Page 169 - Hydrogeology Principles and Practice

HYDC05 12/5/05 5:35 PM Page 152

152 Chapter Five

Fig. 5.11 Methods for the measurement of the physical properties of soils: (a) a neutron probe for measurement of soil moisture content;
(b) a porous pot tensiometer for measurement of pressure head (suction head or tension head); (c) a double-ring infiltrometer for
measurement of infiltration rate.

head and the understanding of their interrelationship neutron probe to give a direct measurement of soil
are important in the understanding of soil water moisture content. As shown in Fig. 5.11a, a radioact-
movement. ive source (for example Am-Be) is lowered into an
The measurement of soil moisture content, θ, augured hole and the impedance of emitted, fast
can be undertaken in the laboratory by gravimetric neutrons, caused by impact with hydrogen nuclei
determination or in the field with a neutron probe. contained in the soil water, is determined from the
The gravimetric method requires a known volume, scatter of slow neurons measured by a detector. By
V , of soil to be removed and the total mass, m , found. calibrating the density of scattered slow neutrons
t t
The sample is then oven-dried at 105ºC until the final with measurements of soil moisture content made
mass of the dried sample, m , is constant. The differ- by gravimetric determination, it is then possible to
s
ence in mass, m − m , before and after drying is equal make repeated measurements of soil moisture con-
t s
to the mass of water originally contained in the tent at various depths in the access tube. The neutron
sample, m . The water content of the soil by mass is probe is calibrated for each soil type investigated,
w
then equal to: with each field site having its own relationship of
probe reading versus soil moisture content, and used
m w to make regular measurements of vertical profiles of
θ = eq. 5.10
m water content.
m s
The field measurement of soil water pressure (the
By calculating the bulk density of the soil sample, ρ , amount of suction) is made with a tensiometer. One
b
from m /V , it is then possible to express the water design is shown in Fig. 5.11b and consists of a ceramic
s t
content by volume, as: pot filled with deionized water. The pressure within
the pot equalizes with the fluid pressure in the sur-
ρ
=
=
θ V w θ m b eq. 5.11 rounding soil and is measured via a hydraulic link to a
V ρ manometer or pressure transducer and data logger.
t w
Porous pots operate best in relatively wet conditions
where ρ is the density of water. up to suctions of about 0.85 bar. In drier conditions
w
A disadvantage of the gravimetric determination is there is a danger of air entering the pot and affect-
that the sample is destroyed providing only a one-off ing pressure transmission. Porous pots also work
measurement. An alternative method is to use a under positive fluid pressure, when a vertical bank of