Two widely used procedures by which radar- and gauge-derived rainfall can be optimally combined are those by the OneRain Corporation and the National Weather Service (NWS). The NWS procedure, called the Multi-sensor Precipitation Estimator (MPE) produces an hourly product on a 4×4 km grid (the HRAP grid). This scheme is used operationally by local NWS offices and by their River Forecast Centers. The OneRain procedure is proprietary and has not been described in the literature. However, it produces a product at 15 min intervals on a 2×2 km Cartesian grid. The Florida Water Management Districts as well as other governmental and private groups use data from the OneRain algorithm. Florida State University (FSU) has employed the MPE scheme to create an historical precipitation database for the Florida Department of Environmental Protection (FDEP). Although the methodologies as well as the time and space resolution of each dataset differ, each is being used to make water management and regulatory decisions. Thus, it is important to know how rainfall values from the two schemes compare to each other.

This paper describes results from a statistical comparison of values from the two schemes. To our knowledge, such a comparison has not been performed previously. We consider the area of the South Florida Water Management District during the four year period 2002-2005. We sum the 15 min. OneRain precipitation values to give hourly values which then are placed onto the 4×4 km NWS HRAP grid. Great care is used to ensure that the original volume of OneRain precipitation is preserved during this transformation.

Comparison results will be presented for yearly intervals, as well as for the component cold and warm seasons. Precipitation types differ during these two seasons (mostly stratiform vs. mostly convective). In addition, differences between the two schemes based on hourly precipitation data will be compared to those of 6 hourly and daily sums which often are used in hydrologic models. In summary, the paper will describe the extent to which the OneRain and MPE products can be used interchangeably, i.e., those situations when differences are expected to be small or large.