Environmental Engineering Reference
In-Depth Information
Figure 6.3 Rainfall and snowfall are contributors to precipitation falling on the earth's surface.
Reprinted with permission of Prentice-Hall from Climatology by John E. Oliver and John J.
Hidore,
©
1984 by Bell & Howell Co.
of 0.6 was applied to the water deficit calculation. However,
in 1955, the 0.6 factor was dropped from the calculations
and moisture index is now defined as follows (Thornthwaite
and Mather, 1955; Thornthwaite and Hare, 1955):
N
=
number of days in the month,
mean monthly air temperature, C,
T a
=
sum for 12 months of the function (T a / 5 ) 1 . 514 ;
I
=
(i.e., 1 month = 1 (T a / 5 ) 1 . 514 ) , based on correlations
to “pan evaporation” measurements, and
100 P
1
a t
=
complex function of the variable I
[i.e., a t =
I m =
PE
(6.1)
10 7 )I 3
10 5 )I 2
( 6 . 75
×
( 7 . 71
×
+
( 1 . 79
×
where:
10 2 )I
+
0 . 492].
I m =
1955 Thornthwaite moisture index,
For a 30-day month and assuming that there are 12 h of
the day for evaporation, the potential evaporation equation
can be written as
P
=
total annual precipitation, and
PE
=
total annual potential evapotranspiration calcu-
lated as the summation of the Thornthwaite (1948)
monthly potential evaporations.
16 . 0 10 T a
I
a t
PE m =
(6.3)
Thornthwaite (1948) incorporated the variables of length
of daylight hours, mean monthly temperature, and an empiri-
cal constant into the prediction of potential evaporation. The
simplicity of the Thornthwaite equation has led to its usage
for the classifications of global climate as well as numerous
other agricultural and engineering applications. The poten-
tial daily evaporation can be written as follows using the
Thornthwaite (1948) equation:
where:
PE m =
potential evaporation, mm/month.
The potential monthly evapotranspiration can be
calculated for each of the 12 months. For 1 year, the
annual potential Thornthwaite evaporation can be written as
follows:
0 . 5333 L
12
N
30
10 T a
I
a t
12
PE d =
(6.2)
PE
=
PE m
(6.4)
i
=
1
where:
The total annual evapotranspiration is used in conjunction
with the total annual precipitation to classify the climate
through use of Eq. 6.1. The climate classification criteria
PE d
=
potential evaporation, mm/day,
L
=
length of daylight, h,
 
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