CALIBRATION OF WATERSHED LAG TIME EQUATION FOR PHILIPPINE HYDROLOGY USING RADARSAT DIGITAL ELEVATION MODEL Fatima Cipriano 1 , Alfredo Mahar Francisco Lagmay 1,2 , Matt Horritt 3 , Enrico Paringit 1,2 , Christopher Uichanco 1 , Jerico Mendoza 1 , Glenn Sabio 1 , Kenneth Nino Punay 1 , Herbert James Taniza 1 , and Mary Rose Oquindo 2 1 Nationwide Operational Assessment of Hazards, P. Velasquez Street, UP Diliman, Quezon City, 1101, Philippines Email: mafcipriano@noah.dost.gov.ph 2 University of the Philippines, Diliman, Quezon City, 1101, Philippines, Email: amfal2@yahoo.com 3 Horritt Consulting, 1 Tredunnock Barn, Llangarron, Ross on Wye, HR9 6PG, United Kingdom Email: matt@horrittconsulting.co.uk KEY WORDS: Hydrology, RADARSAT, digital elevation model, Philippines ABSTRACT: 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. 1. INTRODUCTION The Philippines is one of the most hazard-prone countries in the world when it comes to water-related disasters like floods and landslides. The country has been among the top five most frequently hit by natural disasters, along with China, India, Indonesia, and the United States. According recent studies, the Southeast Asian region, where the country is part of, will more likely experience higher frequency of flooding events in the upcoming years (The World Bank, 2010). In 2009, typhoon Ketsana, locally known as Ondoy, one of the worst typhoons since the 1960's, struck the country (CDRC, 2013) and caused heavy flooding in the country's capital. This calamity resulted in 464 deaths (Rappler, 2014). Similarly, in 2013, typhoon Haiyan, locally known as Yolanda, was the biggest disaster for the country that year (CDRC, 2013) and struck areas in the Visayas region. The region was severely affected because of flooding and other related hazards. Six months after Yolanda, around 24,000 individuals were affected and 6,300 individuals were reported to be dead (Locsin, 2014). Inundation of both urban and rural areas caused by heavy rainfall greatly affect the lives of families and communities. It is essential to understand different hydrological parameters that affect the movement of water in different areas in order to analyze flooding in the country. When it comes to the study on floods, hydrology is one of the important factors that needs to be understood. For this study, a key hydrological parameter, the lag time (TL) is determined using a calibrated equation specific for Philippine setting. The equation will be determined using measurable parameters namely the watershed length, watershed slope, and maximum potential retention derived from the curve number, all of which are available from RADARSAT Digital Elevation Models (DEM). The value of the lag time is essential when it comes to determining river discharge values for flood inundation modeling.