Soil Water Flow - Transport in the Atmosphere-Vegetation-Soil Continuum - page 140

Table 4.1 Systems of units of soil water potential

Expression Name Unit Dimension

Energy/mass Chemical potential J kg -1 L 2 T -2

Energy/volume Soil water potential N m -2 (=Pa) M L -1 T -2

Energy/weight Hydraulic head M L

4.3.1 Hydraulic Head of Groundwater
Groundwater refers to water below the groundwater level, while soil water refers to
water in the vadose zone. Types of energy that may play a role in groundwater are
height, pressure, velocity, osmosis and heat. In general in groundwater, the energy
amounts due to velocity, osmosis and heat are negligible compared to the energy
amounts of height and pressure. For these common situations we may write:

Hhz =+ (4.5)

where H is the hydraulic head (m), h is the soil water pressure head (m) and z is the
elevation or height (m). For pressure head commonly atmospheric pressure is taken
as reference with the value zero. The zero reference for elevation can be taken at any
level. In case of hydrostatic equilibrium and atmospheric pressure at the groundwater
level, the pressure head is equal to the distance to the groundwater level. The water
level in piezometers incorporates both pressure head and elevation. Therefore the
water level is equal to the hydraulic head H at the ilter ( Figure 4.6 ), which makes
piezometers very practical measurement devices.

Question 4.4: In a phreatic aquifer we measure with piezometers the hydraulic heads at
two depths. We take z = 0 at the soil surface. At piezometer 1 (ilter at z = -100 cm) the
water level occurs at z = -80 cm, while at piezometer 2 (ilter at z = -200 cm) the water
level occurs at z = -90 cm.
a) Which pressure heads h occur at the ilter depths?
b) Which hydraulic heads H occur at the piezometers?
c) Does the groundwater at this location low upward or downward?
d) Calculate the depth of the groundwater level, assuming a homogeneous soil below
the groundwater (hint: in such a situation the gradients d H /d z and d h /d z are constant
with depth).

In head diagrams, the elevation, soil water pressure head and hydraulic head are
depicted as function of depth. These diagrams are very useful to interpret piezom-
eter and piezometer data and to determine the direction of low. Figure 4.6 shows
the head diagram belonging to the low situation of Question 4.4 . Both elevation
and pressure head are linear functions with depth. Therefore also the hydraulic head
is a linear function with depth. As H decreases in downward direction, the low is