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Aquifer Resource Index

What does this mean?

The Aquifer Resource Index (ARI) was created by the Southwest Florida Water Management District (SWFWMD) to provide information to the media, residents, local governments and other interested parties about current groundwater conditions and how they are compared to historical records. The underlying purpose of this index is to provide the public with a gauge of groundwater levels in their area, so they can develop an understanding of the severity and cycles of drought and recovery. In order to do this, the groundwater information presented in the monthly document "Hydrologic Conditions" was restructured into a regional context that would be recognizable to the general public. The three regions used in this index are defined by county boundaries.

To read detailed monthly reports, visit SWFWMD’s website: http://www.swfwmd.state.fl.us/waterres/hydro/. In the northern region (Citrus, Hernando, Lake, Marion and Sumter counties) the Floridan Aquifer is at or near land surface, allowing rainfall to easily recharge (replenish) the aquifer system. In the central region, (Hillsborough, Pasco, Pinellas and Polk counties), the Floridan can be unconfined or confined (overlain by clay soils). Where the Floridan is confined, recharge by rainfall is low. In the southern region (Charlotte, DeSoto, Hardee, Highlands, Manatee and Sarasota counties) the Floridan is confined.


How are the data collected? (Methods)

The ARI is derived by comparing current groundwater levels with historic levels for 52 monitoring wells located throughout the three regions of the District. Wells were selected for the network based on whether they had an adequate and reliable period-of-record to calculate weekly percentiles. For each well, the 16th percentile ("low normal") and 84th percentile ("high normal") were chosen because they correspond to the statistical equivalent of one standard deviation above and below the mean. The "normal" range is defined between the 16th and 84th percentiles. The normal range for the northern region is 0–4 feet; 0–6 feet for the central region, and 0–8 feet for the southern region.


Calculations

To determine the ARI for a region, each well is compared weekly to its respective low-normal value and the difference is calculated. The weekly differences are reduced to a monthly value for all monitoring wells within a region, and the resulting ARI value represents how far water levels in the aquifer must rise or fall to reach their respective low-normal values.


Caveats and Limitations

When reviewing index values it is always important to remember that indices provide general presentation of what can be inferred from the voluminous amounts of data from which they are derived. In other words, although indices are valuable for gauging conditions, they cannot convey all of the information that can be found in the database through more complex analyses.


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