CALIBRATION OF WATERSHED LAG TIME EQUATION FOR PHILIPPINE HYDROLOGY USING RADARSAT DIGITAL ELEVATION MODEL

摘要

Widespread flooding is a major problem in the Philippines. The country experiences heavy amount of rainfall throughout the year and several areas are prone to flood hazards because of its unique topography. Human casualties and destruction of infrastructure are some of the damages caused by flooding and the Philippine government has undertaken various efforts to mitigate these hazards. One of the solutions was to create flood hazard maps of different floodplains and use them to predict the possible catastrophic results of different rain scenarios. To produce these maps with accurate output, different input parameters were needed and one of those is calculating hydrological components from topographical data. This paper presents how a calibrated lag time equation was obtained using measurable catchment parameters. Lag time is an essential input in flood mapping and is defined as the duration between the peak rainfall and peak discharge of the watershed. The lag time equation involves measurable parameters, namely, watershed length and watershed slope, which were both available from RADARSAT Digital Elevation Models (DEM). The equation also involves the maximum potential retention of the soil derived from its curve number which was processed from the Philippine Curve Number Map. This approach was based on a similar method developed by CH2M Hill and Horritt for Taiwan, which has a similar set of meteorological and hydrological parameters with the Philippines. Rainfall data from fourteen water level sensors covering 67 storms from all the regions in the country were used to measure actual lag time values. The measured lag time values were plotted against the calculated values obtained from the Natural Resource Conservation Management handbook lag time equation. Regression analysis was used to obtain the final calibrated equation that would be used to calculate the lag time specifically for rivers in the Philippines. The calculated lag time values could then be used as a parameter for modeling different flood scenarios in the country.