Review of the St. Johns River Water Supply Impact Study Final Report (2012) / Chapter Skim
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2 Hydrology and Hydrodynamics
2 Hydrology and Hydrodynamics
Pages 19-36
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From page 19... ...
The purposes of the groundwater modeling were to estimate the response of the underlying aquifer to potential water withdrawals in terms of discharge and aquifer head change, to provide boundary conditions for the mainstem hydrodynamic model, and to provide groundwater data for water budget calculations. Because most of the hydrologic and hydrodynamic modeling efforts were reviewed extensively in NRC (2010)
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From page 20... ...
. The hydrodynamic model was run through a comprehensive set of scenarios to evaluate model sensitivity to different forcing functions.
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From page 21... ...
For example, Base1995PS combines 1995 land-use conditions with 2030 forecast sea-level rise and the upstream projects installed in the present decade, and this combination does not reflect any reasonable historic or forecast condition of the river. Indeed, the purpose of most of these scenarios was neither to forecast nor hindcast3 a possible river condition but to isolate effects and evaluate model sensitivity for different forcing conditions.
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From page 22... ...
GROUNDWATER MODELS Modeling of groundwater flow was used in the WSIS to examine the impact that surface water withdrawals may have on groundwater discharge to the river due to a declining river stage. The models were used mainly in the middle SJR, where surface waters are influenced by groundwater, and to set up groundwater discharge and salinity boundary conditions for the hydrodynamic model.
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From page 23... ...
test whether increased groundwater discharge and chloride load would appreciably alter river conditions if river water levels declined due to surface water withdrawals. Because groundwater discharge is nearly impossible to measure by direct observation, modeling or indirect estimates based on a river basin mass balance are the only practical alternatives.
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From page 24... ...
To clarify the Committee's concerns, it is useful to recognize that the hydrologic modeling effort served two roles in the overall study: (1) Providing time- and space-varying flow rates into the middle and lower SJR as forcing conditions for the hydrodynamic model, which in turn provides water level and flow conditions for the ecological models for those segments, (2)
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From page 25... ...
. The calibration report covers all the key factors involved in designing a working hydrodynamic model, including development of a digital elevation model, grid selection and testing, initial and final calibration results, comparisons with observed data, and confirmation (verification)
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From page 26... ...
. This comparison allows the reader to observe how salinity increases due to water withdrawal, without the complicating factors of the USJR projects, sea level rise, or changes in land use.
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From page 27... ...
(2011) , Volume 2, Chapter 7 where the wide range of water levels at Lake Harney completely obscures all the other data.
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From page 28... ...
. RMSD for daily averaged water surface levels at Lake Harney for Full1995NN and Base1995NN is 4.9 cm.
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From page 29... ...
. Longitudinal Distributions Plots of the longitudinal distribution of mean variables and differences between mean variables in models provide a way to collapse the time variability and emphasize the space variability in model output, e.g., see Figure 2-4.
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From page 30... ...
This approach provides a method for understanding the relationship between the river discharge and the changes associated with different forcing conditions. For example, the District grouped the model results for river discharge using 25 m3 s-1 intervals and plotted key statistical characteristics (median, minimum maximum and quartiles; see Figure 2-5)
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From page 31... ...
For example, Figure 2-7 shows that a salinity of 4 ppt with a return period of ten years has a duration of about three days in the baseline model, but a duration of ten days under the full withdrawal5. FIGURE 2-6 High water levels in Lake Harney over 10 year simulation.
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From page 32... ...
allowed it to develop a linearity test to better understand the relative contributions of different forcing conditions to physical effects and whether or not interactions between different forcing conditions lead to nonlinear behavior. Linear behavior allows different forcing conditions and model response to be analyzed individually, whereas nonlinear behavior requires different forcing conditions to be analyzed together.
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From page 33... ...
. FIGURE 2-9 Linearity test for salinity effects in Future Conditions scenarios.
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From page 34... ...
The principal limitation of the District's uncertainty analysis is that it focuses on evaluating model performance uncertainty relative to the hindcast conditions and does not address the underlying uncertainty in forecast conditions. The District's focus on hindcast uncertainty is commensurate with the state-of-the-art in hydrodynamic modeling; however, future land-use conditions, future sea-level rise, and permitted water withdrawal restrictions are also subject to considerable uncertainty.
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From page 35... ...
It appears likely that uncertainties in forecast conditions may be the critical unknown. From results shown during the committee meetings, the Committee concludes that there are two major competing effects -- sea-level rise and increased runoff due to future land use changes such as development -- that both affect water surface levels and salinity.
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From page 36... ...
2010. Hydrodynamic Modeling Results.
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