Geoscience Reference
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soil column
head diagram
height z
H
h
z
Unsaturated
groundwater level
Saturated
z, h, H
Figure 4.9 Head diagram of variably saturated soil at hydrostatic equilibrium.
Piezometer
Tensiometer
z = 0
Cup
x 1 cm
x 2 cm
Groundwater level
Filter
Figure 4.10 Setup to measure the hydraulic head in saturated soil with a piezometer
and in unsaturated soil with a tensiometer.
we should use tensiometers, which have a porous cup that is impermeable for air, at
least in the occurring pressure head range. Of course the cup should conduct water in
order to adjust the water pressure head inside the porous cup to the soil water pressure
head around the cup ( Figure 4.10 ).
Question 4.6: Consider the piezometer and tensiometer as depicted in Figure 4.10 .
a) How large is the hydraulic head H in case of the piezometer and in case of the tensi-
ometer? Take z = 0 at the soil surface.
b) Does the soil water between porous cup and groundwater level low upward or
downward?
c) Can we use a tensiometer to measure the hydraulic head in saturated soils?
In general the pressure head proile will deviate from the 1:1 line. Figure 4.11
shows pressure head proiles above the water table in case of hydrostatic equilibrium
(no low), downward low (iniltration) and upward low (capillary rise).
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