Agriculture Reference
In-Depth Information
may provide better estimates of the pan coefficient than would longer term
average data. Updated pan evaporation data were obtained from NOAA's
Climatological Data Sheets. To evaluate possible trends, pan evaporation
data were plotted graphically, and regression analysis was used to determine
whether the regression coefficient (i.e., the slope) of the best-fit linear model
was significantly different from zero. All statistical analyzes were performed
by using the statistical software package StatMost Version 3.6 [19].
8.2.2 REFERENCE EVAPOTRANSPIRATION (ET)
The long-term monthly reference ET was estimated by using the Penman -
Monteith equation [10]:
900
⎛
⎞
(
)
(
)
0.408
RG
−+γ
uee
−
⎜
⎟
n
⎝
⎠
2
s
a
(4)
T
+
273
ET
=
(
)
o
+γ
1
+
0.34
u
2
where: ET
o
= ET (mm day
-1
); ∆ = Slope of the vapor pressure curve (kPa°C
-1
); R
n
=
Net radiation at the crop surface (MJ m
-
2
day
-1
); G = Soil heat flux density (MJ m
-2
day
-1
);
a = Mean air density at constant pressure (kg m
-3
); Cp = Specific heat at con-
stant pressure (MJ kg
-
1
°C
-1
); e
s
-e
a
= Vapor pressure deficit (kPa); e
s
= Saturation vapor
pressure (kPa); e
a
= Actual vapor pressure (kPa); r
a
= Aerodynamic resistance (s m
-1
);
r
s
= The bulk surface resistance (s m
-1
);
λ
= Latent heat of vaporization (MJ kg
-1
);
γ =
The psychrometric constant (kPa°C
-1
).
Equation (4) applies specifi cally to a hypothetical grass reference crop
with an assumed crop height of 0.12 m, a fi xed surface resistance of 70 sec/m
and a solar refl ectivity of 0.23. The FAO recommends using the Penman-
Monteith method over all other methods even when local data are missing.
Studies have shown that using estimation procedures for missing data
with the Penman-Monteith equation will generally provide more accurate
estimates of ET
o
than will other available methods requiring less data input
[10].
Of the various climate parameters needed to calculate ET
o
with Eq. (4),
only air temperature (T) and wind speed (u) were available for all seven experi-
mental substations in Puerto Rico; however, wind speed was not measured
consistently. For example, in the case of Lajas, wind speed data were avail-
able only during the following years: 1963, 1966 to 1969, 1971 to 1978, 1983 to
1985 and 1987 to 1990. Wind speeds were measured at 0.33 m above the ground
and therefore needed to be adjusted to the two-meter value (u
2
) using the
logarithmic adjustment equation presented by Allen et al. [10].
Relative humidity (needed to estimate actual vapor pressure) is measured at the
substations by using a sling psychrometer, but only once in 24 h; thus, these data do
not represent daily average values. Therefore, the actual vapor pressure was derived
from the dew point temperature (Tdew). Long-term average dew point temperature
ρ
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