To estimate runoff we have to consider:
a. Debit of ciliwung at Depok can be calculated using rating curve as follows : debit Q = 21.5(TMA+0.22)^1.5 m3/s , TMA = water level when proclaim alert :
Alert I (Siaga I), TMA > 350 cm equivalent to debit 154 m3/s or 555 335 m3/h.
Alert II (Siaga II), TMA > 270 cm equivalent to debit 386 202 m3/h
Alert III ( Siaga III) , TMA > 200 cm is equivalent to debit 71.1 m3/s or 256 017 m3/h.
In an ideal condition (no sedimentation) maximum debit of Ciliwung river at smallest cross section is 600 000 m3 /h. Today debit Ciliwung river at smallest cross section is about 300 000 m3 /h due to sedimentation and houses at the river bank.
b. Floods is cause by high rainfall intensity (> 15 mm/h) that last for hours. Intensity of rainfall 15 mm/h is equivalent to 150 000 m3 / km2.h. Hence if it last for more than 2 hour and extended over more than 50 km2 on Ciliwung basin than the amount of water will be more than 1 500 000 m3, so that Ciliwung will overflow and cause floods. If the intensity is greater than 20 mm/h for more than 3 hours, and extended over more than 100 km2 , the amount of water is more than 6 millions m3.
Runoff could be estimated using system dynamics that based on generalized Lorenz’s differential equations:
dXi/dt = fi ( X1, X2, …) ; i = 1,2,3,4,…
fi can be determined empirically using system dynamics software Vensim , with the rate of rainfall at downstream Ciliwung basin , and river water level / debit at Depok as inputs to the system and the debit at Manggarai as output of the system.
The input debit from the river is given by rating curve:
Q = 21.5(TMA+0.22)^1.5 m3/s
The input from rainfall can be estimated from pentad prediction and spatial and temporal rainfall distribution.
The output debit can be determined empirically i.e. the debit of the river plus a parameter time square root of rainfall intensity.
From the system dynamics we can show precipitation and debit of Ciliwung at Depok as the input the runoff as a function of time and the output debit.
The accuracy of Floods forecasting of Ciliwung can be increased if the space and time distribution of rainfall can be obtained. We also need topography, hydrology data of Ciliwung, and the dynamics of atmosphere.