HYDRAULIC LOAD CELL LYSIMETERS

H.C. Korven

Member CSAE

Research Station

Research Branch, Canada Agriculture

Swift Current, Saskatchewan

W.L. Pelton

Research Station

Research Branch, Canada Agriculture

Swift Current, Saskatchewan

INTRODUCTION

A study designed to measure the effectof advection(2) required the measurement of evapotranspiration at seventeensites. Since lysimeters measure evapotranspiration directly, it was decided toinstall a lysimcter at each site. Becauselow cost of each unit was paramount in amultiple lysimeter installation, thehydraulic load cell lysimeter was selected.The purpose of this paper is to describeand compare the performance of twotypes of hydraulic load cell.

DESIGN

A good review of lysimeters, includingthe hydraulic load cells, has been provided by Tanner (6). The first attempt bythe authors was to support the lysimeteron three bags made from 30-cm diameterbutyl-nylon pipe connected to a 3-welldifferential mercury-water manometer (6)located in a manhole below ground surface. Since it was difficult to space threerectangular bags under a circular tank0.86 m in diameter and to keep a supplyof mercury in the reservoirs if the tankleaned, it was decided to simplify thedevice by using a coil of 5-cm diameterlay-flat butyl tubing (Figure 1) connectedto a single well differential mercurymanometer (7). The differential mercury-water manometer was temperaturesensitive, and correcting for temperatureeffects did not prove successful.

Figure 1. Hydraulic system of 5-cm diameterlay-flat butyl tubing in a coil.

RECEIVED FOR PUBLICATION DEC. 2, 1971

33

Tube Load Cell and Standpipe

In the spring of 1968 the mercury-water manometer was replaced with astandpipe, and the butyl tubing wasfolded back and forth (Figure 2) to increase the area of contact so the height ofwater column would be about 2 m.

Temperature effects were corrected bymeans of a dummy standpipe (1) andboth the lysimeter and dummy standpipewere housed in a shaded box. This systemproduced more reliable readings than thecoiled tube and mercury-water manometer, but it was difficult and time-consuming to remove the air from thetube. Another weakness suggested is theproblem of the folds in the tubing causingchanges in the area of contact (7).

\

Figure 2. Hydraulic system of 5-cm diameterlay-flat butyl tube folded back andforth.

Diaphragm Load Cell and Standpipe

One means of overcoming the weaknesses of the tube load cell is to replace itwith a diaphragm or "waffle" such as theone described by Middleton and Jensen(4). Middleton's waffle was constructedcompletely of rubber and was 5 cm high.The design reported herein differs fromMiddleton's in that the rubber sheets

were clamped between metal hoops andwas only 1.2 cm high.

Each diaphragm load cell was made oftwo sheets of 1.6 mm nylon reinforcedbutyl rubber clamped between threemetal hoops (Figure 3). The top andcentre hoops were made of 12.7 mm x

12.7 mm square rod, and the bottomhoop of 3.2 mm x 12.7 mm flat iron. Theoutside diameter of the hoops was 12.7mm less than the inside diameter of the

outside tarik. The inlet was located in the

centre hoop, and the outlet, or air escapevent, in the top shev... The diaphragmload cell was connected to 6.4 mm

copper tubing that ran '- 'ontally for adistance of 3 m and the.i up out of theground. The top 0.75 m of the standpipewas glass of the same diameter for sighting purposes. The standpipe was supported in 3.2 mm x 12.7 mm x 25.4 mmchannel to which was cemented a metric

tape. The dummy tube for temperaturecorrection as suggested by Hanks andShawcroft (1) was also supported in asimilar channel iron complete with ametric tape.

Figure 3. General view of diaphragm.

INSTALLATION

Filling the Lysimeters

The procedure during the initial fillingof the lysimeters was as follows. A suction plate was centred in the bottom ofthe lysimeter in a sand envelope about 5cm thick. The suction plate consisted of aporous ceramic disc 30 cm in diameterbacked by a neoprene diaphragm towhich was attached a thick-walled suction

hose with an inside diameter of 1 cm. The

plate was placed horizontally in the sandlayer, with the ceramic disc up. The hosewas led along the floor and then brought

CANADIAN AGRICULTURAL ENGINEERING, VOL. 14, NO. 1, JUNE, 1972

up along the inside of the tank so that apump could be attached to remove anybuildup of water in the bottom of thelysimeter. Soil from a hole of the samedimensions as the inside of the lysimeterwas removed in 15-cm layers and stockpiled on plywood sheets. Each layer,starting with the lowest, was placed in thelysimeter and tamped to the same volumeas it had occupied in the field.

Hydraulic Load Cell Installation

The tube load cell was filled withliquid and the air removed before beingfolded back and forth in the bottom ofthe outside tank which was a corrugatedCulvert 0.91 m in diameter and 1.4 mlong with a 3-mm thick steel sheet weldedin the end to form the floor.

The diaphragm was lowered into thetank, connected to the copper tubing andfilled with liquid from the top of thestandpipe from a pressurized container.During filling, the centre of the top sheetwas held up so the air could escapethrough the vent. When the liquid levelreached the vent, the pressure was released, and the top sheet lowered slowlywhile the vent was being closed.

A sheet of plywood 2.5 cm thick wasplaced on top of the tube or diaphragm.A hole was cut in the centre of the

plywood sheet to provide clearance forthe vent in the top sheet of the diaphragm.

The inside tank was then lowered(Figure 4), centred, and held in a verticalposition with three equally spaced horizontal wire ties. The inside tank was alsoof corrugated galvanized iron 0.86 m indiameter x 1.22 m deep and weighedbetween 1132 and 1225 kg. The cost ofeach complete lysimeter installation(Figure 5) was about $250.

THREE EQUALLY SPACED

HORIZONTAL WIRE TIES-

1968 and 1969. In 1970 and 1971 thecrop studied was wheat (Manitou, Triti-cum aestivum) so calibrations were conducted in the spring and after harvest.Since three caUbrations were available in1968 and 1969, the spring 1971 calibration data were grouped with the twocalibrations in 1970 to make a group ofthree calibrations.

MANOMETER

(6.4 mm DIA GLASS)

-NOTE- METRIC TAPEBEHIND BOTH TUBES.

mmw^v^rw^r: %- ws^mw^

OUTSIDE TANK —

II cm OIA, X 130 cm

INSIDE TANK

86 cm OIA. x 122 cm

HYDRAULIC LOAD CELL'

(TUBE OR 0IAPHRAGM)

,DUMMY TUBE

LYSIMETER INSTALLATION

Figure 5. Lysimeter installation.

ASSESSMENT OF PERFORMANCE

During the first year of assessment,1968, the 16 lysimeters were supportedon tube load cells filled with water.

Diaphragm load cells were installed under10 of 16 lysimeters in the spring of 1969and the liquid was changed from water toa 1 : 1 antifreeze — water solution in all

lysimeters. The diaphragm load cellproved easy to install and functionedsatisfactorily so in the spring of 1970 theremaining six lysimeters were equippedwith diaphragms filled with antifreezesolution.

Figure 4. Lysimeter being lowered into outsidetank.

Sensitivity

The sensitivity of the lysimeter wasmeasured by calibrating each lysimeter inthe field. The change in liquid level in thestandpipe was read for each weight onboth adding and subtracting sequences.Six 11.3-kg lead weights were used whichwere equivalent to about a 12-cm depthof water on the lysimeter.

The lysimeters were calibrated in thespring and after the first and second cutsof alfalfa (Rambler, Medicago media) in

The calibration data were more vari

able when the lysimeter was supported onthe tube than when on the diaphragm(Table I). The change of liquid fromwater to 1 : 1 antifreeze — water mixture

decreased the sensitivity, but the standarddeviation remained about the same. Since

the liquid level in the two standpipes canbe read to 0.5 mm, a change in weightequivalent to a depth of water of 1.0 mm± 0.04 could be measured.

Soil Characteristics

The soil moisture and temperature inthe field and lysimeter were measuredand compared at one site (No. 16) during1971, by the psychrometer technique (3,5). The psychrometer units were installedin the spring of 1971 at depths of 15, 45,75, and 105 cm in the lysimeter and atdepths of 15, 30, 60, 90, and 120 cm inthe field. The psychrometers were placedat the same depths in the field as thoseused when measuring soil moisture by theneutron meter, whereas it was notpossible to place a unit at the 120-cmdepth in the lysimeter because of thesand layer and porous plate. The units in

CANADIAN AGRICULTURAL ENGINEERING, VOL. 14, NO. 1, JUNE, 1972 34

TABLE I COMPARISON OF TUBE AND DIAPHRAGM LOAD CELLS

Year 1968 1969 1970 & 71

Number of lysimeters 16 6 10 16

Type of hydraulic load cell tube tube diaphragm diaphragm

Fluid water 1 : 1 antifreeze - water

Mean sensitivity of three calibrations 0.94 0.98 1.04 1.05

(mm water/mm change in manometer)

Standard deviation ±0.07 +0.06 ±0.04 ±0.04

Standard deviation/mean, % 7.4 6.1 3.8 3.8

the lysimeter were therefore raised 15cm, except for the top one which wasplaced at 15 cm. Li comparing the two,the average of the 30- and 60-cm depthsin the field was compared to the 45-cmdepth in the lysimeter.

The soil tension data as measured bythe psychrometer units are not includedbecause values as high as 48 bars weremeasured, indicating improper functioning of the equipment. There was, however, a tendency for the values to behigher (soil drier) in the lysimeter than inthe field. The soil in the lysimeter wasgenerally warmer than the soil in thefield, with a greater difference in temperature being exhibited in the early and late

portions of the season. The 75-cm depthwas selected as typical (Figure 6).

Water Buildup

The lysimeters were checked forexcess water by pumping the porous platein the spring and after heavy irrigations.This pumping indicated little, if any,buildup of a water table in the lysimeters.

Crop Yield

The alfalfa hay yield was based on twocuts per season and the wheat yields ontotal production (straw and grain). Threesquare-meter samples were taken in thevicinity of each lysimeter to comparewith the yield from each lysimeter.

16-

Soil Temperature

75 cm depth Lysimeter

14

12

°c10

10 20

JUNE

30 10 20

JULY

30

v Field

9 19

AUGUST

Figure 6. Soil temperature graphs for the 75-cm depth in the lysimeter and field.

The yield data showed differences butnot in a consistent direction. There was atrend for the yield of alfalfa from thefirst cut to be higher from the lysimeterthan from the field and the reverse in thesecond cut. The mean ratio lysimeteryield : field yield for the 13 irrigated sitesvaried from 1.00 to 1.22 for the first cutand 0.50 to 0.81 for the second cut. Themean ratio for the total yield varied from0.89 to 0.91. The mean ratio in 1970 forwheat was 0.82.

These differences in yield may not beentirely due to growing conditions in thelysimeter. The lysimeters had to beseeded by hand whereas the field wasseeded by machine. The better yield inthe lysimeters at the first cut may be dueto the greater difference in temperaturein the early part of the season.

CONCLUSIONS

The diaphragm with 1 : 1 antifreeze -water mixture is recommended when low

cost is critical for a multiple lysimeterinstallation because:

1. It is easier to remove the air from thediaphragm than from the tube duringthe filling procedure.

2. The data are less variable when usingthe diaphragm than when using thetube.

3. The use of an antifreeze-water solutionextends the length of season for readings and simplifies field operationsbecause the system need not bedrained for winter.

The soil in the lysimeter was slightlywarmer than the soil in the field and

there was no buildup of a water table inthe lysimeter.

SUMMARY

Two hydraulic-type lysimeter systems,a five-centimeter lay-flat butyl tubing anda diaphragm of nylon reinforced butylrubber sheets clamped between ironhoops, are described and evaluated. Theassessment covers a four-year period(1968-71) and includes field calibrationswith 11.3-kilogram weights, the use ofwater versus antifreeze-water solution,and soil temperature measurements bythe psychrometer method during the lastyear of study.

The soil in the lysimeter was warmerthan the soil in the field and there was no

indication of excess water in the lysimeter. The ratio of lysimeter yield : fieldyield varied from 0.50 to 1.2. The dia-

35 CANADIAN AGRICULTURAL ENGINEERING, VOL. 14, NO. 1, JUNE, 1972

phragm is recommended over the tubebecause there was less variability in thereadings. The use of the antifreeze-watermixture extends the length of season forreadings and simplifies field operationsbecause it is not necessary to drain thesystem. The diaphragm with the antifreeze solution measures evapotranspira-tion with a sensitivity of one millimeterdepth of water on the lysimeter to within± 0.04 millimeter.

1.

REFERENCES

Hanks, R.J. and R.W. Shawcroft.1965. An economical lysimeter for

evapotranspiration studies. Agron. J.57: 634-636.

2. Korven, H.C. and W.L. Pelton. 1967.Advection in Southwest Saskat

chewan. Can. Agr. Eng. 9: 88-90, 6124.

3. Korven, H.C. and S.A. Taylor. 1959.The Peltier effect and its use fordetermining relative activity of soilwater. Can. J. Soil Sci. 39: 76-85.

4. Middleton, J.E. and M.C. Jensen. 7.1969. Hydraulic weighing lysimeter.Wash. Agr. Exp. Sta., Coll. of Agr.,Wash. State Univ. Circ. 506.

5. Rawlins, S.L. and F.N. Dalton. 1967.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 14, NO. 1, JUNE, 1972

Psychrometric measurement of soilwater potential without precisetemperature control. Soil Sci. Soc.Amer.Proc. 31: 297-301.

Tanner, C.B. 1967. Measurement ofevapotranspiration, p. 534-574. InR.M. Hagan, H.R. Haise and T.W.Edminster (ed.) Irrigation of agricultural lands. Agronomy 11, AmericanSociety of Agronomy, Madison,Wisconsin.

van Bavel, C.H.M. and L.J. Fritschen.Construction and evaluation of asimple hydraulic lysimeter. Mineo-graphed report, U.S. Water Conservation Laboratory, Tempe, Arizona,U.S.A.

36