PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest
PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest
PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest
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may bias the results . Meteorological method s<br />
must be tested in the areas of anticipated us e<br />
(Slatyer and Mcllroy 1961) . Such testing has<br />
not been accomplished in forested areas .<br />
Thus, weighing lysimetry appears to be th e<br />
only method capable of yielding the neede d<br />
short period accurate rates .<br />
Weighing lysimeters have been used to determine<br />
evapotranspiration of agriculture<br />
crops (McIlroy and Angus 1963, Pruitt and<br />
Angus 1960, Harrold and Dreibelbis 1951, Va n<br />
Bavel and Meyers 1962), but they haven't<br />
been used for natural vegetation, such a s<br />
brush and trees, which have extensive root<br />
systems .<br />
The use of weighing lysimeters to deter -<br />
mine evapotranspiration of trees is feasibl e<br />
only if the trees are uniformly spaced such as<br />
in a plantation . Installation is easier when the<br />
root system is naturally restricted, thereby<br />
reducing the size of the lysimeter and it s<br />
deadweight. Under these conditions, it would<br />
be possible to obtain accurate short period<br />
evapotranspiration rates, thus enabling a<br />
mechanistic examination of evapotranspiration<br />
in relation to the determining meteorological<br />
and soil moisture conditions .<br />
Establishment of evapotranspiration from a<br />
single tree or group of trees in a specific location<br />
is important but does not yield answer s<br />
for other types of vegetation in different<br />
climatic zones . Installation of lysimeters in<br />
many locations is not possible and may not be<br />
feasible because of the cost . However, meteorological<br />
methods if tested can be employed to<br />
establish evapotranspiration rates wher e<br />
lysimeters are not feasible. Results fro m<br />
meteorological methods can be tested against<br />
the results from a properly installed weighing<br />
lysimeter. This has been accomplished for<br />
agriculture crops (Tanner 1967, Pruitt 1963 ,<br />
Fritschen and Van Bavel 1963, Fritsche n<br />
1965) . However, differences in scale factor s<br />
such as canopy height, density, and roughnes s<br />
demand additional testing before these<br />
methods can be used over natural vegetation .<br />
The purpose of this paper is to discuss th e<br />
establishment of the weighing lysimeter in a<br />
Douglas-fir forest for the primary purpose of<br />
studying the complex relation between evapotranspiration<br />
rates and the determining<br />
meteorological and soil moisture conditions .<br />
In later stages the installation will be used t o<br />
evaluate meteorological methods for determining<br />
evapotranspiration to be used i n<br />
other areas .<br />
The Site<br />
The lysimeter installation is located on the<br />
Cedar River Watershed near Seattle, Washing -<br />
ton. The soil is a Barneston, gravelly, loamy<br />
sand originating from glacial outwash laid<br />
down at the end of the Vashon glacial period<br />
(Poulson and Miller 1952) and generally restricts<br />
the root system above the 3-foot depth<br />
(Gessel and Cole 1965) . The lateral extent of<br />
the root system is largely restricted to th e<br />
basic spacing of the trees . The area is relatively<br />
level and has a uniform canopy density<br />
making it desirable for micrometeorological<br />
investigations .<br />
The trees are 35-year-old Douglas-fir (Pseudotsuga<br />
menziesii) which regenerated naturally<br />
after logging. The average tree spacing i s<br />
5.8 m, resulting in 231 trees per 4,041 m 2<br />
consisting mostly of Douglas-fir with a fe w<br />
hemlock (Tsuga heterophylla) and maple .<br />
Trees in the 5 to 30 cm d.b.h. classes occur<br />
with the greatest frequency . Ground vegetation<br />
consists of bracken fern (Pteridiu m<br />
aquilinum), salal (Gaultheria shallon), red<br />
huckleberry (Vaccinium parvifolium), an d<br />
mosses, mainly (Eurhynchium oregonum) .<br />
The Lysimeter<br />
The lysimeter was constructed around th e<br />
root ball of a 35-year-old, 28 m tall an d<br />
38 cm diameter Douglas-fir tree . The lysimeter<br />
consists of two right cylinder containers,<br />
one located within the other . The inner -<br />
most container in which the tree is located i s<br />
366 cm in diameter and 122 cm deep, havin g<br />
a surface area of 10 .5 m 2 (fig. 1). The container<br />
at "field capacity " with tree and soi l<br />
weighs 28,900 kg (fig . 2) .<br />
The inner container is resting on a hydraulic<br />
transducer located on the bottom of th e<br />
outer container. The hydraulic transduce r<br />
consists of eleven 15 m lengths of 6 .35 c m<br />
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