaturated soils, a coon approac is to nuerically solve te Ricards equation, wic usually requires te paraeterization of te soil water retention curve (Capbell, 985; Jarvis, 99). Tere are two ain approaces for obtaining te soil water retention paraeters: te first is to experientally deterine te soil water retention and fit a water retention function to te experiental data (e.g., Brooks and Corey, 966; Capbell, 974; van Genucten, 98; Vogel and Cislerova, 988; Durner, 994; Kosugi, 996), and te second is to derive te soil water retention paraeters fro knowledge of basic soil pysical properties by using pedotransfer functions (Rawls et al., 982; Leij et al., 996; Scaap et al., 999, 2; Acutis and Donatelli, 23). Te paraeters obtained fro te fitting of water retention curves are listed in Tabel and te corresponding water retention curves are sown in Fig.9 and Fig.. Only te results for layer I (fro 5 to 25c) and III (fro 35 to 55c) are sown: te results fro oter soil depts in te stand odel were siilar. ISBN:

5 Table Paraeters obtained fro te fitting of water retention curve for layer I Model Equation Paraeters R 2 λ Brooks and Corey (964) van Genucten S =, > e b S e =, b (98) + ( α) Kosugi (994) Voluetric water content (c3/c3) b S = e n (=-/n) S e θ s =.4328 θ r = 3.2e-7 b = λ =.8778 θ s = θ r = 5.546e-7 α = n = ln θ s = = θ Q r =.5789e-6 σ = σ =.7528 easured BC odel VG odel LN odel Pressure ead (c) Fig. 9 Fitting of water retention curve (soil layer I) Table 2 Paraeters obtained fro te fitting of water retention curve for layer III Model Equation Paraeters R 2 Brooks and Corey (964) van Genucten λ S =, > e b S =, e b (98) + ( α) Kosugi (994) b S = e n (=-/n) S e ln = Q σ θ s =.3875 θ r =.49e-5 b =.23 λ =.4396 θ s =.2932 θ r =.759e-5 α = n =.954 θ s =.375 θ r =.6772e-6 = σ = Voluetric water content (c3/c3) easured BC odel VG odel LN odel Pressure ead (c) Fig. Fitting of water retention curve (soil layer III) Fitting result are presented in Table and 2. 4 Conclusion A fast and accurate etod for easuring retention curves as been developed in wic te LabVIEW progra is used for te coputer anageent of tensioeters fro a pysical odel. Tis LabVIEW-based test syste as been an effective developent and ceckout tool for our laboratory drainage box. Tis type of testing would ave been a difficult task witout LabVIEW, wic siplified data collection and analysis, decreased testing tie, and reduced te aount of operator interruption. LabVIEW also eliinated te need for separate data acquisition, spreadseet, and ateatical software packages. Wit its nuerous features, LabVIEW ade ipleenting tis new test and easureent etod easy. Regarding te paraeters obtained fro te fitting of water retention curve, fro te figure and table we get te ipression tat VG and LN odels give better fitting tan BC odel on average, wile VG and LN odels are not so different in precision of te fitting. Te (θ ), K(θ ) functions ad been establised wit accuracy and te results are global satisfactory. References: [] Brooks, R.H., and A.T. Corey: Hydraulic properties of porous edia Hydrol. Paper 3. Colorado State Univ., Fort Collins, CO, USA, 964. [2] Buta C., Maftei C., - Analysis of ydrodiaics properties of soils in a laboratory tank odel, Lucrări ştiinţifice, Anul XLIX Vol I, Seria Horticultură, Ed. Ion Ionescu de la Brad, Iaşi, 28. [3] van Genucten, M.: A closed-for equation for predicting te ydraulic conductivity of unsaturated soils. Soil Sci. Soc. A. J.44: , 98. ISBN:

6 [4] Kao, C.: Fonctionneent ydraulique des nappes superficielles de fonds de vallées en interaction avec le réseau ydrograpique. Tese pou obtenir le grade de docteur, ENGREF, Paris, 22. [5] Kosugi, K.: Lognoral distribution odel for unsaturated soil ydraulic properties. Water Resour. Res.32: , 996. [6] National Instruents Corp, LabVIEW User Manual,. Austin, Texas, U.S.A. April 23 Edition. [7] Paris, A.: Etude du transfert d eau et de solutes dans un sol a nappe superficielle drainee artificielleent. Tese pou obtenir le grade de docteur, ENGREF, Paris, 24. ISBN: