Time Domain Reflectometry Soil Moisture Measurement for Hydrological Modelling

Transcription

1 Time Domain Reflectometry Soil Moisture Measurement for Hydrological Modelling Sustainable Urban Water Workshop June 4, 2015 Xinbao Yu, Ph.D., P.E., Assistant Professor University of Texas at Arlington Arlington, TX,

2 TDR History Electrical Engineering (cable tester) Detect cable breaks Agriculture and Hydrology Volumetric moisture content Topp- father of TDR Geotechnical Engineering Purdue University Research Group Water content and density, ASTM standard Various Area of Civil Engineering

3 Current TDR Research at UTA Thermo-TDR Soil moisture, dry density, thermal properties Suction-TDR Soil moisture and suction

4 TDR Basics

5 RG58 A TDR System

6 Different TDR devices TDR Instruments (a) 1502C (Tektronix Inc., Beaverton, OR); (b) TRIME-FM (IMKO, Ettlingen, Germany); (c) TRASE System I (Soil Moisture Equipment Corp., Goleta, CA); (d) TDR100 (Campbell Scientific Inc., Logan, UT)

7 TDR probes TDR Instruments (cont.) Different type of TDR probes Design of your own probes for your research needs

8 Data flow A Field Monitoring System

9 What TDR measures? (Apparent) Dielectric constant: K a Electrical conductivity: EC b K a L = L a p 2 Apparent Length, L a EC b 1 V s = 1 C V f Relative Voltage (V) V s /2 V f Scaled Distance (m) Note: dielectric constant K = ε ε o permittivity

10 PMTDR Data Acquisition

11 Signal Analyses Detection of reflections Topp et al. (1982) Baker and Allmaras (1990)

12 APPLICATION EXAMPLES Measurement of Moisture Content and Density Infiltration Monitoring

13 Moisture Content and Density Earthwork control Pavement

14 Current Methods Water Content Oven Dry (ASTM D 2216) Microwave (ASTM D 4643) Nuclear Method (D 3017) Calcium Carbide Gas Pressure (ASTM D 4944) Density Nuclear Method (ASTM D 2922) Sand Cone (ASTM D 1556) Rubber Balloon (ASTM D 2167) Drive Cylinder ( ASTM D2937)

15 Nuclear Method the State of Art Advantages: Easy to use and accurate for most situations Disadvantages: Regulation, nuclear radiation; biased by hydrogen and certain types of ions; different sampling zone (a) Backscatter (b) Direct Transmission Different Modes of Nuclear Gauge (from Hausmaan, 1990)

16 Information from TDR Signal 1.25 (a) Relative Voltage (V) Increasing K a w=2.1% r d=1648 kg/m 3 w=5.2% r d=1655 kg/m 3 w=9.3% r d=1658 kg/m 3 Increasing EC b w=14.0% r d=1653 kg/m Scaled Distance (m) Constant dry density; changing water content

17 Information in TDR Signal 1.25 (b) Relative Voltage (V) Increasing K a w=10.7% r d=1256 kg/m 3 w=10.7% r d=1490 kg/m 3 w=10.6% r d=1583 kg/m 3 w=10.4% r d=1871 kg/m Scaled Distance (m) Increasing EC b Constant water content ; changing dry density

18 Soil Dielectric Constant, K a Soil dielectric constant, K a Predominantly decided by water content Topp s equation relates K a to volumetric water content (widely applied in the current practice) θ = Ka Ka Ka K θ = β a ( 1 n) K β a, s K β a, s β a, air K nk β a, air Air Water Soil Solids

19 Utilizing Electrical Conductivity Air Water Soil Solids K a =81 K a =1 K a =3-5 ρ K w = a + bw a ρ d Soil Phases Dielectric Constant Analogy EC a =0 EC w EC s ρw ECb = c + ρ d dw Electrical Conductivity

20 One-Step TDR Method versus Nuclear Method Water Content Location No. TDR Oven Dry Nuclear Glacial Till, INDOT Division of Research Testing Pavement

21 Water Content Comparisons Gravimetric Water Content by TDR :1 Line 1:1 Line +1% 1:1 Line -1% One-Step Method Oven Dry Gravimetric Water Content

22 Density Comparisons Dry Density by One-Step Method (kg/m 3 ) One-Step Method :1 Line 1:1 +3% Line :1-3% Line Dry Density by Total Density and Oven Dry Water Content (kg/m 3 )

23 Rainfall Infiltration Monitoring Flexible Flat Strip TDR Sensor Coiled TDR strip Sensor Installation TDR strip Sensor in the Field 23 Cross Section View Sampling Area

24 Sensor Calibration Photo of calibration experiment for the TDR strip sensor

25 Seepage Monitoring Photo of the sand tank seepage monitoring Measured TDR signals during the seepage experiment

26 Infiltration Monitoring Measured TDR signals during the precipitation test Test set up for monitoring infiltration process

27 Conclusions TDR can accurately measure soil moisture content and dry density. TDR can monitor rainfall infiltration. TDR probe sensor and flat strip sensor will be used for soil moisture and infiltration monitoring. 27

28 Thank you for listening!