Soil Water Content & Soil Water Potential

Transcription

1 Soil Water Content & Soil Water Potential ICT International

2 Before We Start

3 Outline Soil Water Content Sensors Soil Water Potential Sensors Inferring Soil Water Potential from Plant Water Potential

4 Soil Water Content The total amount of water in the soil Gravimetric versus Volumetric Water Content Gravimetric Volumetric = Gravimetric x Bulk Density = %VWC

5 Relatively Easy to Measure Two common classes of field techniques: Neutron Moisture Measurement Dielectric Measurement All field techniques are indirect: NOTHING measures water in the field directly

6 Dielectric Measurement Method Dielectric permittivity (ε) = the ability to hold charge EM Pulse Voltage Storage Time to Hold Charge Output Dielectric Permittivity

7 Dielectric Measurement Method Dielectric permittivity (ε) = the ability to hold charge ε dry soil = ~4 ε saturated soil = ~40 to 50

8 IMPORTANT! We are measuring the ability of a substance to hold charge, the dielectric permittivity We are NOT directly measuring water content We infer water content from dielectric permittivity

9 Dielectric Sensors Time Domain Reflectrometery (TDR) Standing Wave Technique (MP406) Capacitance (Frequency Domain)

10 Common Capacitance Sensors Inserted into PVC Access Tube Push & Read Sentek Triscan Sentek Diviner Aquaterr Direct Soil Contact Decagon Devices 10HS

11 Sensor Outputs MOST COMMON OUTPUT: % VWC: percent volumetric water content dielectric permittivity Raw output; for example millivolt (mv), raw counts etc

12 Where Does %VWC Come From? Going from dielectric to volumetric water content

13 Where Does %VWC Come From? Raw output mv Output Brown Loam (%) Fancy Equation % VWC = a o + a 1 χ + a 2 χ 2 + a 3 χ 3 = χ χ χ 3 Where χ = MP406 output in volts Lab based measurement

14 Importance of Calibration Relying on factory calibration conversions relies on uniformity across: Dielectric Permittivity Sensor Uniformity Factory Soil Volumetric Water Content

15 Importance of Calibration Relying on factory calibration conversions relies on uniformity across: Dielectric Permittivity Sensor Uniformity Factory Soil Volumetric Water Content ERRORS! ERRORS! ERRORS!

16 Errors in Dielectric Permittivity Bulk density (how much water is pact in ) Dielectric permittivity of water changes with temperature: ε free water = 80 at 20 C High clay content affects bound water Salinity

17 Errors in Sensor Uniformity NB: Check the manufacturer s stated accuracy on spec sheet

18 Errors in Factory Calibration Assuming calibration was done correctly Calibration only done on a generic soil type BULK DENSITY CHANGES! Ask manufacturer about calibration

19 Take Home Message For accurate, precise, repeatable, soil volumetric water content measurements, you must CALIBRATE your sensors.

20 Soil Water Potential Water Potential Definition: The energy required to transport a quantity of water from the sample to a reference pool of pure, free water Pure Water High Potential Other Material Low Potential

21 Soil Water Potential Water Potential Units of Measurement: High Potential Low Potential pf J/kg MPa Bars Field Capacity Wilting Point Air Dry Oven Dry

22 Moisture Retention Curves Soil water content sensors can measure soil water potential.

23 Gypsum Blocks Standard matrix equilibrates with soil Electrical resistance proportional to water content of matrix Inexpensive, but poor stability, accuracy and response Sensitive to salts in soil

24 MPS-2 Capacitance Sensor Standard matrix equilibrates with soil Water content of matrix is measured by capacitance Stable (not subject to salts and dissolution) Good accuracy from to -0.5 MPa

25 TM229 Thermal Matric Sensor Robust (ceramic with embedded heater and temperature sensor) Large measurement range (wet and dry end) Stable (not subject to salts and dissolution Requires complex temperature correction and calibration

26 Tensiometers Equilibrates water under tension with soil water through a porous cup Measures tension of water Highest accuracy of any sensor in wet range Limited to potentials from 0 to MPa Significant maintenance requirements

27 Lab Based Measurements Pressure Extractor WP4C Dewpoint Potentiameter

28 Plant Water Potential PSY1 Stem Psychrometer

29 Pre-Dawn Plant Water Potential Unpublished Data - Courtesy of Alec Downey and Alvaro Arias, species: coffee, location: Costa Rica

30 Pre-Dawn Plant Water Potential 0.0 MPa -0.3 MPa -1.0 MPa -1.4 MPa -1.7 MPa -0.1 MPa 0.0 MPa

31 PSY1 Stem Psychrometer

32 The Psychometric Method Condensation Dew Point = Condensation

33 The Psychometric Method Temperature = 25 C Dew Point = 23 C Relative Humidity = 89 % Water Potential = MPa Temperature = 25 C Dew Point = 16 C Relative Humidity = 57 % Water Potential = MPa

34 The Psychometric Method Ambient Temperature C-thermocouple Dew Point Corrections

35 Step 1: Dry Bulb Reference Temperature difference equals close to zero Electronic Dry Bulb Offset (EDBO)

36 Step 2: ΔT: Chamber vs Stem Temperature ΔT is temperature difference between chamber air and plant stem

37 Step 3: Peltier Cooling Pulse Condense water onto C-thermocouple for 10 seconds

38 Step 4: Wet Bulb Depression Measure how cold the C-thermocouple was in order to condense water How cold was C-thermocouple in order to change water from vapor to liquid phase Dew Point

39 Step 5: Ambient Temperature Measure temperature of chamber for ambient temperature

40 Review: The Psychometric Method Ambient Temperature C-thermocouple Dew Point Corrections

41 Review: The Psychometric Method STEP 1: Dry Bulb Reference STEP 2: ΔT STEP 3: Peltier Cooling Pulse STEP 4: Wet Bulb Depression STEP 5: Ambient Temperature Correction Correction Condensation Dew Point Ambient Temp

42 Exact Relationship Unpublished Data - Courtesy George Koch Northern Arizona University

43 Exact Relationship Stem Water Potential (MPa, PSWC) Temperature Correction 1:1 Relationship Between PSWC and PSY1 Data and information from Mike Dixon, Guelph University Stem Water Potential (MPa, PSY1)

44 ICT International Pty Ltd Solutions for soil, plant and environmental research INTERNATIONAL Phone: Fax: PO Box 503, Armidale, NSW, Australia, 2350