At some locations, values of percent sunshine hours (S) are available that
can be used to estimate R,. Fritz and MacDonald (1949) proposed a
relationship of the form
R, = (0.35 + 0.61S) Ro, (8)
Values of R, calculated from the above equation are subject to more error
than measured values of Rs, but when averaged over several days to a
month, should not be in error by more than 5% to 10%.
Where local orographic features do not strongly influence cloud cover,
daily measurements of R, at a single station can be used over large areas
for ET estimates over 5 and 10 day periods. In Florida, areas within 5 to 9
km of the coast may have significantly different cloud cover than the
central portion of the state. In these coastal areas, Rs values for stations in
the center of the state should be used with caution.
Total incoming radiation, R,, can also be estimated from cloud cover
data presented by NOAA for several locations in Florida (or obtained
from other methods). Readers are referred to a publication by Dooren-
bos and Pruitt (1977) for details on the procedure for estimating R, from
cloud cover.
Since wind speed is measured at many different heights above the
ground surface, and since the Penman equation requires wind speed at a
height of 2 m, wind speed will normally need to be adjusted to a height of
2 m.
u2 = uz (9)
where u2 = wind speed at height of 2 meters in km/day
uz = wind speed at height z in km/day
z = height of wind measurement in m.
The working Penman equation for potential ET in mm/day becomes
ET= A [(1 a) R, t T4(0.56 0.08 Va)
1.42RA 0.42)1/
Rso \
+ ^ [0.263 (ea ed)(0.5 + 0.0062u2)] (10)
A+y
The above equation, along with the discussed procedures for estimat-
ing R, and adjusting wind speed, is considered the most accurate method
available for estimating potential ETfrom a vegetated surface. The other
methods which are discussed briefly in the following paragraphs are less
accurate and should be used only when data are not available for the
Penman equation.
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