Developing a Procedure for a Calibrated High-Resolution Precipitation Database (CHPD) for Florida

A Calibrated High-Resolution Precipitation Database (CHPD) is a dream tool for Total Maximum Daily Loading (TMDL) modelers, as well as all water resource related surface water (SW), ground water (GW) or integrated SW-GW modelers. With the software support of the National Oceanic and Atmospheric Administration (NOAA), funding support of the Environmental Protection Agency (EPA) and academic support from Florida State University (FSU), Florida is the first State to have successfully completed the construction of a practical CHPD.Precipitation input for a watershed model typically has relied on weather data obtained from weather stations sparsely distributed in a given watershed. A popular method for precipitation computation is called the Thiessen method. This method assigns an area called a Thiessen polygon around each weather station or gauge. The Thiessen polygon is an area in which every point within the polygon is closer to the particular gauge than to any other. In effect, the precipitation in the entire polygon is assumed to be uniform and equal to the gauge value. This method does not answer the question about what really happens to rainfall between weather stations. In addition, this method can not give the true rainfall distribution for an isolated or fast moving storm – which may cover one side of a street, but not the other, and change its intensity and coverage along its passage.Hourly Doppler radar-derived rainfall (NEXRAD) estimates possess superior temporal and spatial resolution compared to gauges and can be applied to watershed modeling in TMDL projects. The Doppler-derived precipitation estimates are produced hourly on a 4-kilometer by 4-kilometer grid covering the nation.Hourly radar-derived rainfall data provide excellent spatial and temporal coverage. However, gauges provide more accurate values if the data are carefully quality controlled. Thus, it is appropriate to optimally combine the two data sources so that the strengths of each are realized.This optimal combination has been completed to produce a historical precipitation database for Florida. Correlation coefficients have been calculated between the resulting hourly optimally combined radar estimates versus coincident independent hourly gauge data. The results generally are quite good but imply that errors may be greater for warm season high precipitation scenarios. This aspect is being corrected in a final version of the data set that is currently being prepared.This paper introduces the development of a Calibrated High-resolution Precipitation Database (CHPD) to support the watershed modeling requirements of the TMDL program in the Florida Department of Environmental Protection (FDEP). Approximately ten years of historical precipitation data are required to calibrate the rainfall driven models.This precipitation database also will be released through FDEP's computer world-wide-web server. Furthermore, the database will be used in a graphical user's interface as a module in the WAMView watershed model.