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

7.4 ppm . The lysimeters at Coshocton, Ohi o
(Harrold and Dreibelbis 1951), have a sensitivity
of 2,268 g or 0 .25 mm of water or
38 ppm and the lysimeter at Davis, Californi a
(Pruitt and Angus 1960), has a sensitivity o f
907 g or 0 .03 mm of water or 20 ppm .
The Weather Station
Located adjacent to the lysimeter tree an d
on a tower 33.5 m in the air are the meteorological
sensors of the weather station . Thes e
consist of a solarimeter, a net radiometer, air
temperature sensor, vapor pressure sensor ,
wind direction and speed sensors, and a
tipping bucket rain gauge . In addition to thes e
parameters, soil temperature is measured a t
four depths inside and outside of the lysimeter.
The signal from these sensors is re -
corded automatically on a digital magneti c
tape data logging system. At the present time ,
five of the signals are being integrated continuously
: solar radiation, net radiation, rain -
fall, windspeed, and the drainage from th e
lysimeter. The integrals of these signals an d
the other parameters are recorded on the
magnetic tape at hourly intervals to conserv e
the magnetic tape's supply . With hourly
records, the tape supply will last for 30 days .
The magnetic tape (6 .4 mm) is converted t o
12.7 mm computer compatible tape and the n
analyzed at the University of Washingto n
Computer Center with the Burroughs 550 0
computer .
The Proposed Uses of the
Lysimeter Facility
Since the lysimeter was installed during th e
summer of 1970 and the completed facility
was not completely checked out unti l
recently, very little data is available for discussion.
However, it may be enlightening to
discuss the proposed uses of the facility .
An Evapotranspirimeter
The lysimeter installation is expected to
yield short period rates of evapotranspiratio n
from the 28 m Douglas-fir tree . The accurac y
of the measured evapotranspiration is withou t
much question . How representative the dat a
are is questionable unless the water potential
of the root mass is kept equal to the wate r
potential of the root mass of adjacent trees .
Since the root mass is restricted in a 366 c m
diameter container, the amount of soil avail -
able for water and nutrient extraction by thi s
tree is slightly less than adjacent trees . Durin g
the summer months, water should be with -
drawn from the lysimeter container at a faster
rate than from the adjacent soil . Therefore ,
water will have to be added to the lysimeter
container to maintain a water supply equal t o
that in the adjacent area. During the winter
months, the opposite will be true . The bottom
of the lysimeter container inhibits dee p
percolation . When rainfall exceeds evapotranspiration,
water will have to be removed fro m
the container in order to prevent a buildup of
water in the bottom of the container .
Evapotranspiration as a Function of Potentia l
In order to study the mechanism of transpiration
from a Douglas-fir tree, the evapotranspiration
rates will be determined in relation
to the soil-water potential and the atmosph
eric evaporative demand . During thes e
studies, soil moisture potential will be deter -
mined with a series of thermocouple psychrometers
installed in the lysimeter container .
The atmospheric evaporative demand will b e
calculated from meteorological parameters .
Tissue Volume Change s
Dendrometer bands will be installed at various
locations on the tree to study the swellin g
and shrinking of the tree in relation to water
potential and evapotranspiration . At the sam e
time, stomata aperture and plant stress will b e
determined using leaf resistance meters an d
thermocouple psychrometers or Scholande r
bombs.
Test Cuvette Technique
While the detailed studies of evapotran -
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