would be brought about by withdrawals, including changes in river flow, stage, salinity, and water age (i.e., the length of time a water parcel remains in the river).
The Committee found the work of the hydrology and hydrodynamics (H&H) workgroup on building, testing, and analyzing its hydrologic and hydrodynamic models, including efforts to quantify the propagation of data uncertainty into hydrodynamic model uncertainty, to be state-of-the-art science. The District is building their WSIS analyses on a hydrodynamic foundation that is well-tested, robust, and well-understood. The H&H workgroup could further improve its efforts by comprehensively synthesizing its model results in its final report. This would put into context the relationships between key mechanisms of the river system and their responses to forecast conditions. In particular, the workgroup should pay attention to two major competing effects—sea-level rise and increased runoff due to future land use changes such as urban and suburban development—that affect water surface levels and salinity, and the uncertainties associated with these effects should be discussed. This “bigpicture view” of the river should be directed at non-modelers and non-hydrologists so that they can better understand the implications of the extensive modeling studies.
A previous report of the Committee noted the limitations of the surface water hydrology modeling program HSPF and the steady-state groundwater flow models based on MODFLOW. Because HSPF has limited value for wetlands, the District was urged to (and subsequently did) continue developing the Hydroperiod Tool and analyzing water level data from transects used to develop regulations on minimum flows and levels (MFLs) to determine the correspondence between river stage and wetland hydroperiod and thus the potential responses of different wetland types to water withdrawals. In the future, the District also should develop a groundwater model that simulates the full interaction of the river with the surficial aquifer system and the Upper Floridan aquifer under both steady state and transient conditions. This should include an uncertainty analysis for groundwater discharge to the river based on hydraulic conductivity, which may have uncertainties of an order of magnitude or more for basins the size of the St. Johns.
Seven environmental workgroups used information from the H&H results in combination with hydroecological models of possible effects to predict the potential impacts of water withdrawals on (1) wetland vegetation, (2) soil biogeochemical processes, (3) plankton communities, (4) submersed aquatic vegetation (SAV), (5) freshwater and estuarine benthos, (6) fish, and (7) wetlands wildlife. Each workgroup was asked to characterize potential environmental effects of water withdrawals using three criteria: persistence, strength, and diversity. Persistence was defined in terms of recovery time relative to the return interval for conditions causing a given effect; strength was defined in terms of both the intensity and scale (geographic area affected); and diversity was defined in terms of the range of environmental attributes showing effects. Based on the three criteria, the District developed five categories of effects ranging from negligible to extreme. For each ranking, the workgroups assigned an uncertainty level (ranging from very low to very high) defined with reference to (1) the availability of a predictive model, (2) supporting evidence, and (3) understanding of the mechanism for an effect. These categories for levels of effect and uncertainty were defined in an effort to obtain consistency among the workgroups in assessing effects—a strategy condoned by