7 - UZ flow Flashcards
Moisture content facts
Specific yield of an aquifer is the same as the drainable porosity
Moisture content is equal to effective porosity in the saturated zone
Moisture content doesnt show how dry a rock is; water saturation is better in this way
Maximum value for moisture content is effective porosity
Maximum value for water saturation is 1
Surface tension
Liquids behave as if covered by an elastic membrane in a constant state of tension.
A molecule of liquid some way below the surface has equal forces of attraction.
A molecule at the surface is attracted to the liquid by stronger forces than to the gas.
Net attraction causes surface to contract.
Larger contact angle on rough surface - Wettability
Neutron Probe?
Neutron probe is the oldest and most established method of in situ moisture content measurement.
Neutrons are emitted from a radioactive source.
Neutrons are dramatically slowed down after colliding with hydrogen atoms. The probe also contains a slow neutron counter.
The density of slow neutrons is related to the local prevalence of hydrogen atoms, which in turn is related to the prevalence of water.
Slow neutron count is related to moisture content by a calibration relationship.
Disadvantages are that it cannot be continually logged and also requires lots of health and safety precautions due to radioactive source.
Also the sphere of observation decreases under dryer conditions.
Dialectric methods
The dielectric constant is a property relating to the amount of electric energy a material can store.
Typical values are:
Water = 80
Soil solids = 5 to 6
Air = 1
Therefore, a measure of the bulk soil dielectric constant can be related to the water content.
There are three methods to measure:
Time domain reflectometry
Impedance technique
Capacitance technique
A major advantage of dialectic methods over neutron probe is that can be continuously logged.
A disadvantage is that they generally require in situ calibration. This is normally done by a comparison to close proximity neutron probe measurement.
Profile probe
Advantage is the ability to continuously log moisture content at multiple depths using just one probe.
Profile probe is typically about a metre long.
Profile probe measures soil dialectric constant using the capacitance method.
It creates a 100 MHz signal (similar to FM radio). The signal is applied to pairs of stainless steel rings,
which transmit an electromagnetic field extending about 100 mm into the soil.
The field passes easily through the access tube walls, but less easily through any air gaps.
The dialectric constant of the soil has a strong influence on the applied electromagnetic field resulting in a voltage output, which, given an appropriate calibration relationship, can be used to measure soil moisture content.
Tensiometers
Tensiometer consists of a porous cup connected through a tube to a pressure transducer, with all parts filled with water.
When initially placed in soil, tensiometers tends to be at atmospheric pressure.
Because unsaturated soil is less than atmospheric pressure, soil sucks water out of the porous cup.
This reduces the pressure in the tensiometer, which is recorded by the pressure transducer.
The measured pressure is easily translatable to the pressure head.
The measuring range of tensiometers is limited by the boiling point of water
Also, if the soil subsequently becomes wetter, the tensiometer sucks in soil water, which contains dissolved gas. This can lead to additionally problems
If the soil remains too dry for long periods of time, the tensiometer water is sucked out by the soil and a pressure reading of zero is observed
Porous material sensors (General)
Another method of measuring pressure head is as follows: A porous block is inserted into the soil.
The block absorbs water from the soil until the soil pore water and block pore water reach pressure equilibrium.
The moisture content of the block is measured, by a variety of techniques (e.g. dialectric method).
Note that measured moisture content is of the block, not the soil.
Moisture content of block is translated to an estimate of pressure head using a relationship for the block, determined a priori in a laboratory.
The pressure head of the block is assumed to be that of soil as with tensiometers.
These are particularly good for dry conditions as they do not suffer from problems associated with boiling and drying out (as with tensiometers).
However, moisture content of block tends becomes insensitive to pressure head changes during wet conditions.
Porous material sensors (ER)
Electrical resistance sensors:
Gypsum block is moulded around two electrodes
Electrical resistance of block is proportional to water content
Gypsum block slowly dissolves, to maintain a saturated concentration of calcium sulphate, so that the electric conductivity is insensitive to changes in solute concentrations
At high moisture content they are insensitive to pressure head
