Hydrograph modeling with rational modified method

¹ Planning and Management of Coastal Area and Watershed, Faculty of Geography, Universitas Gadjah Mada, Indonesia  
² Department of Environmental Geography, Faculty of Geography, Universitas Gadjah Mada, Indonesia  

^* Corresponding author: megayulisetyawidasmara@gmail.com

Abstract

Water, a vital natural resource for a human being, could bring negative effects such as flood and landslides. The best way to show the hydrological process is called “model”. One of them is Modified Rational Method (MRM). There are several types of MRM base on its equation modification. Hydrological mass balance or kinematic wave in order to route the flow. With this model modification, the output of the model is not only peak discharge but also unit hydrograph. Model modification was done in the calculation of peak discharges by assigning the C value (coefficient runoff), A value (area) and land characteristic (soil texture, Manning roughness coefficient, and saturation coefficient in the pixel basis. PCRaster software allows us to perform discharge calculation on each pixel. Flow accumulation by using kinematic wave was done to get the unit hydrograph. Three (3) flood events were used to run the model validation, i.e. January 21, January 22, and February 10, 2016. Each event has different rainfall characteristics. The result of this model was DRO hydrograph. Based on the baseflow separation of the observed hydrograph as well as the hydrograph from the model, we found that the flow through the outlet during discharge recession is the base flow. The accuracy value is quite good, i.e. 10–30 %. The result of the model shows a different response between direct runoff and base flow, while time to peak was faster than the recession time.