CAPACITANCE AND TDR TECHNIQUES

are often grouped togetherbecause they both measure thedielectric permittivity of thesurrounding medium. In fact, it

is not uncommon for individuals to confusethe two, suggesting that a given probemeasures water content based on TDR whenit is actually uses capacitance. With that inmind, we will try to clarify the differencebetween the two techniques.

The capacitance technique determines thedielectric permittivity of a medium bymeasuring the charge time of a capacitorwhich uses that medium as a dielectric.

We first define a relationship between thetime, t, it takes to charge a capacitor from astarting voltage, Vi, to a voltage V, with anapplied voltage, Vf

[1]

where R is the series resistance and C the iscapacitance (Fig. 1).

Fig. 1. Simple capacitor circuit.

Short technical report: Florian Wichern, Eike Luedeling,Maher Nagieb and Andreas BuerkertUniversity of Kassel, D-37213 Witzenhausen (Germany)

Materials and methods

Five ECH2O Soil Moisture Probes, connected to a datalogger with periodic readings every 30 min (CR10,Campbell Scientific, Shepshed, England) were tested

on a periodically irrigated, man-made terrace soil profile atthe oasis of Balad Seet (23.19° N, 57.39° E, 980 m a.s.l., Fig. 1and 2) in the Northern Omani Jabal Akhdar Mountains.The selected 4m2 irrigation basin was planted with alfalfa(Medicago sativa L.). Four of the probes were placed at 0.1m

Testing of ECH2O Soil Moisture Probesin Oasis Agriculture of Oman

Register for eligibility toreceive a FREE Em5ECH20 Loggerwhen you order up to10 ECH2O probesbefore November 15th

2003. (See back cover for details.)

DECAGON echo@decagon.com1

Pros and Cons ofCapacitance and TDR

TDR measurements arerelatively insensitive totemperature and salinity.(Still salinity must notcompletely attenuate thereflected TDR signal.)

Capacitance sensors aremuch less expensive andsimpler than TDR becausethey use standard circuitryand time measurement isless demanding.

Capacitance sensors aremore sensitive to salinityand temperature. (This isremedied with a soil specificcalibration.)

Capacitance sensorscontinue to read even whensoil electrical conductivity istoo high for a TDRmeasurement.

Capacitance sensorresolution is better than TDRbecause of noise generatedin the TDR process.

For both methods, inferiorprobe-soil contact (air gaps)from poor probe installationaffect accuracy more thanthe choice method. e

The change in voltage across the capacitorover time, is illustrated in Fig. 2. If theresistance and voltage ratio are held constant,then the charge time of the capacitor, t, isrelated to the capacitance according to

[2]

Fig. 2. Charging of capacitor when switch inFig. 1 closes.For a parallel plate capacitor, the capacitanceis a function of the dielectric permittivity (k)of the medium between the capacitor platesand can be calculated from

[3]where A is the area of the plates and S is theseparation between the plates. Because A andS are also fixed values, the charge time on thecapacitor is a simple linear function (ideally)of the dielectric permittivity of thesurrounding medium

CONTINUED PAGE 7

NotesDECAGON

SPECIAL FALLEDITIONSoils

Capacitance and TDR probe: Differences?In thisissue:

•Capaci-tance &TDRprobe.Which?

•Soilsmoistureprobes inoasis.

•GeeWaterFluxmeter.

•SierraNevadasoils.

•Profilewatercontentdrylandwheat.

•Citrusirrigation.

CONTINUED PAGE 2

to 76, Fig. 4). We were, however, unable to investigate thisinput voltage induced error in more detail. The standarddeviation of the pooled measurements from probe 1 to 4was much higher immediately after irrigation, with 35mmof water than thereafter (data not shown). This might havebeen due to irregularities in the soil structure and surface ofthe plot. It was surprising that the moisture readings ofprobe 1 and 2 seemed to be continuously affected by the dayand night cycle with increasing soil moisture readingsaround noon (Fig. 5). Probes 3 and 4, in contrast, did notshow such differences of which the causes remain unknown.

ConclusionsIf properly connected to a data logger and in close contactwith the soil, the ECH2O Soil Moisture Probes seem to offera low-cost, automatic and sturdy method to monitor soilmoisture under field conditions. For soil moisturemeasurements at lower depth, however, the probes will haveto be placed after digging pits. This will necessarily lead tothe destruction of the soil structure and subsequentalteration of the hydraulic conductivity around and abovethe probes. Our findings confirm the reliability of the probereadings but also indicate that soil-specific calibrations arehelpful to increase their precision which otherwise dependson the factory calibrations. The causes for the differences inthe readings of the four probes also merit furtherinvestigation. e

Omandepth with probe 1 and 2 being placed from the top (soilsurface) and probe 3 and 4 being pushed in laterally after apit had been dug and a small hole was made with a steelblade. Probe 5 was also pushed in laterally from within thepit at 0.8 m profile depth. All probes were positioned about0.2 m inside the border of the irrigated basin. The soiltexture was clayey at the top (0 to 0.6 m) and sandy in thebottom layer (< 0.6m). While placing probe 1 to 4 at 0.1meither from the surface or laterally was relatively quick and aclose connection with the sourrounding soil material waseasily obtained, placement of probe 5 at 0.8m in the bone-dry, compacted soil was almost impossible. After this probehad been finally pushed into the profile, we were unable tobring it into close contact with the surrounding soilmaterial. The subsequently reported data therefore onlyinclude measurements of probe 1 to 4. During an initialcalibration period, the averaged voltage output reading ofprobe 1 to 4 was regressed against the gravimetricallydetermined volumetric water content at 0 to 0.2m. Theregression equation obtained (y = 0.0508x-11.48, r2=0.964;Fig. 3) was then used to derive the estimated volumetricwater readings for all four probes. Soil moisture curves wereplotted for a 27-day period, comprising three irrigationevents.

ResultsProbe readings were parallel and steady as long as batteryvoltage did not fall below 11 V. When this happened,erroneous increases in sensor readings were noticed (days 73

Testing of ECH2O Soil Moisture Probes in Oasis Agriculture of Oman Continued from cover

Figure 1. Overview of the mountain oasis of Balad Seet(Oman). The arrow indicates the position of the experimentalsite.

Figure 2. Soil profile of theexperimental terrace site 30

S

2

FROM GERMANY5 July 2002

To: Dr. Colin S. CampbellEnvironmental BiophysicistDecagon Devices

Dear Colin,

“Many thanks once more foryour valuable help in setting upour soil water measuring datalogger with the ECH20 Probes inOman. The setup is working finefor many weeks now and I amattaching a report on ourresults for your use. We willcertainly recommend theprobes to our colleagues formonitoring surface soil watercontents.

In our experiment there stillseems to be some dilferencesin the readings from thedifferent probes at the samedepth in the same plot. Thesecould reflect natural variationsin the soil’s water holdingcapacity even if this appears abit unlikely given thehomogeneity of the substrate.”

Many thanks and sincerelyyours,Andreas Buerkert

Institute of Crop ScienceUniversity of KasselD-37213 WITZENHAUSEN(Germany)

Figure 5. Time course of soil water content (%) at 0.1mseparated for each of the four probes after an irrigation of35mm. Note the peaks around noon. Balad Seet, Oman(2002).

Figure 4. Time course of soil water content (%) at 0.1m depthafter four irrigation events of 35mm (indicated by arrows) atBalad Seet, Oman (2002).

Figure 3. Regression between ECH20 Soil Moisture Probereadings and gravimetrically determined volumentric watercontent (%).

3

DECAGON

vadose zone water fluxmeter withdivergence control”] designed the tooland tested it in the laboratory and inthe field, in remote areas, ranging fromdesert sites in Nevada to a teaplantation in Sri Lanka.

Measure drainage rates.Quantifying drainage from irrigatedlands, watersheds, and waste sites etc.,is important for a variety of reasons,not the least of which is growingeconomic and environmental concernabout excess use of water andchemicals [e.g., fertilizers/pesticides].Yet there is a scarcity of reliable fluxmeasurements available. The waterfluxmeter described in this researchprovides a direct way to measuredrainage rates in the vadose zonewithout relying on imprecise indirectmethods, which depend on theuncertainties of water contents, waterpotentials or ground-water headmeasurements.

Minimizing drainage the goal.Turfgrass managers, landfill operators,and farmers are all becoming sensitizedto the need for minimizing drainage.Yet there is no standard method todetect or control drainage. The waterfluxmeter provides a first step towardmethodology that may be useful forwide application to routinemeasurement of soil drainage under avariety of soil and climatic conditionsthroughout the world. e

FluxmeterGeeWaterThe

RESEARCHERS HAVE

DESIGNED a simple,inexpensive instrument for measuring water drainage from

agricultural fields that will allowfarmers worldwide to grow crops moreefficiently. The device providesdrainage information that helpsfarmers estimate the amount of waterlost below the root zone and can alsobe used to detect if excess fertilizer orpesticide is being leached fromthe soil.

No farmer speculation.The water “fluxmeter” draws excesswater from the shallow vadose zoneinto a small bucket connected to an

Glendon GeeSenior Staff ScientistBattelle’s Pacific Northwest Div.

Research Interests� Carbon Sequestration in Arid Environments.� Water Harvesting and Phytoremediation Techniques.� Soil Physics Applications to Waste Site Problems.� Remediation of High Nitrate Groundwaters.� Water Balance Modeling.� Sensor Development- Vadose Zone Water Flux Sensors.� Waste Site Alternative Surface Cover Systems.� Vadose Zone Flow and Transport Analysis.Education� Ph.D., Soil Physics, WSU, 1966� B.S., Physics, USU, 1961Affiliations� ARPACS - Certified Professional Soil Scientist.� Soil Science Society of America.� American Geophysical Union.� American Society for Testing & Materials.� Sigma Xi.

Glendon Gee

OTHERRESOURCES:

Glendon,Gee, WaterResourcesResearch,

Vol. 38,No. 8,

10.1029/2001

“The water fluxmeterprovides a first steptoward methodology

that may be useful forwide application to

routine measurement ofsoil drainage under a

variety of soil andclimatic conditions

throughout the world.”

electric wire that records the waterlevel, producing a record of the waterdraining from the land. Gee et al.[“A

4

DECAGON

SERVICE TO SOILS RESEARCHINNOVATIVE INSTRUMENTS

20YEARS

s o i l s

D E C A G O N : O U R R O O T S A R E I N S O I L S

The Gee Water Fluxmeter isconstructed of 3 pieces of PVCpipe. This Fluxmeter is nowinstalled at a Decagonexperimental site. It can beconnected directly to Decagon’sEm5 or Em5R loggers. Em5Rallows radio retrieval of data.

top opening

width

21cm

length ofdivergence

control zone

59cm

total height

169cm

length ofbase

110cm

bottomopening

width

16cm

Dimensions

Z DECAGON installed a water flux

meter (WFM) in a turf grass field to

see if there was any water moving

beyond the root zone under our

irrigation scheme. For the first three

weeks, the WFM did not register a

single tip. Although we were confident

Mini-Flux meterA

Divergence control zoneB

Funnel 1- conicaldirector conducts waterto inert fiberglass wickand through therestriction channel.C

Funnel 2- directscollected water to thetipping bucket gauge.D

Funnel 3- directs tippingbucket water to thewater supply collectionbottle.E

Collection tube back tosurface.F

The field readout anddata collection device isan EM5/Em5R.

A

B

C

D

F

the system was in working order, we

still wondered how much water would

have to be applied for it to move

beyond the root zone of the grass. Our

answer came when a mistake was

made watering a nearby tree. The

graph shows the cumulative

infiltration measured by the WFM

during a 10 h accidental flood

irrigation event.

The Gee W

ater Fluxmeter

5X The apparatus can be placed in a normal hole and can be used for any soil type or climactic condition.

Sierra Nevada10 May 2002To: wp4@decagon.comFrom: Liukang XuSubject: excellent instrumentWP4

Dear Sir:

We just brought a WP4 abouttwo weeks ago, I used it this

week to obtain soil moistureretention curve. And it turnedout that the result is excellent.

See the attached results.

The texture of the soil is about 48% sand, 42%silt and 10% clay, and sample was from the foothill of Sierra Nevada of California. Also this machine is very easy to use and verystable. Every morning I check it with the standardKCL solution, never need to adjust the calibrationin the past week. My supervisor, Dr. Dennis Baldocchi, told methat we should share this good result with you. Also I would like to thank Dr. C. S. Campbell forhis valuable information on how to use thismachine and how to prepare the samples.Thanks.

Liukang Xu PhDEnvironmental Science, Policy and

ManagementUC Berkeley

Berkeley, California

WASHINGTONECH2Oprobesprofile watercontent indrylandwheatsystems.Armen Kemanian,Ph.D. candidate, BiologicalSystems Engineering,Washington State University

In the fall of 2000, we starteda research project to monitorspatial and temporal variabilityof soil water content. Weinstalled 5 research sitescontaining water content,temperature, and radiationsensors in a dry land system ofwheat and legumes. TheECH

2O water content sensors

were installed vertically atdepths of 1 to 5 feet in the soilto give us a profile of the soilwater content.

The ECH2O probes were

useful in showing the earlierdepletion of the water in thesouthern slopes as comparedwith tophills (or summit) andnorthern slopes, and werepowerful tools for trackingwater content deep in theprofile and then for doingwater balances.

Work was done by Agronomy Department

Cunningham Farm of WSU-USDA.

m

m

j Soil Texture—The

“teaching” textural triangle

allows you to select an area

on the triangle and

determine the % sand,

silt and clay, together

with the textural

classification.

6

XSoil moisture release curvefrom the Tonzi Ranch. TakenFeb. 8, 2002 by Dr. DennisBaldocci.

ZSome of theresearch sites forthe ECH2O probes

on theCunninghamFarm, WSU.

DECAGON

FloridaCitrusResearch

[4]

Soil probes are not parallel plate capacitors, but the linear relationshipshown in Eq. [4] still holds.Time domain reflectometry (TDR) determines the dielectricpermittivity of a medium by measuring the time it takes for anelectromagnetic wave to propagate along a transmission line that issurrounded by the medium. The transit time (t) for anelectromagnetic pulse to travel the length of a transmission line andreturn is related to the dielectric permittivity of the medium, k, by the

following equation

[5]

where L is the length of the transmission line and c is the speed oflight in free space (3 x 108 ms-1). Thus, the dielectric permittivity iscalculated

[6]

The dielectric constant is therefore proportional to the square of thetransit time. e

What is the difference between acapacitance probe and TDR probe? continued from cover

FOR THE SOIL used in [our]calibration, Field Capacity isapproximately 7.5 %

volumetrically. Wilting point isassumed to be approximately 1%volume but could be as high as 1.5%.

This is an extremely narrow range ofvalues, making a very low value forplant-available soil water. This is why,although Florida gets a large (>50inches) amount of average annualrainfall, we need probes that offer ahigh degree of accuracy in this narrowrange during our dry season of October-June.

Thank you for the opportunity towork with your product. The [ECH20]

Scheduled Irrigation for citrus.

k Upper and lower dotted lines represent field capacityand permanent wilting point, respecitively, in thesand soil.

sensors respond very well in our sandsoils. They would make great sensorsfor scheduling irrigation andmonitoring soil moisture.

I have attached a file containing thefield calibration data and a plot of mydata and the linear calibrationprovided. The calibration providedextremely low values and good linearfit was made which will make [ECH20]sensors very useful in our soils.

Kelly T. MorganUniversity of FloridaCitrus Researchand Education CenterLake Alfred, FL

7

BRegister for eligibility to receive a FREE Em5 ECH20 Logger

when you order up to 10 ECH2O probes before November 15th 2003. (See back cover for details.)

XKelly T. Morgan (L) and Colin Campbell (R)

during a Decagon customer visit andproduct demonstration at the University ofFlorida Citrus Research Center.

Special offer is good only when youregister at a Decagon ASA booth.

Indianapolis, IndianaNovember 10–14booths 320, 401, & 403 C

DECAGON800-755-2751

fax 509-332-5158echo@decagon.com

www.decagon.com/echo/

We cannot accept payment at the booth (rules).Limitation of 5 registrations per individual

(non-transferable).

FREE$375LOGGER

5 channels, no radiowith purchase of10 ECH2O probes.

Register for eligibility to receive aFREE Em5 ECH20 Loggerwhen you order up to10 ECH2O probesbefore November 15th 2003.

Prsrt StdU.S. Postage

PAIDSpokane, WAPermit #589

DECAGON950 NE Nelson CourtPullman, Washington 99163

8

Register for eligibility to receivea FREE Em5 ECH20 Loggerwhen you order up to10 ECH2O probesbefore November 15th 2003.

+(See

details

below))

Soils

Note

s