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links both systems. One of the many advantages of an integrated model is that it<br />

simulates the entire hydrologic system. It represents the “state-of-art” tool in assessing<br />

changes due to rainfall, drainage alterations, <strong>and</strong> withdrawals. The model code used is<br />

called the Integrated Hydrologic Model (IHM) which combines the HSPF surface water<br />

code <strong>and</strong> the MODFLOW ground-water code using interprocessor software. More<br />

details on the application of this model to estimating declines in flow at <strong>Weeki</strong> <strong>Wachee</strong><br />

can be found in Appendix 2-2.<br />

The INTB model is a regional simulation <strong>and</strong> has been calibrated to meet global<br />

metrics. The model is calibrated using a daily integration step for a transient 10-year<br />

period from 1989-1998. Model-wide mean error for all wells in both the surficial <strong>and</strong><br />

Upper Floridan aquifers is less than 0.2 feet. Mean absolute error was less than two<br />

feet for both the surficial aquifer system (SAS) <strong>and</strong> UFA. Total stream flow <strong>and</strong> spring<br />

flow mean error averaged for the model domain are each less than 10 percent.<br />

The INTB was used to assess groundwater impacts resulting from groundwater<br />

withdrawals from 110 mgd pumpage within the Northern West-Central Florida<br />

Groundwater Basin (NWCFGWB) <strong>and</strong> 224 mgd pumpage from the Central West-<br />

Central Florida Groundwater Basin. The domain of the INTB model is shown in Figure<br />

2-16 <strong>and</strong> the simulation period was 1993-1998. The model was adjusted to reflect<br />

recharge from septic tank leachate <strong>and</strong> for the non-consumptive groundwater<br />

withdrawals associated with mining limestone The results indicate that <strong>Weeki</strong> <strong>Wachee</strong><br />

discharge was reduced by an average of 21.2 cfs over the five-year simulation period.<br />

A second model (Northern District Model, NDM. Hydrogeologic, 2008) of even greater<br />

spatial extent (11,220 mi 2 ) was calibrated to steady-state 1995 <strong>and</strong> the results<br />

compared to pre-development conditions (zero withdrawals). Based on the impacts of<br />

the 1995 groundwater withdrawals (450 mgd) over the NDM domain, predicted<br />

reduction in the <strong>Weeki</strong> <strong>Wachee</strong> Springs discharge was 9.7 cfs.<br />

____________________________________________________________________________________________<br />

Proposed <strong>Minimum</strong> <strong>Flows</strong> <strong>and</strong> Levels for <strong>Weeki</strong> <strong>Wachee</strong> <strong>River</strong> Page 33 of 164<br />

Watershed Characteristics

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