Lecture 17 Soil Water (4)

Soil Water Measurement

• Soil moisture

• Soil water potential

• Infiltration rate

Soil Moisture Measurement

1. Gravimetric method: Coring soil samples of known volume and determining their weight loss when dried in an oven at 105C.

• 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

Soil Moisture Measurement2. 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

Soil Moisture Measurement3. 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

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

Soil Moisture Measurement5. 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

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.

Soil Water Potential Measurement1. 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

http://www.soilmeasurement.com/tensimeters.html

Tensiometers

Figure 5.27, Jones

Soil Water Potential Measurement2. 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

http://www.sowacs.com/sensors/gypsum.html

Gypsum Block

Infiltration measurements 1. Infiltrometers

Measures the rate of water entering the soil surface, i.e., mm/h

2. Permeameters: Measures infiltration at a given depth

3. Throughflow pitsMeasures infiltration at different depths