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considered small.

For Way Awi (Figure 5), using different time steps

also show different results for time to peak. Using time

step of 10, 30 and 60 minutes resulted in aver age time to

peak of 30, 30 and 60 minutes respectively. Again,

smaller time step gives more reasonable results in indi-

cating time to peak.

For Way Kuala Garuntang, in addition to time step, the

unit hydrographs are also made into three groups ac-

cording to the months. The groups are for December,

January, as well as February and March events (Figure

6). The average time to peak for time step of 10 minutes

is 30, 60 and 60 minutes for December, January and

February-March events respectively. While the average

time to peak for time steps of 10 minutes for overall

events is 60 minutes. For time step of 30 minutes, the

average time to peak is 30, 60, 60 and 60 minutes for

December, January, February-March and overall events

respectively. The average time to peak for time step of 60

minutes is 60 minutes for December, January, February-

March and overall events. For a larger catchment such as

Way Kuala Garuntang, smaller time steps confirm time

to peak as resulted from larger time step. In this case, it is

predicted that the appropriate time to peak for Way

Kuala Garu ntang is 60 minutes.

3.3. Time Steps and Peak Discharges

In addition to time to peak, another important issue with

regard to unit hydrog raph is the peak discharge. For Way

Simpur (Figure 4) peak discharges for all time steps are

in the range of 0.5 - 2.4 m3/s, where the average peak

discharges for time steps 10, 30 and 60 minutes are 1.8

m3/s, 1.2 m3/s and 1 m3/s respectively. Please note that in

fact there are seven peak discharges in the unit hydro-

graphs for Way Simpur using time step 60 minutes (Fig-

ure 4(c)), which seems to be sorted into two groups be-

cause five of them are in the range of 1.023 - 1.028 m3/s

and the other two are 0.931 and 0.937 m3/s. Therefore it

looks like there are only two curves as the outcome of

seven flood events (Figure 4(c)).

For Way Awi (Figure 5) the average peak discharges

for time steps 10, 30 and 60 minutes are 4.1 m3/s, 3.7

m3/s and 1.8 m3/s respectively. The results show the

smaller the time step the larger the peak discharge. This

happen because only selected flood events which have

high rainfall intensity during short time interval are able

to be utilized in con structing unit hydrograph. Therefore,

the rainfall intensity is larger at smaller time step which

impacts on larger peak discharge.

The average peak discharges for Way Kuala Garun-

tang for December, January and February-March events

(Figure 6) using time step 10 minutes are 7.7 m3/s, 4.5

m3/s and 4.2 m3/s respectively, using time step 30 min-

utes are 5.8 m3/s, 4.7 m3/s and 7.4 m3/s respectively and

using time step 60 minutes are 4 m3/s, 4.6 m3/s and 5.2

m3/s respectively. While the trend of average peak dis-

charges seem opposite for time step 10 minutes, the trend

of those for other time steps shows there is an increase of

average peak discharges toward the peak of wet season.

For February-March flood events there were only two

out of six flood events which were able to be utilized in

unit hydrograph using time step 10 minutes, and there

were only six out of eight even ts for January flood even ts

could be utilized for 10 minute time step hydrograph.

Meanwhile, all three flood events in December could be

used for 10 minute time step hydrograph. Therefore the

results from using time step 10 minutes show inconsis-

tent trend with regard to the wetter season as the lack of

data.

3.4. The Average of Unit Hydrographs for

Different Time Steps

Comparing the results between those three catchments

there is a general trend of average peak discharge, i.e. the

higher the time step, the lower the average peak dis-

charge (Figure 7). This trend does not fully work for

Way Garuntang as the peak discharge using time step 10

minutes is lower compare to that using time step 30 and

60 minutes. Considering overall events for Way Kuala

Garuntang, average peak discharges for time steps 10, 30

and 60 minutes are 4.64 m3/s, 5.2 m3/s and 4.7 m3/s. Al-

though in nearly all unit hydrographs, peak discharges

resulted from using time step 10 minutes are larg er com-

pared to peak discharges resulted from using larger time

step. This may happen because the method in calculating

the average peak discharge is so simple, that is simply

taking the average of the events for particular time step,

both for the discharge and time to peak. Furthermore,

peak discharge is closely related to time to peak. Using

small time step, time to peak may vary significantly be-

tween 10 to 60 minutes. Considering peak discharges

which occur at various time to peak, this may result in

low average of peak discharge as in the case of average

peak discharge of Way Kuala Garuntang using time step

10 minutes.

4. Conclusions

This study shows the impact of time steps on un it hydro-

graphs with regard to time to peaks and peak discharges.

In general, smaller time step gives more accurate resulted

unit hydrographs. It was observed that the average time

to peaks for Way Simpur are 20, 30 and 60 minutes using

time steps 10, 30 and 60 minutes respectively. The aver-

age time to peaks for Way Awi are 30, 30 and 60 minutes

using time steps 10, 30 and 60 minutes respectively. And

he average time to peaks for Way Kuala Garuntang are t

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