| Summary: | The designed flood hydrograph can be found with certain return period
using a unit hydrograph method. It needs the inputs of designed rainfall in the
form of rainfall distribution as a time function. Determining the designed rainfall
distribution needs data on rainfall duration and hourly distribution pattern, which
can be collected by conducting a study on automatic rainfall data. However, if the
data required were not available in the observed region, a rainfall distributionbased
approach can be applied. The inappropriate determination of rainfall
duration and rainfall distribution pattern using the approach can cause either
overestimated or underestimated results of designed discharge, which are not
expected in each designing process.
In the study, the rainfall distribution-based approach and its rainfall
duration used Tadashi Tanimoto rainfall distribution model, Alternating Block
Method (ABM) and rainfall distribution patterns from other researchers including
Edy Sukoso (2004), Ika Tyasning Yudiyanti (2006) and Sobriyah (2003) that can
be applied in the observed catchments, i.e. Progo, Bogowonto, Kupang, Pemali
and Lusi. The accuracy of rainfall distribution used was seen from the deviation
of designed flood obtained from its comparison with the observed designed flood
discharge from the frequency analysis.
Result of the analysis on all the rainfall distributions and rainfall duration
shows that for Progo at Badran the smallest deviation was Tadashi Tanimoto, for
Bogowonto at Pungangan it was the rainfall distribution of Bogowonto, for
Kupang at Kuripan Kidul it was Ika Tyasning Yudiyanti rainfall distribution in
Wuryantoro subcatchment, for Pemali at Bantarkawung it was ABM rainfall
distribution, and for Lusi at Tawangharjo it was Ika Tyasning Yudiyanti rainfall
distribution in Temon subcatchment. Rainfall distribution indicating the mostly
overestimated designed discharge in Progo, Pemali and Kupang was ABM, and
that in Lusi was Envelope curve rainfall distribution. Result of the underestimated
discharge in Progo catchment was Temon subcatchment rainfall distribution and
for Bogowonto, Kupang and Pemali catchments was Alang subcatchment rainfall
distribution.
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