
ZENO
®
-3200 USER MANUAL
Coastal Environmental Systems (206) 682-6048 Page D-20
13 Longitude of the standard time zone meridian
14 Radiation sensor code: 0 for net radiometer, 1 for solar
Outputs: 1 Average potential evapotranspiration rate in mm/hr
2 Most recent potential evapotranspiration rate in mm/hr
E1 Process Error Code
Assumptions: The ZENO must be set to the standard time period -- not to daylight
saving time.
Typical values of the vegetation height are 0.1 m for grass, 0.3 m for
alfalfa. The vegetation canopy height must be greater than 0.05 m, and
less than the wind measurement height.
If a solar radiation sensor is specified, the model estimates the cloud
cover, and hence the net radiation, using that value. If the ZENO starts
up at night, it cannot estimate the cloud cover until sunrise the next day.
In the interim, the ZENO will output a null value of -1000.0.
The Penman-Monteith equation is used in the following form:
E
RG ceTer
npsH
*
(1)
where the symbols indicate:
E potential evapotranspiration rate, to be determined
latent heat of evaporation of water
rate of change of saturation vapor pressure of water with temperature
R
n
net radiative transfer of heat into the ground
G rate of loss of heat into the ground through conduction
density of air
c
p
specific heat capacity of air at constant pressure
e
s
(T) saturation vapor pressure at temperature T
e measured vapor pressure
r
H
resistance of radiative and convective transfer of heat in air
* modified psychrometer constant
The calculation requires that the ZENO time clock not be set to daylight saving time, unless a net
radiation measurement is available.
Net Radiation
If a net radiometer is available, it measures the net radiation R
n
directly. If only solar radiation is
measured, the net radiation is calculated as described in this section.
The ground conduction G is parameterized as 0.1R
n
when R
n
is positive, and 0.5R
n
when R
n
is negative.
If the solar radiation S
t
is measured, the net radiation is calculated in the following way. The quoted
hourly accuracy of this fit (developed and tested in coastal, desert, mountain and interior valley regions in
California) is 10% for solar angle
> 10º. The fit, based upon well-watered grass, was:
RScTcT
nta s
089 1 1
44
.
(2)
where the new symbols are:
short-wave surface albedo
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