Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
Areal reduction factors
Computation of the areal reduction factor for the Gibe III catchment at duration of 1-, 3-, 5-, 7-, 10- and 15day
has been carried out applying the U.S. Weather Bureau method 12,15 . This estimate is performed
generating a record of areal rainfall for the chosen duration having multiplied each event by the relative
Thiessen weighting factor.
Areal reduction factor is determined dividing the mean of the annual maxima of the areal precipitation over
the entire basin by the mean of the annual maxima of point precipitation at each station for any year.
Years of observation where some station data are missing have been disregarded in the areal reduction
factors definition.
The areal reduction factors so computed are given in the following table:
Table 5.18: Gibe III catchment – Areal Reduction Factors
Rain Duration (days)
1 3 5 7 10 15
ARF 0.44 0.59 0.65 0.67 0.72 0.79
Areal rainfall curves
Precipitation data are derived from daily rainfall dataset, but daily rainfall is recorded at a conventional time
and therefore the dataset is not referred to an unrestricted 24hr period. Consequently results of the
elaborations have been adjusted adopting the corrective factors proposed by Weiss and expressed with the
following formula 7,8 :
⎛ t ⎞
R = 1+
0.
13⎜
⎟
⎝ Δt
⎠
where:
−1
t is the given duration;
t Ä is the time step of the original record, that is 1 day.
Must to be noticed that for 1 day duration the depth of rainfall is to be multiplied by 1.13, that means an
increase of 13%. Obviously, for longer durations the errors tends to disappear. Table 5.19 summarizes the
Weiss correction factors.
Table 5.19: Gibe III catchment – Weiss correction factors
WEISS
FACTORS
Rain Duration (day)
1 3 5 7 10 15
1.13 1.043 1.026 1.019 1.013 1.009
15 Gill T. D., “Transformation of areal rainfall to point rainfall by estimating the area reduction factors, using radar data,
for Texas”, Texas A&M University, May 2005
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