Learn More: Radar-Based Rainfall Estimates

What does this mean?

Radar-based rainfall data provide an estimation of rainfall distribution which is based on data from multiple weather radar towers located throughout the state. Bias-correction algorithms are then applied which use supplemental rain gauge data. These algorithms are needed to resolve discontinuities which may occur at the limits of coverage of individual radar sites. The rainfall data used by the Water Atlas were provided by the Southwest Florida Water Management District (SWFWMD) and the St. Johns River Water Management District (SJRWMD). The data can be visualized as a grid of estimated rainfall amounts, with each grid cell representing a 4 km2 area (2 km x 2km). The SWFWMD and SJRWMD provide monthly rainfall estimates for each grid cell within each district's geographic boundary, with a lag of about one month. Data are available for the period from 1995 until the present.

SJRWMD interactive radar rainfall map

How are the data collected? (Methods)

Rainfall data are received on a monthly basis and the individual grid cell values are copied into database tables. These tables are then used to calculate rainfall statistics for different time periods (monthly, annual, period-of-record) and for different geographic areas: single grid cells, basins, watersheds, and/or WBIDs (watershed-based land areas associated with regulatory activity under the Clean Water Act). Rainfall statistics are calculated and stored in the database so that they may be displayed graphically in charts and graphs and shown on choropleth maps. A choropleth map uses colors or patterns to represent ranges of values associated with a particular phenomenon.


The following calculations are made for each level of geography (grid cell, basin, watershed, WBID, e.g.):

  • Rainfall for each month of the year, for the entire period of record: Maximum, upper quartile, median, lower quartile, and minimum.
  • Rainfall for each year during the period of record: Total, median
  • Monthly rainfall totals for the most recent year

rainfall boxplot

Caveats and Limitations

  1. Intent: This component displays rainfall estimates and is designed to allow temporal and spatial comparison of rainfall amounts, not exact rainfall quantities.
  2. Geographic Extent: Grid data is provided for the entire area within the two water districts' boundary. The geographic area of some Water Atlas sites includes portions of more than one Water Management District and therefore the data displayed comes from more than one source.
  3. Data Aggregation: At a minimum, Water Atlas sites have data aggregated at the geographic level of pixel and WBID (water body ID); some sites have data aggregated for basins and watersheds in addition.
  4. Data Integrity: There are known quality problems with the 2004 data, as there were some issues with the source NexRad imagery that year, which underreported rainfall by appx 15% due to problems with the National Weather Service source radars for that year. Every radar-based rainfall product will have the same errors for that year.
  5. Questions: For questions about SWFWMD-supplied data, please contact Margit Crowell, (352) 796-7211 x4310 or Margit.Crowell@WaterMatters.org. For questions about SJRWMD-supplied data, please contact Phillip Benson, Bureau of Water Resource Information, (386) 329-4500 or PBenson@sjrwmd.com