OCTOBER SEVENTH CARIBBEAN ISLANDS WATER RESOURCES CONGRESS 2007 HYDROLOGIC STUDIES USING VFLO IN WESTERN PUERTO RICO* Alejandra Rojas 1 and Eric W. Harmsen 2 ABSTRACT: Real-time prediction of flash flooding can save money and save lives. The goal of this project is to develop real-time flash flood prediction capabilities within the Collaborative Adaptive Sensing of the Atmosphere (CASA) Testbed located in western Puerto Rico. Project objectives include: configuration and calibration of a hydrologic simulation model capable of accepting real-time radar data from the CASA distributed collaborative adaptive radar network as input, augmentation of the current river stage and flow monitoring network on the Aæasco, Yagüez and Guanajibo rivers; evaluation of the small-scale variation of rainfall within the study area; validation of the rainfall estimates from the CASA distributed collaborative adaptive radar network; and development and testing of a real-time flood forecasting system for western Puerto Rico. An initial step in developing an Alarm Prediction System is to enhance forecasting skill, to understand the dynamics of the system and how the system response is affected by the spatial resolution and input uncertainty in complex topography and tropical environments. This paper provides some preliminary results related to the configuration and calibration of the hydrologic simulation model. KEYWORDS: Distributed models, Vflo, runoff, rainfall interpolation methods. INTRODUCTION In this study we will use the distributed hydrological model Vflo (Vieux and Associates, Inc., 2004) to simulate flash flooding in western Puerto Rico (Figure 1). The region of study includes the Ro Guanajibo, Ro Grande de Aæasco and Ro Yagüez basins which compass approximately 800 km 2 and include the Guayo, Yahuecas and Prieto reservoirs as boundary condition on the east and the Mayagüez Bay on the west. The flood plain associated with the Ro Grande de Aæasco is characterized by an alluvial fan having an area of 41.5 km 2 and 0.08% average slope. Most of the input data for the Vflo model was prepared in ArcGIS 9.2, using the USGS 30 m x 30 m digital elevation model (DEM). Overland slope, flow direction, and stream locations were determined from the DEM and resized to 200 meters spatial resolution. During this step the streams were !burned" into the model grid using a multi-step process in ArcGIS, in which the flow direction is forced to follow to rivers. This step is necessary because the flow direction calculation tends not to be accurate in low slope areas (e.g., Ro Grande de Aæasco and Ro Guanajibo floodplains). The final resized digital elevation model has a correct flow direction according to the hydrological maps of the topographic quadrangles (Figure 1). The flow direction and the rivers delineated were calculated with Arc Hydro Tools and ArcGIS. With the flow direction information, it was possible to create the stream network. First, the flow accumulation was calculated, which counts the number of cells which contribute flow to each cell. A cell located at a watershed outlet would have a flow accumulation from the total cells in the watershed. Next, a stream definition was made assuming that flow accumulation from 90 cells will produce a river. 1 Ph.D. Graduate Research Assistant, University of Puerto Rico, Mayagüez, Civil Engineering Department. alejandra_rojas@yahoo.com 2 Professor, University of Puerto Rico, Mayagüez, Agriculture and Biosystems Engineering Department. eharmsen@uprm.edu * Funding for this research was provided by NSF-CASA. 81