Review of the St. Johns River Water Supply Impact Study Report 3 (2010) / Chapter Skim
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Chapter 2 Review of Modeling Methods and Results
Chapter 2 Review of Modeling Methods and Results
Pages 12-23
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From page 12... ...
in order to project future populations, and thus water demand, for public water supply utility service areas. Special algorithms were used at the parcel level to reallocate growth from high growth parcels having spatial constraints to lower growth parcels, while still conforming to the county-level projections.
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From page 13... ...
Indeed, the land use shown for 2004 in Figure 2-1 does not seem to reflect projected changes in population growth. Thus, although the Committee does not have any technical issues concerning the approach used by the District to estimate 2030 land use/land cover conditions, it recommends that the District revisit and update the population and resulting land use projections in future periodic reviews (e.g., the District's water supply plans and the water supply assessments made by the District every five years)
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From page 14... ...
Where only daily rainfall records were available, disaggregation from daily to hourly values was accomplished using the software package WDMUtil, which is part of the EPA BASINS package and can be considered state-of-the-science. Although more comprehensive Doppler rainfall data are available, the District chose to use the rain gage data due to their longer period of availability.
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From page 15... ...
However, the existing Doppler radar sets that the District did not use in their hydrologic modeling efforts could be used to help quantify the uncertainty associated with distribution of point-gage rainfall data across the Thiessen network. A further criticism regarding the use of NWS rain gages is discussed below relative to the calibration of the watershed hydrology model.
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From page 16... ...
The model has been demonstrated to provide reasonable results during the calibration period. The District chose to use data from the NWS rainfall gages rather than NEXRAD Doppler radar data for their calibration study because they are using a 32-year NWS rainfall gage record for scenario analysis.
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From page 17... ...
RIVER HYDRODYNAMICS The District provided an extensive PowerPoint presentation on the river hydrodynamic model development and calibration. It is clear that they are proceeding carefully and analyzing how model implementation choices affect results.
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From page 18... ...
It is well known that coarser model grids induce a damping error – "numerical dissipation" – that has the same effect on the model as the turbulent dissipation produced by the bottom boundary. The small bottom roughness height produced by calibration indicates that numerical dissipation is of the same order of magnitude or larger than physical dissipation produced by boundary-driven turbulence.
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From page 19... ...
This section focuses on resolving comments made previously by the Committee about the assumption of constant chloride concentrations in groundwater input to the river channel over time and about the use of a steady state, non-density-dependent groundwater model. Groundwater Salinity An earlier criticism of the District's analysis of groundwater was their assumption of a constant average chloride concentration value for a given location in their groundwater modeling.
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From page 20... ...
Using well level and river stage data from four locations in the middle basin, the EFDC model was run with both the steady state and transient groundwater flux (using 10 years of transient groundwater discharge data)
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From page 21... ...
Thus, despite the initial (and intuitive) assumption that water withdrawals would reduce river flows and stage, the modeling revealed that both variables in fact would increase under the full withdrawal condition, assuming management of the upper basin to bring water back into the system and the 2030 land use condition.
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From page 22... ...
For example, at Cocoa there was a 1.5-ft rise in stage at the 11-12 ft mark because the upper basin project and the 2030 land use more than compensated for any loss in stage from water withdrawal. The modeled results indicate that the effect of the upper basin projects would taper out as stage rises, because the projects will store water in the upper basin at high flows.
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From page 23... ...
. It should be noted that the modeling studies predicted that full withdrawal plus the Ocklawaha withdrawal would lead to a 9.3 percent decrease in flow in the Ocklawaha River basin compared to the 1995 baseline condition.
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