The St. Johns River Water Management District in northeast Florida is studying the feasibility of withdrawing water from the St. Johns River for the purpose of augmenting future public water supply. The District requested that its Water Supply Impact Study (WSIS) be reviewed by a committee of the National Research Council (NRC) as it progresses. This third report from the NRC committee focuses on the hydrology and hydrodynamics workgroup. A
brief summary of the report’s major conclusions and recommendations is presented below.

The main output of the hydrologic and hydrodynamic models is to predict stage, flow, and salinity at various points along the river given potential changes in water withdrawals—information which will then be used by the six ecological workgroups to better understand impacts. The committee is generally satisfied that the modeling approach of the hydrology and hydrodynamics workgroup reflects the state of the science and available data and information. As in previous reports, the criticisms mentioned in this report are intended to improve the workgroup’s efforts as the modeling evolves to support future water supply planning in an adaptive management framework.

Modeling of the St. Johns River watershed, which is integral to understanding the hydrologic response of the river to changes in water withdrawal, relies heavily on accurate estimates of future land use. The committee cautions that the land use relationships developed for the current WSIS may not hold in the future, especially if the actual rate of population increase or its impact on the hydrologic response of the resulting change in land use is significantly different from the current forecast. It is strongly recommended that the District revisit and update the population and resulting land use projections in future periodic reviews.

The hydrologic model was calibrated using observed meteorology with fixed (1995) land use over the decade 1995 to 2006, such that the model’s reliability is limited outside its calibrated time span (e.g., for the 2030 conditions). Because insight can be obtained by a quantitative evaluation of the model outside its calibration range using newer data, it is recommended that the District apply the model (without further calibration) to 2009–2010 land
use conditions and 2009–2010 observed rainfall and streamflow to provide a basic understanding of how the model behaves for a case outside the calibration range. In addition, whether the hydrologic model can adequately quantify confounding processes outside the calibration range is unknown. Confounding processes are processes whose effects are large but in the opposite direction such that they tend to cancel each other out. Even with the best possible model, confounding processes outside the calibration range can lead to uncertainty in the prediction that is larger than the magnitude of the predicted impact. The District’s analysis of model results across the scenarios should carefully consider which scenarios have confounding processes and
which do not.

The hydrologic and hydrodynamic models provide reasonable approximations of the major fluxes through the watershed, with the exception of some wetlands hydrologic processes. The following concerns are noted. First, the District should consider supplementing rain gage data with NEXRAD Doppler data for scenario testing. Second, due to a changing climate, decisions based on model predictions using historic rainfall conditions should be revisited as the state-of-the-science improves. Finally, as computational power increases over the next decade, the grid resolution of the hydrodynamic model should increase to 4X or even 16X in order to not limit the model’s ability to represent physical/ecological dynamics driven by salinity gradients.

For reasons detailed in the report, the HSPF model used to predict hydrologic changes for the different water withdrawal scenarios has limited value for wetlands. The District is urged to continue developing the Hydroperiod Tool and analyzing the empirical water level data available from minimum flow and levels (MFL) transects in order to determine the correspondence between river stage and wetland hydroperiod. These tools and data have the potential to provide considerable insight into the response of the different wetland types to water withdrawals.

Results of the hydrologic/hydrodynamic simulations, expressed in terms of changes in flow and water stage, were generated for various scenarios at two locations in the watershed. The modeling revealed that flow and stage would generally increase under the proposed full withdrawal condition, assuming management of the upper basin to bring water back into the system and the 2030 land use condition (which would increase the contribution of stormwater to river flow). Given these results, which were unexpected at the onset of the WSIS, the committee urges that as much attention be given to potential water quality and other environmental impacts of future increases in flows and levels and in the temporal distribution and routing of flows as to the potential for decreases in flow and levels.