An Analysis of Excessive Rainfall Events and Suwannee River Streamflow Response

The Suwannee River Basin, located in South Georgia and North Florida, is a hydrologically complex area, located in a region with karst topography, high agricultural demands, and where a high percentage of the Floridan Aquifer is not protected by an overlying clay layer. These conditions allow for rapid recharge to the aquifer during times of abundant rainfall. While the Sacramento Soil Moisture Accounting Model (SAC-SMA), which is used by the National Weather Service (NWS) River Forecast Centers (RFCs) to produce life-saving forecasts for the public, attempts to model the wetting and drying process of the soil, it does not account for deep groundwater processes. Consequently, the uniqueness of the Suwannee River Basin groundwater-surface water interaction results in model simulations that overestimate river response during these high rainfall events.

The Suwannee River near Ellaville, FL is located near the confluence of the Withlacoochee, Alapaha, and Suwannee Rivers, and is a NWS flood forecast location. There have been multiple instances of “false alarms” in recent years, where RFC forecasters have issued forecasts that crest much higher than what actually occurs. This recurring “over forecast” issue generates mistrust between the NWS, their partners, and the public that lives along those rivers. Routing the higher level forecasts downstream continues this over forecast to communities for the entire lower Suwannee River system.

This study attempts to find the locations and extent of the surface water loss along the Suwannee River system due to groundwater demand that occurred during three high flow events from 2009 through 2015. An analysis of historic surface streamflow and historical groundwater data across the Suwannee River drainage system will lead to a greater understanding of when groundwater levels will hamper surface streamflow response. The results of this investigation will allow hydrologic forecasters to improve models within the Community Hydrologic Prediction System (CHPS) by accounting for the loss of routed flow due to groundwater demand, as well as create forecast simulations that are more in line with observations. These results will also improve communication and usefulness of river forecasts to better serve partners with their decision making.