Review of the St. Johns River Water Supply Impact Study: Report 3 (2010)Water Science and Technology Board
A proposed withdrawal of 262 million gallons of water per day from the St. Johns River in northeast Florida -- intended to help satisfy increasing demand on public water supplies -- may not decrease the river's average flow and water level, according to research conducted by the St. Johns River Water Management District. The unexpected finding assumes management of the upper river basin to bring water back into the system and depends on projected land use patterns that would increase the contribution of stormwater to river flow. In its ongoing review of scientific aspects of the District's Water Supply Impact Study, the National Research Council urges the Water Management District to give as much attention to water quality and environmental impacts from potential increases in water flow and levels as from potential decreases.
The surprising model predictions come from the District's hydrology and hydrodynamics workgroup. The National Research Council committee conducting this review is generally satisfied that the workgroup's modeling approach reflects the state of the science and available data and information. Six other ecological workgroups will use information from the hydrologic and hydrodynamic simulations to better understand potential impacts throughout the river basin.
- How to handle a changing climate in hydrologic modeling remains an open and important research question. 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). Insight can be obtained by a quantitative evaluation of the model outside its calibration range using newer data.
- Predicted land use may be the critical driver on water flow and levels. As urban land use expands, increasing impervious surfaces tend to decrease water fluxes through the vadose zone (from the top of the ground surface to the water table) and increase surface water flow. The net effect is to decrease transpiration losses and thus increase the surface water available for withdrawal.
- Projected population growth through the planning period (to the year 2030) is expected to result in increased residential and commercial/industrial land use while open, range, forest, and agricultural land areas in the drainage basin decrease. These land use relationships, developed for the current Water Supply Impact Study, 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.
- The HSPF hydrological model (hydrologic simulation program - Fortran) has limited value for modeling wetlands. Continued development of the Hydroperiod Tool, which estimates daily water depth using the difference between ground surface and water surface elevations, along with analysis of the empirical water level data available from minimum flow and levels (MFL) transects would better determine the correspondence between river stage and wetland hydroperiod.