PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

Here v is the volume of water crossing a uni t
area per unit time (cm/min) ,
K is the hydraulic conductivity (cm/min) ,
is the soil water pressure or suctio n
(hydraulic head) (cm) ,
0 is the soil water content (volume of
water per unit volume of soil), an d
z is the height above datum (referenc e
level) of the point under consideratio n
(cm) .
By introducing the concept of soil-wate r
diffusivity, defined b y
purposes of calculations . Soil moisture specified
as a function of time is required at th e
upper and lower boundaries of the soil column
. Their solution permits the simultaneous
calculation o f
(Dj +
Dj+) )
2 (oJ+1 zj+1 - 01 zj
This expression approximates D a8 and hence
v, the volume flux of water, apart from th e
constant K (cf. equation 2) .
D = -K a alP d ,
where D is measured in c m 2 /min, Childs and
Collis-George (1950) cast the Darcy equatio n
in a form similar to Fick 's law of diffusion :
v = -Da 8 -K (2 )
Applying the equation of continuity ,
ad + av
at az
= 0 (which implies that there are n o
sources or sinks of water in the system), one
obtains the full Richards soil flow equation :
30 __ a ad aK
at az (D az ) + a z
(3 )
Soil moisture is obtained as a function o f
depth (z) and time (t) by integrating (3) .
Accordingly the Richards model is compatibl e
with our larger soil-plant-atmosphere model ,
which requires soil moisture at various depth s
and times as an input to a submodel whic h
represents the uptake of water by the roots o f
a tree. Our current uptake model is that of
Gardner (1960), a diffusion-type flow equation
in which time rate of change of soil wate r
concentration (0) at any point (z) in the soi l
is related to the moisture gradient betwee n
that point and an absorbing root .
Since D and K depend on 0, (3) is non -
linear in 0, and a solution is possible only i f
numerical approximations are used . Remso n
et al . (1965) developed a computer program
which gives approximate results when initial
moisture content is specified at each of n+ 1
equally spaced points or nodes into which a
soil column zn centimeters thick is divided for
Available
Observational Data
At the Allan Thompson Research Center ,
Cedar River Watershed, the downward flow o f
water through soil profiles has been measure d
for several years by a tension lysimeter syste m
(Gessel and Cole 1965) . A prescribed suctio n
on the porous lysimeter plate causes drainage
through the plate to approximate the drainag e
in the adjacent soil . Results are recorded i n
permanent form on paper tape by an automatic
data recording system . Our data com e
from a series of controlled experiments, carried
out in 1970, which were designed t o
study the effects of rainfall on wetting front s
in the rooting zone of a Douglas-fir stand .
Lysimeter plates were installed at depths o f
11 and 25 cm and rainfall was simulated by a
below-canopy sprinkler system . Soil moisture
was measured by tensiometer at a point mid -
way between the soil surface and the uppe r
lysimeter plate, and water flux through th e
lysimeter was recorded by a flow meter connected
to the data recorder .
For the present study only the data for th e
11-cm plate were used. Since an abrup t
change in the physical characteristics of th e
soil occurs between the 11- and 25-cm levels ,
a different set of estimates of the conductivity
and diffusivity parameters is require d
for the lower layer . A model coupling th e
flow of water through the two layers has bee n
developed but has not yet been tested .
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