Impact of Using Radar Rainfall Data in Water Budgets for South Florida Stormwater Treatment Areas

The South Florida Water Management District (District) currently operates 6 Stormwater Treatment Areas (STAs) located in the Everglades Agricultural Area (EAA) covering a total of 16,200 hectares. STAs are large wetland filter marshes that receive agricultural runoff in an effort to reduce suspended solids and nutrients prior to discharge to environmentally sensitive waters in the Everglades. One of the tools used to assess and manage these areas is an annual water budget analysis and report. The District is acquiring rain gage-adjusted, 15-minute NEXRAD radar rainfall data from a vendor, in conjunction with the other Florida water management districts (Huebner, et al, 2003). The NEXRAD data provide 2 km by 2 km areal estimates of rainfall. In South Florida, rainfall usually comprises up to approximately 10 percent of an STA's annual inflow. It is not unusual for annual water budget errors to be on the same order (Abtew, 2005; Liyanage and Huebner, 2005). Due to the nature of storm cells during the wet season (June through October) in South Florida, using rain-gage values of rainfall for treatment areas that span up to 6,700 hectares has the potential to introduce a significant error in the water budget. Using radar rainfall data for this purpose introduces its own set of errors associated with this rainfall estimation technology. Using NEXRAD and rain-gage rainfall data and other water budget data for three STAs over a two-year period, this analysis concludes that, for the years and STAs examined, NEXRAD data provided results that were consistent with data from the rain gages in annual water budget calculations and represented spatial variation of rainfall across an STA's footprint.