WSP09428

• File Number: 8449.926
• Description: South Metro Water Supply Study
• State: CO
• Basin: South Platte
• Date: 2/1/2004
• Author: SMWSS Board
• Title: South Metro Water Supply Study
• Water Supply Pro - Doc Type: Publication

. Storage Coefficients The confined and unconfined storage coeflicients initially \vere based
on values specified in the S8-74 model. and were moditied locally during model calibration
to impro....e model tit to obser....ed historical drawdo\\TI data. The initial unconfined storage
....alue (specific yield) and contined specific storage were set at 0.17 and 2xlO~. respectively.
These storage values apply only to the permeable sand bodies \\ithin each of the aquifer
units. therefore they needed to be scaled based on the estimated thickness of the sand layers
divided by the total aquifer thickness (aquifer top elevation minus bottom elevation); this
ratio was derived from the values specitied in the S8-74 model. The final values used in the
South Metro model are described in Section 3.2.2. .\fodel Calibration.

. Leakage Between Layers While each of the aquifer units are explicitly simulated as
individual layers. the aquitard uniL~ between each pernleable layer are implicitly simulated
utilizing a leakage model between layers. In MODFLO\\'2000. the leakage was simulated by
multiplying the hydraulic gradient between layers by an effecti....e vertical conductance
bet\\:een layers. The vertical conductance was computed as a layer-thickness~weighted
harmonic average of the vertical hydraulic conductivities of the 1\\"0 aquifers and the \"Crtical
hydraulic conductivity of the intervening aquitard I. Again. the vertical conductance values
as specified in the S8-74 model were used. and those values were modified locally as
required during the model calibration process to improve the tit between model predictions
and observed historical heads and drav..dov-TIs (see Section 3.2.2).

. Transmissh:it). I Hydraulic Conducth.ity The transmissivity of each layer is computed as
the aquifer thickness times the hydraulic conductivity. Therefore. hydraulic conductivity was
specified. and MODFLOW internally computed the transmissivity as required (using the
difference bct\....een the aquifer top and bottom elevations as the thickness). In the South
:\1etro area. all the transmissivity values provided by the South Metro water providers
(derived from pumping tests) were directly specified in the model. Conductivities for other
gridblocks inside the South Metro region but without directly measured transmissivities were
obtained by interpolating between the mea.-'.ured values. Outside the South Metro region. the
aquifer was zoned based on the calibrated S8-74 conductivity distribution. and the final
values for each of these zones \....ere adjusted (and in some cases smaller subzones Wcre
added) during the calibration process to minimize the difference bet\\:een observed and
modeled heads and drawdO\\TIs. The final best-tit values used in the South Metro model are
sho\\TI in Section 3.2.2. A/ode! Calibration.

3.2.1.3 External Boundar)" Conditions for the Model - There are three external primary
boundary conditions used in the South Metro model;

. Aquifer perimeter boundaries.
. Areal recharge boundaries. and
. Alluvial aquifer - bedrock aquifer interface boundaries.

I In the 58.74 model de\'C~lopcd In MODFLOW96, the \enical conductance is compufed outside the mlldel and pro\ided a.~
input In MODFlO\V2000. which ....as the platform for the South :-'letro model. the user sp~'Cifies as inputs the HnicaJ
conducti\'il)-. for each layer and the intef','cnmg aqunard. and the code computes the \erlieaJ conductance.

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