Outline 12/9/2010. Two Variables are Needed to Describe the State of Water. Definition: Volumetric Water Content Definition: Gravimetric water content

Transcription

1 12/9/21 Generating Soil Moisture Characteristic Curves with Vapor Pressure Methods Douglas R. Cobos, Ph.D. Decagon Devices and Washington State University Outline Introduction Defining water content, water potential and Soil Moisture Characteristic Curve (SMCC) Measuring water potential and water content Generating SMCC Sample preparation Data analysis Applications Automated SMCC instruments Aquasorp for dry soils Hyprop for moist soils Two Variables are Needed to Describe the State of Water Soil Moisture Characteristic Curve (SMCC) Water content Quantity Extent Heat content Charge and Related Measures and and Water potential Quality Intensity Temperature Voltage Relates water content to.5 Clay water potential in a soil Loam.4 Different for each soil Sand.3 Used for.2 Converting from water content to water potential.1 (and vice versa) Finding plant available water Water Potential (kpa) Estimating specific surface area of soil Predicting soil swelling Many other uses er Content Wate Definition: Volumetric Water Content Definition: Gravimetric water content Separate into constituent parts V w V T Air Water is volumetric water content (VWC), V w is the volume of water V T is total sample volume 15% 35% 35% VWC Gravimetric water content (w) w m w m m d m mass w water d dry solids Soil 5% 1

2 12/9/21 Definition: Water Potential Energy required, per quantity of water, to transport, an infinitesimal quantity of water from the sample to a reference pool of pure, free water Water Potential: important points Energy per unit mass, volume, or weight of water We use units of pressure (Bars, MPa, kpa, meters H 2 O,) Differential property A reference must be specified (pure, free water is the reference; its water potential is zero) The water potential in soil is almost always less than zero Water Potential is influenced by: Binding of water to the soil matrix Solutes in the water Pressure on the water (hydrostatic or pneumatic) Position of water in a gravitational field Only the first two can affect the SMCC Total = Sum of Components = m + o + g + p m matric adsorption forces o osmotic solutes g gravitational position p pressure hydrostatic or pneum. Measuring water potential Liquid equilibrium techniques Work best in the wet range of water potential Only measure matric potential Important for understanding liquid water flow Tensiometer ( to -.85 MPa) Pressure plates ( to -.5 MPa) Water Potential and Relative Humidity Relative humidity (h r ) and water potential (Ψ) related by the Kelvin equation: RT ln M w h r R is universal gas constant M w is molecular mass of water T is temperature t Condition Water Potential (MPa) Relative Humidity (h r ) Pure, free water 1. Field Capacity Permanent wilting point

3 12/9/21 Vapor Equilibrium Methods Only work well in the dry end of soil water potential Give total water potential (matric + osmotic) Best for plant water relations or other biological systems In non-saline soils, osmotic component is negligible, so total water potential matric potential Vapor Equilibrium Methods Thermocouple psychrometer Dew point hygrometer Chilled Mirror Dew Point Method Cool mirror until dew forms Detect dew optically Dew Point Method for Measuring Water Potential Gives total water potential (matric + osmotic) Measure mirror temperature Measure sample temperature with IR thermometer Water potential is approximately linearly related to Ts - Td Mirror Optical Sensor Infrared Sensor Sample Fan Range is to -3 Mpa Accuracy is about ±.5 MPa WP4C - Verification with standards Measuring Water Content: Gravimetric (w) Technique Measured Su uction (MPa) measurements 1:1 line Suction of Standard (MPa) w = (moist soil mass dry soil mass)/dry soil mass 3

4 12/9/21 Field measurements of water content All field methods measure Volumetric water content Volumetric vs. Gravimetric Water Content Volumetric Water Content (VWC) Water volume per unit total volume Gravimetric Water Content Soil bulk Density, b (GWC) Water weight m d per unit dry soil weight b VT w b w Outline Introduction Defining water content, water potential and Soil Moisture Characteristic Curve (SMCC) Measuring water potential and water content Generating SMCC Sample preparation Data analysis Applications Automated SMCC instruments Aquasorp for dry soils Hyprop for moist soils 1. Air dry soil 2. Grind and/or sieve with 2 mm sieve (if necessary) 3. Fill 1-12 stainless steel sample cups ~1/2 full of dry soil - Weigh out same mass of soil in each cup - ~2-7 g depending on density 4. Add ascending amount of DI water to each sample -, 1, 2, 4, 6, 8, 1, 14, 18, 22 drops of water works well 4

5 12/9/21 5. Mix samples thoroughly 6. Cap samples and allow to equilibrate at least overnight 7. Done! Preparing samples: the precise method Calculate amount of water to add Must know soil texture Clay Loam Sand Water Potential (kpa) Water Content Texture w 1.5 w ad Sand.8.3 loamy sand.15.5 sandy loam sandy clay loam Loam sandy clay silt loam Silt clay loam silty clay loam silty clay Clay Preparing samples: the precise method Must use larger samples Subsample into stainless steel cups Detailed instructions: Water-Content-of-Soils-with-the-WP4C.pdf Measure water potential with the WP4C Measure the water content Dry in a 15 C oven for 24 hours Insert sample Seal chamber Weigh moist samples Weigh dry samples Wait ~5 min. and read the result w = (moist soil mass dry soil mass)/dry soil mass 5

6 ( 12/9/21 Data formatting Plot water potential vs..14 water content.12 Log 1 water potential.1 vs. water content.8 becomes a straight line.6 X intercept is oven dry water potential (-1 MPa) Water Content Water Potential (-MPa) Does Sample Density Matter? In the dry range, no Density effects negligible on samples drier than -.1 MPa Water is held mainly on particle surfaces and in very small pores not affected by packing In the wet range, yes Packing can strongly affect water potential because it affects sizes of capillaries holding the water WP4C pushes into this range WP4C improved wet end accuracy WP4C improved wet end accuracy rement error (kpa) Measu WP4C wet end accuracy KCl solution water potential (kpa) WP4C #1 WP4C #2 WP4C #3 WP4C #4 c water content gravimetric Soil B water potential ( kpa) WP4C dewpoint T5 tensiometer etric water content gravime WP4C improved wet end accuracy Soil B water potential ( kpa) WP4C dewpoint T5 tensiometer Outline Introduction Defining water content, water potential and Soil Moisture Characteristic Curve (SMCC) Measuring water potential and water content Generating SMCC Sample preparation Data analysis Applications Automated SMCC instruments Aquasorp for dry soils Hyprop for moist soils 6

7 12/9/21 What can we do with the SWCC? Estimate water potential from a measured water content Estimate water content from a measured water potential Estimate plant available water in soil Estimate specific surface of soil Evaluate the expansiveness of a soil Water Conten Estimate WP from WC or WC from WP Water Potential (-MPa) w a bln( ) a w exp b Plant Available Water Soil Specific Surface Area from a Moisture Characteristic Lower limit of plant available water is water content at -1.5 MPa (Permanent Wilting Point) Drained upper limit (Field Capacity) Profile specific (-.1 to -.33 MPa) Often 2X PWP Can use tensiometer or pressure plate in this range ce Area (m2/g) EGME Surfac y = x x R 2 = Slope of Semilog plot Expansive Soil Classification from McKeen(1992) Moisture Characteristics for a Range of Soil Texture Class Slope Expansion I > -6 special case II -6to -1 highh III -1 to -13 medium IV -13 to -2 low V < -2 nonexpansive Log1 (M MPa) 3 Dune Sand Bentonite 2.5 Missouri Texas 2 New Mexico Water Content 7

8 12/9/21 Coefficients for Semi-Log Fit Expansive Soil Classification from McKeen(1992) Soil Slope Intercept Texture a b r 2 Dune Sand sand New Mexico clay loam Missouri silty clay loam Texas clay Bentonite clay Class Slope Expansion Sample I > -6 special case II -6to -1 high III -1 to -13 medium IV -13 to -2 low Clay V < -2 nonexpansive Bentonite -5.4 Silty clay loam Clay loam Sand -224 Outline Introduction Defining water content, water potential and Soil Moisture Characteristic Curve (SMCC) Measuring water potential and water content Generating SMCC Sample preparation Data analysis Applications Automated SMCC instruments Aquasorp for dry soils Hyprop for moist soils AquaSorp: Automatic SWCC Generation for Dry Soils Internal Workings of AquaSorp A Chi Plot of Dry Range Soil Water Characteristics t (g/1 g) Water Content Bentonite Palouse B New Mexico Walla Walla Royal LS L-Soil ln(-ln(aw)) 8

9 12/9/21 New Mexico Clay WP4 and Aquasorp Comparison HyProp for wet end SMCC and hydraulic conductivity function 14 t (g/1 g) Water Content MPa MPa ln(-ln(aw)) HyProp for wet end SMCC and hydraulic conductivity function Take home SMCC Hydraulic conductivity The SWCC relates water content to water potential for a given soil Each different soil has a different SWCC Knowledge of the SWCC gives valuable information about a soil Vapor pressure methods are the only methods available to construct SMCCs below about -.5 MPa There are quick and easy methods to develop dryend SMCCs (manual and automated) For more information on instruments, techniques, or lab services Thanks! doug@decagon.com colin@decagon.com Or see p// 9