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

.0500
.040 0
z .030 0
E
L.)
0 .020 0
.0140
I
4 0 20 .0 40 .0 60 .0 80 . 0 100 . 0 120 . 0 146 . 0
ELAPSED TIME . (MIN . ]
Figure 4 . Precipitation (PI, observed soil water flow (F), and modele d
soil water flow (M) at 11 cm . Lysimeter suction was 146 cm of water .
tion provides a satisfactory description of th e
flow of water in the upper rooting zone o f
our Douglas-fir stand . Since the Everett
gravelly sandy loam is extremely variable, our
results probably should not be extrapolate d
much beyond the local area in which the y
were obtained .
We have not been able to account for th e
25 percent discrepancy between predicted
and observed peak flows . A number of possible
explanations have occurred to us . It can
be argued that the lysimeter system is not a
perfect device for measuring water flow in a
vertical column of soil . For example, in the
early stages of a flow experiment, acceleratio n
of flow due to lysimeter suction may be
greater than that due to gravity. Perhaps more
significant may be variations in flow associated
with the lack of homogeneity of an i n
situ forest soil. We suspect that stones or
roots in soil above the lysimeter plate diverte d
water toward or away from the plate . Perhaps
more important, our estimates of initial value s
of 0 at several nodes are not accurate.
It is probably more important to emphasiz e
that the field experiments described here wer e
conducted for purposes rather far removed
from our present one of evaluating th e
Richards equation as a possible component of
a soil-plant-atmosphere model . We also wish
to point out that the Richards equation has
not received adequate evaluation in field situations,
perhaps because the stringent under -
lying assumptions are rarely satisfied . Th e
rather close correspondence between model
and observation that we have obtained suggests
that the model may be more robust tha n
has been generally believed .
Acknowledgments
The work reported in this paper was supported
by National Science Foundation Grant
No. GB-20963 to the Coniferous Forest Biome ,
U .S. Analysis of Ecosystems, Internationa l
Biological Program . This is Contribution No .
27 to the Coniferous Forest Biome .
100