Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 166

Vacuum gauge

∆ z 1

Soil surface

∆ z 2

Porous ceramic cup
Figure 4.27 Tensiometer with vacuum gauge.

With z = 0 at the soil surface and without osmotic head, the hydraulic head equals:

Hh = +

gauge ∆ 1

z (4.45)

Whereas there is no resistance to low in piezometers, so that they are always instan-
taneously at equilibrium with the soil water at the lower open end, this is not neces-
sarily true for tensiometers. The porous cup usually presents considerable resistance
to low and the water pressure inside may adjust only slowly to changes in the soil
water pressure head at the cup. Also, if the hydraulic head in the soil is not uni-
form, the tensiometer will indicate only an average of the soil hydraulic head around
the cup.

Question 4.18: The cups of tensiometers 1 and 2 are at a depth of 60 and 80 cm, respec-
tively. The gauges are 20 cm above the soil surface. The gauge of tensiometer 1 indi-
cates h gauge = -90 cm.
a) Draw the potential diagram, assuming that the water in the soil is at hydrostatic equi-
librium.
b) Calculate the gauge reading of tensiometer 2.

Question 4.19: At another moment the tensiometers of the former question indicate
h gauge = -90 cm for tensiometer 1 and h gauge = -100 cm for tensiometer 2.
a) Draw the potential diagram for this new situation, assuming H is linear with z .
b) What is the height of the groundwater table?
c) How can you easily determine the difference in hydraulic potential of the soil water
at the two cups?

Instead of vacuum gauges, mercury manometers can be used to measure the pres-
sure head of the liquid in the tensiometer ( Figure 4.28 ). The pressure head in the cup
relative to atmospheric pressure can be calculated by starting at the lat air-mercury