Instruction manual

10
apple or kiwifruit, and so we have been able to use the same set of probes for periods of
up to 3 months. In other species this may not be the case and then some reasoned
assessment, or independent measure of transpiration, should be made to determine just
how long the probes remain useful.
1.4 Converting Heat-Pulse Velocity to Sapflow
Once the corrected heat-pulse velocity,
V
c
, has been determined, the next step is to relate
it to the actual sap flow. Marshall’s (1958) analysis showed that if the sap and woody
matrix are considered to form a homogeneous medium, then the sap flux density,
J
[m s
-
1
], can be calculated from
J
P
M
V
c
=
+
(
.
)
0
33
(3)
where
P
[kg m
-3
] is the wood density (oven dry weight of wood/green volume) and
M
is
the moisture content ((wet weight - oven dry weight)/oven dry weight) of sapwood. The
density and moisture content of the sapwood are both physical properties of the woody
matrix, and they can be determined easily from trunk cores. The factor 0.33 in Eq (3) is
the specific heat of dry wood, which is assumed to be constant. In our analysis, we use an
alternative expression for
J
, which was developed by Edwards and Warrick (1984) by
considering the sapwood to comprise 3 phases of gas, solid and liquid with appropriate
physical and thermal properties. The working equation is given by
J
F
F
V
M
L
c
=
+
(
.
)
0
505
(4)
where
F
M
and
F
L
are the volume fractions of wood and water, respectively. The factor
0.505 is related to the thermal properties of the woody matrix, and is assumed to be
constant within and between species.
1.5. Measuring the Volume fractions, F
M
and F
L
Volume fractions
F
M
and
F
L
implicit in Eq. (4) are determined from the Archimede’s
principle, in the following manner. Firstly, a core sample is taken and its fresh weight,
M
F
[kg], is determined. This weight is equal to the mass of water and the mass of dry wood,