1. Soil Water Measurement
Lecture 17 Soil Water (4)
Soil Water Measurement
Soil moisture
Soil water potential
Infiltration rate

2. Soil Moisture Measurement
Gravimetric method:
Coring soil samples of known volume and determining
their weight loss when dried in an oven at 105C.
Standard method
Widely used
Laborious and time consuming
Prone to errors in sampling and repeated weighing
Can’t distinguish between structural and non-structural water
Clays may not be totally dried while organic matter may oxidize

3. Soil Moisture Measurement
2. Neutron Probe:
Measuring soil water content from the energy reduction of neutrons released in the soil. A radioactive source of fast (high energy) neutrons is released at a given depth of the soil and the number of neutrons which are slowed or thermalized by the collisions with hydrogen nuclei, mainly in soil water, is measured by a detector. (Figure 5.28 Jones)
Measuring soil water content in a radius of about 10 cm for wet soil and 25 cm for dry soil
Not useful for top layer of soil (0-5 cm)
Needs to be calibrated against gravimetric method
Fast and little labour
Accurate for measuring changes in water content rather than the absolute value
Precautions for radioactivity

4. Soil Moisture Measurement
3. Time-domain reflectometry (TDR):
Determine soil water content from measuring the dielectric constant of the soil between two lines. The higher the soil water content, the larger the dielectric constant. The dielectric constant is measured between two transmission lines (rods) inserted in the soil either horizontally or vertically.
Good for taking the average soil water content at a depth when rods are installed horizontally
Good for taking the average soil water content of a layer of soil when rods are installed vertically
Accuracy is comparable to the gravimetric method
Allows for non-manual, continuous monitoring
Non-radioactive

5. Soil Moisture Measurement
4. Capacitance probe:
Determine soil water content from measuring the dielectric constant of the soil at a point. A probe is inserted to the soil and the tip is in contact of soil where the dielectric constant is measured
Point measurement
Particularly useful for top layers of soil
Sensitive to local inhomogeneities and air gaps between probe and soil.
Needs calibration using gravmetric method
Non-radioavtive

6. Soil Moisture Measurement
5. Synthetic aperture radar (SAR) :
Inferring soil water from active microwave (RADAR, RAdio Detection And Ranging) interaction with the soil surface. Wet soil surfaces have higher dielectric constants and therefore higher reflectivity for radar
Areal average
Depth depends on the radar wavelength
Useful for detecting soil moisture change and spatial distribution rather than the absolute value
SAR is also strongly affected by soil surface roughness

7. Soil Water Potential Measurement
Water potential is related to soil moisture, but is also unique measure of the effects of soil water on water movement and extraction of water by plants.

8. Soil Water Potential Measurement
Tensiometers:
Measures soil water suction. It comprises of a porous pot filled with deionized water and inserted to the soil. The pressure within the pot equalizes with the tension or pressure in the surrounding soil is measured using either of pressure transducer or a manometer.
Oldest and widely used
Often used for unsaturated soil
Can measure total water potential below water table,
i.e., piezometer
Lowest suction to measure is 80kPa, below which air
may enter into the porous pot, introducing errors.
Doesn’t work well in clays

9. Tensiometers
Figure 5.27, Jones

10. Soil Water Potential Measurement
2. Resistance block:
Deriving soil water potential from the electrical resistance of a block of porous material buried in the soil and in equilibrium in water suction with the soil.
Can work at very low water potentials (as low as –1500 kPa)
Useful for clays
Sensitive to temperature and salinity
Gradual change in resistance over time
Compatibility of physical characteristics between the soil and the burried block
Gypsum
Block

11. Infiltration measurements
Infiltrometers
Measures the rate of water entering the soil surface, i.e., mm/h
Permeameters:
Measures infiltration at a given depth
3. Throughflow pits
Measures infiltration at different depths