Lecture 5 Measurement of Soil Water

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A. Measurement of Soil Water Content (q )

1. Gravimetric

(a) dry soil at 110^o C

% water = {(wet wt - dry wt)/dry wt} x 100

(b) reference point

- standard method

- defines "dry" soil

2. Neutron Probe

 

water content related to slow neutrons measured

 

B. Soil Water Potential (y )

1. Tensiometers

 

range = 0 to -0.8 bars

2. Resistance blocks (water content or potential)

 

H₂O = k/r, where r = electrical resistance

range = -1 to -15 bars

3. Thermocouple Psychrometer

(a) cooling current (Peltier effect)

= water drop forms on thermocouple

(b) water evaporates

= cools thermocouple

(c) rate of evaporation (cooling) is proportional to vapor pressure (P/P_o)

(d) y_T = RTln(P/P_o)

(e) used on plants and soil

(f) range = about -50 to -10,000⁺ kPa

C. Content vs Potential

1. Tension or Suction plate

Ü fix potential (y ) with hanging water column

Ü measure water content (gravimetric)

ü obtain soil moisture/potential curves

· range = 0 to -0.8 bars

2. Pressure plate

· like suction plate except use pressure to generate lower potentials

i.e. -15 bars = 225 psi

· completes "field moisture" range

- range = -80 to -1500⁺ kPa

 

Ü fix potential (y ) with pressure

Ü measure % water (gravimetric)

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