Model Construction

The Groundwater Modeling System (GMS) 10.2.4,a commerciallyavailable com-

prehensivegraphical userenvironment for performing groundwater simulations, was

usedtocreateaconceptualmodeloflowerScottsCreekusingGIS,field,andlaboratory

data.

EnvironmentalSystemsResearchInstitute(ESRI)ArcMap10.4was usedtocreate

GIS data including the model boundary and locations of wells, piezometers, and

exploratory boreholes. All GIS data were assigned the NAD 1983 California State

Plane Zone III FIPS 0403 (meters) projected coordinate system. GIS data were

imported into GMS and converted to featureobjects. In GMS,feature objectswere

used to build coverages (group of feature objects) including a sources and sinks

coverage and a coverage for each alluvial layer. All coverage data were entered in

units of meters and days.

The sources and sinks coverage included the irrigation wells and the boundary

conditions of the model. The screened interval of each well was specified using

information obtained from well completion reports. The Archibald Well screened

intervalwas from 4to -23.5 m,the Pump House Well screened intervalwas from-11

to -25.3 m, the VFDWell screened interval wasfrom -9.8 to -28 m, and the Queseria

Well screened interval was from 0.5 to -27.5 m. The grid around all four wells was

refined and setto abase size of 15m,a bias of 1.1, and amaximum size of 120 m.

Three alluvial layer coverages were created, each representing one hydrostrati-

General-Head Boundary

General-Head Boundary No-Flow Boundary

River Boundary

Figure 4.52: Plan view of the three-dimensional (3D)finite-difference groundwater flow model (MODFLOW 2000) grid of lower Scotts Creek showing the boundary and initial conditions (starting head contours (m)) of the model.

Monitoring Well

Bedrock Aquiclude

Pumping Well

Piezomete

r

Stream

Middle Aquita

rd

Lower Semi-Conf

ined Aquifer

Potentiomet

ric Surf

ace

Upper Unconfined Aquifer

Figure 4.53: Sc hematic of conceptua l mo del of the groundw ater system on lo w er Scotts Creek.

Figure 4.54: Schematic of general-head boundary (excerpted from the online user- supported help database for XMS software).

Figure 4.55: Schematic of river boundary streambedconductance components for an individualcell (excerpted fromMcDonald and Harbaugh, 1988).

interpolated from a 1.5 meter digital elevation model (DEM) of the Scotts Creek

watershed. The elevationof the bottomof layer 1, and the topand bottom elevations

of layers 2 (aquitard) and 3 (lower semi-confined aquifer), were interpolated from

exploratory borehole data and information from well completion reports that were

entered into GMS as2D scatter points. Similarly, hydraulic conductivities of layers

1 and 2 were interpolated from laboratory permeameter test data. The hydraulic

conductivity and specific storage of layer 3 were set at constant values of 9.16 m/d

and3.5 × 10−5 m−1, respectively, average values from pumping testcurve-matching

analyses. The specific storageof layer 2was set at a constantvalue of3.5 × 10−4 _m−1

(one order of magnitude greater than layer 3) and the specific yield of layer 1 was

set at a constant value of 0.3. All three layers were isotropic. Starting heads were

interpolated frommeasured water table elevation data recorded inAugust 2017 (see

Appendix I for 2D scatterdata). The interpolation scheme used throughout was the

inverse distanceweighted(IDW) methodusingthe constantnodalfunction(Shepard’s

method) method which is givenby the equation

n

X

F(x,y) = wifi, (4.19)

i=1

where n isthe number of points used to interpolate, fi arethedataset values atthe

points, andwi arethe weight functions assigned at each point calculatedaccording to

equation −p r wi = _n , (4.20) P h−_jp j=1

where p isan arbitrary positive realnumber called theweighting exponent(default

mostby measured valuesclosest tothe predictionlocation andless by more distant

points. Theconstant nodalfunctionform of the IDWmethod was usedbecause of its

simplicity,while still retaining allthe functionality needed for the application.

The model boundary was split into four distinct arcs and each was assigned a

boundarycondition. Thenorthernmostarc(Archibald Creek)was definedas ageneral

headboundary,theeasternmostarc(SwantonRoad)wasdefinedasano-flowboundary,

the southernmost arc (Queseria Creek) was definedasa generalhead boundary, and

the westernmost arc (Scotts Creek) was modeled as a river boundary. All of the

boundaries (exceptthe no-flow boundary) were assigned a conductance of 0.34 m2_/d.

Additionally, head-stage wasdefined atthe upstream and downstream nodes of each

generalhead boundary arc, and head-stage and bottom elevationwere definedatsix

locations (upstream,downstream, and the four instream piezometer locations)along

the river boundary using observed instream piezometer data. Table 4.11 provides

a summary of the head-stage and bottom elevation of individual nodes along each

boundary arc.

Table 4.11: Head-stage and bottomelevation in meters of nodesalong the boundaries of the MODFLOW model. Dash (–) indicates data are not applicable.

Node ID Head-Stage (m) Bottom Elevation (m)

Archibald Upstream 12.0 – Archibald Downstream 6.8 – Queseria Upstream 4.0 – Queseria Downstream 2.3 – Scotts Upstream 6.8 6.2 Scotts PH-up 5.4 4.8 Scotts PH-down 4.9 4.3 Scotts VFD-up 4.4 3.8 Scotts VFD-down 4.1 3.5 Scotts Downstream 2.3 1.7

62 columns, 22 rows, and 3 layers oriented in a north-south direction was created.

The grid generated included4,092 cells. The active zones of the modelwere defined

by activating cells in the coverages. The active grid included 2,398 cells. The 3D

grid model wasconverted to a MODFLOW 2000 numericalmodel. The MODFLOW

global optionswere inspected to ensure thatthe conceptual model data were assigned

to the appropriate cells. Starting heads representative of steadystateconditions were

generated by runninga transient simulationwith 75stress periods (75 days) and no

groundwater pumping. The model generated heads forthe 75th time step were set as

the starting heads forall subsequent transientsimulations.