Agriculture Reference
In-Depth Information
Hydraulic Head of Water in the Field
Equation 6.4 is often expressed in head units. Take, for example, the case where
a pipe open at both ends is installed in the soil to a depth
z
below the soil sur-
face. This is called a
piezometer
. If the soil is saturated at point A (fig. 6.2a), we
may wish to know the water potential in head units at this point. Noting the con-
tinuum between pressure potential and matric potential, when the soil is satu-
rated, we define the
piezometric head h
as
h
p
z
6.1.2
(6.5)
where
p
is the positive pressure in the water at point A. The negative sign appears
in this equation because, by convention,
z
is measured positively upward from the
reference level.
Alternatively, a tensiometer installed in unsaturated soil at a depth
z
below
the soil surface (point B in fig. 6.2b) will indicate that the water is at a negative
pressure or
suction
. Water in the tensiometer equilibrates with the soil water sur-
rounding the porous pot (at point B) such that the head
h
of the soil water is
given by
h
p
z
(6.6)
Head gradients are used in calculating the rate of water movement through
soil in the field (section 6.3). But before considering rates of water flow, we should
examine the relationship between
and
.
Figure 6.2
(a) A piezometer showing soil water at positive pressure in saturated soil. (b) A tensiometer
measuring the negative pressure of soil water in unsaturated soil (redrawn from White
1997).
(a)
(b)
Soil surface
z
h
h
B
z
Porous
pot
-p
p
A
Soil water at point A
Soil water at point B
below the water table
above the water table
Head h = p - z
under negative pressure
- p
Head h = - p - z
