Compared to conventional rain gauge networks, the Weather Surveillance Radar-1988 Doppler provides precipitation estimates at enhanced spatial and temporal resolution that River Forecast Centers can use to improve streamflow forecasts. This study documents differences between radar-derived (stage III) mean areal precipitation (MAPX) and rain gauge-derived mean areal precipitation (MAP). The area of study is the headwaters of the Flint River basin, specifically the Culloden basin located in central Georgia south of Atlanta, with a drainage area of 1853 mi 2. The timing of radar installations in the southeast United States provided overlapping data for only 2 yr (Jun 1996-Jul 1998). The MAP and MAPX products being examined were prepared using procedures identical to those employed operationally at the National Weather Service's Southeast River Forecast Center. Results show that the radar (MAPX) underestimates gauge-derived rainfall (MAP) by ϳ38% at the end of the 2-yr period. This underestimate is most pronounced during the winter months of November-April when MAPX underestimates MAP by ϳ50%. Comparisons during the summer (May-Oct) indicate that MAPX is similar to MAP. The underestimation of winter rainfall likely is due to several factors: the inappropriate combination of radar values in areas of overlapping coverage, the radar beam overshooting the tops of stratiform rainfall, an inappropriate Z-R relationship, faulty radar calibration, and too few hourly rain gauges to prepare an accurate stage II bias adjustment factor and quality control the stage III product.

In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 a geodetic datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly called Sea Level Datum of 1929.

Lake Okeechobee, located in southern Florida, is an important component in the regional hydrologic system. Currently, the Southeast River Forecast Center (SERFC) is setting up a forecasting scheme for Lake Okeechobee and its major inflows. An important aspect in calibrating the system is estimating the depth of direct precipitation over the water surface. Within this project, National Weather Service (NWS) and South Florida Water Management District (SFWMD) surface gauges, along with stage III multisensor precipitation estimates, are used to create time series of mean areal precipitation (MAP). The computed MAP values are compared in order to find the relative differences between them, and to determine the utility of using each data source for calibration and in future operations. It was found that the SFWMD gauge-based MAP was the most useful data source, because it had a suitable period of record and the SFWMD gauges had a better spatial sampling of precipitation over the lake surface. The radar-based stage III estimates were not found to be a useful source of data, despite the superior spatial sampling resolution, because they had too short a period of record and a number of changes in the processing algorithms made the associated MAP nonhomogeneous and inappropriate for model calibration.

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