Indirect evidence that subsidence may be related SIMULATION OF GROUND-WATER FLOW to ground-water withdrawals includes water-level declines greater than 100 ft, subsurface collapse of well A numerical ground-water flow model of the two casings in the South Pleasant Valley subbasin and regional aquifer systems (upper aquifers and lower South Oxnard Plain subarea, required repeated leveling aquifers) in the Santa Clara–Calleguas Basin was of irrigated fields for proper drainage, degraded developed to simulate steady-state predevelopment operation of drainage ditches in agricultural areas, and conditions prior to 1891 and transient conditions for lowering of levees along the Calleguas Creek in the the development period January 1891–December 1993. South Pleasant Valley subbasin. In the Las Posas Valley The model simulations provided information and South Pleasant Valley subbasins, water-level concerning predevelopment hydrologic conditions and declines of 50 to 100 ft have occurred in the upper- aquifer response to changes in pumpage and recharge aquifer system, and declines of about 25 to 300 ft or through time. Simulations were made using the three- more have occurred in the lower-aquifer system since dimensional finite-difference ground-water flow model the early 1900s (figs. 13 and 14). Owing to large water- (MODFLOW) developed by McDonald and Harbaugh level declines, the area of probable subsidence may be (1988). Additional packages were incorporated into the larger than that delineated by Ventura County and may ground-water flow model to simulate the routing of include the Las Posas Valley subbasin and the streamflow (Prudic, 1989), land subsidence (Leake and remainder of the Pleasant Valley subbasin. By 1992, Prudic, 1991), and faults as horizontal barriers to total subsidence in the Oxnard Plain subbasin could ground-water flow (Hsieh and Freckleton, 1993). exceed the 2.6 ft measured during 1939–78 along the Transient simulations were calibrated for the coastal traverse. Although the amount of subsidence period of historical systematic data collection, which from various sources remains unknown, ground-water generally spans from the 1920s through 1993. The withdrawals and oil and gas production probably are most important period of the calibration spans the major causes of subsidence in the Oxnard Plain period of reported pumpage (1984–93). Simulation subbasin, and tectonic activity probably is a minor results and model calibration provided insight into the cause. conceptual model of the regional flow system, and into Water released by compaction of layers of fine- the limitations and potential future refinements of the grained deposits within the upper- and lower-aquifer regional-scale model. The model also was used to systems can be a significant additional one-time source analyze the distribution of flow and changes in storage of water to adjacent producing coarse-grained layers in during 1984–93, to project future ground-water flow, the aquifer systems. Geochemistry data (Izbicki, and to evaluate alternatives to future projected ground- 1996a, fig. 3) and geophysical data (EM and natural water flow. The analysis allowed assessment of water- gamma logs in Appendix 5) indicate that fine-grained resources management alternatives and of the effect beds may be a significant source of the poor-quality that implementation of selected alternatives and water in areas such as the South Oxnard Plain subarea geologic controls might have on recharge, coastal in the coastal region between the Hueneme and Mugu landward flow (seawater intrusion), land subsidence, submarine canyons where saline fine-grained layers ground-water movement, and overall resource and seasonal pumpage may collectively contribute to management under climatically varying conditions that poor-quality water. affect supply and demand.

Simulation of Ground-Water Flow 75 Model Framework used to evaluate the FGMA management goals. Reichard’s model was used to simulate the flow of The orientation, areal and temporal ground water and to generate response surfaces for use discretization, vertical layering, areal extent, and in an optimization model. In turn, the optimization internal structural boundaries constitute the framework model was used to test different ground-water and of the numerical ground-water flow model developed surface-water allocation schemes that would satisfy for this study. The model is an extension and water demands and minimize coastal landward flow refinement of the previously developed regional (seawater intrusion). Nishikawa (1997) completed a models and, as such, represents the RASA Program cross-sectional transport model of a vertical section contribution to the continuing effort to evaluate and through the Hueneme submarine canyon to test manage the ground- and surface-water resources of the alternative conceptual models of seawater intrusion for Santa Clara–Calleguas Basin. Model attributes and predevelopment conditions and for 1929–93 developed related data have been added to the Geographic conditions. A numerical wellbore hydraulic model of Information System (GIS) completed by the RASA an aquifer test in the lower-aquifer system in the South Program (Predmore and others, 1997). The metadata Pleasant Valley subarea was completed to test that describe and document these additional GIS alternative conceptual models of the vertical coverages are summarized in Appendix 1. The flow of distribution of hydraulic properties (Hanson and information used to estimate and assemble the input Nishikawa, 1992, 1996). data for the Recharge Package, Streamflow Package, and Well Package of the ground-water model is Model Grid summarized in the flowcharts in Appendix 6. The model grid is oriented at N. 27° W. and Previous Models contains 60 rows and 100 columns discretized into square cells with sides 0.5 mi in length (figs. 7, 16, and Previous models of the area include basinwide A1.4). Average values of aquifer properties and initial digital Theissan-Weber Polygon superposition hydraulic head are assigned to each cell; average initial simulations of historical transient hydraulic and hydraulic head for each cell is assigned at the center, or water-quality conditions for 1950–67 (California node, of each cell. The model contains two layers, one Department of Water Resources, 1974a,b, 1975), and each for the upper- and lower-aquifer systems. The two numerical subregional ground-water flow models of the model layers were made identical in areal extent lower-aquifer system in the East and West Las Posas everywhere in the landward part of the model domain Valley subareas (CH2M HILL, 1993) and the upper- (fig. 16). The top of the upper layer is aligned with the and lower-aquifer systems in the Santa Rosa Valley bottom of the fine-grained layers that separate the subarea (Johnson and Yoon, 1987). More recently, semiperched shallow aquifer from the upper-aquifer Reichard (1995) completed an extended and enhanced system throughout the Northwest and South Oxnard digital model based on the original Theissan-Weber Plain subareas. The top of the upper layer is coincident Polygon model. Reichard extended this model areally with the land surface throughout the remainder of the to include the offshore coastal areas; like the regional upper layer. The bottom of the upper layer and the top model, it simulates the upper- and lower-aquifer of the lower layer are coincident with the bottom of the systems in the Oxnard Plain subareas, the lower- Mugu aquifer. This boundary generally occurs at a aquifer system in the Las Posas Valley and Pleasant depth of 400 ft in the Oxnard Plain subareas. The Valley subareas, and the upper-aquifer system in the bottom of the lower layer is coincident with the bottom Santa Clara River Valley subareas. The model uses of the Fox Canyon aquifer throughout most of the estimates of recharge and pumpage for the historical model area (figs. 7A and 8). simulation period (1984–89), which is the base period

76 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California The model was extended offshore farther in the Formation outside of the active flow system in the northwest corner of the lower layer than previous lower layer are constant-flux boundaries (described models (California Department of Water Resources, later in this section). The constant-flux boundaries are 1974a,b, 1975; Reichard, 1995). The areal extent of the specified flows that change with every season (stress layers was based on the outcrop areas on the geologic period) of each year for the period of simulation. map (Weber and others, 1976) on land, and the seaward The offshore boundary in both layers is extent was based on bathymetry and submarine represented as a strong source-sink boundary; this outcrops estimated from geology maps (Kennedy and boundary is located at the geographic location of the others, 1987). The upper layer (upper-aquifer system) seawater intrusion front identified by Greene and (fig. 16A) is an active flow region covering 374 mi2, of others (1978). This boundary is represented in the which about 27 percent is offshore. The lower layer model as a general-head boundary simulating inflow (lower-aquifer system) (fig. 16B) is an active flow (source) of water from outside the model area or region of 464.5 mi2, of which about 41 percent is discharge (sink) of water from the boundary model offshore. cells to outside the model area. Flow at this boundary is proportional to the hydraulic-head difference between Temporal Discretization the equivalent freshwater head of the ocean along the submarine outcrops and the head of the model cells that The model was used to simulate the period from are coincident with the boundary (fig. 16). Flow at this January 1891 through December 1993. This 103-year boundary is also proportional to the hydraulic historical simulation of ground-water and surface- conductance. Hydraulic conductance was determined water flow was temporally discretized into 3-month during model calibration and represents the periods (stress periods) that represent the four seasons impediment to flow at the seawater intrusion boundary within a calendar year. For computational purposes, in each layer. For the purposes of this report, coastal streamflow, recharge, and pumpage from wells are inflow along this boundary is termed coastal landward specified for each season of every year. Each season flow (a surrogate for seawater intrusion) and outflow is was discretized into 12 equal time steps to estimate termed coastal seaward flow. flow and heads throughout the model. The coastal flow of water through the submarine canyon outcrops is, in part, dependent on the equivalent Model Boundaries freshwater head of seawater and the location of the The perimeter of the active flow region within freshwater/seawater interface. On the basis of EM and the model represents the approximate limit of the natural gamma logs (figure A5.1 in Appendix 5), the ground-water flow system. The boundary is intrusion and movement of seawater occurs largely represented by a combination of no-flow, constant-flux, along the coarse-grained basal layers above regional and general-head boundaries. Except where unconformities. Chloride-concentration data, mountain-front recharge enters the model along the geophysical logs, and cross-section transport modeling boundaries of the landward active flow region of the Hueneme submarine canyon (Nishikawa, 1997) (fig. 17A), the landward model cells along this outer indicate that seawater intrusion is characterized by a boundary of both model layers are represented as a relatively sharp front restricted to selected coarse- no-flow boundary. No-flow boundaries occur where grained layers. Simulation of the seawater-interface there is no flow of water between the active flow-region boundary in this model assumed a position of the model cells and the adjacent areas. The bottom of the interface that is between the submarine outcrop and the lower layer is also represented as a no-flow boundary; coast. The interface location for the current model was this layer generally is coincident with the base of the inferred from the location estimated by Green and Fox Canyon aquifer except in the Santa Rosa Valley, others (1978) for the lower-aquifer system (fig. 16B), East Las Posas, and parts of the Pleasant Valley transport model simulations (Nishikawa, 1997), and subareas. These no-flow boundaries represent the geochemical data from coastal monitoring wells contact with non-water-bearing rocks. Mountain-front (Izbicki, 1996a). The limitations of this assumption are recharge that enters along stream channels in the upper further discussed in the “Model Uncertainty, layer and at the outcrops of the Santa Barbara Sensitivity, and Limitations” section.

Simulation of Ground-Water Flow 77 T 2 N T 4 N 45' ° (0 - 10%) (10 - 25%) (25 - 50%) (50 - 75%) (75 - 100%) 118 Piru R18W Lake Model-grid boundary of flow region and subareas Inferred seawater interface - assigned to Lower-Aquifer system (Green and others, 1978) egetation per cell Acres of riparian 6-40 0-80 0-120 v 0-16 1 4 8 120 - 160 (percent of cell area) San Pedro (5) on of horizontal flow barriers and associated San Cayento 5MILES Two colors used to distinguish adjacent faults he Santa Clara-Calleguas ground -water basin, Ventura County, Horizontal flow barrier Fault name and conductance index Fault and selected name

KILOMETERS River 5 Camarillo (4) Extension (4) Springville Bailey (5) ° Pitas Point 0 0

119

Clara Bailey (5)

Santa

Fox Canyon (2) k

e

e

r

C

s

a

u

g EXPLANATION

e

l

Paula l Santa a C Oil Wells (4) Subbasin names are given in figure 1 Depth below sea level, in feet Hydrologic Unit boundary River and selected streams Selected bathymetry contours – Ground-water subbasin boundary –

-98 R22W -66 Club (1) Country 6 -6 0 -23 15' ° 119

-33 , Lower-aquifer system. (7) 0.086 (8) 0.043 (9) 0.0086 B (10) 0.00086 (11) 0.000086 -66 Pitas Point n ea -656 c O c Boundary head altitude is 3.75Boundary feet conductance is 1,296 feet squared per day

General-head boundary cells i -230 2 if

(1) 43,200 (2)(3) 86.4 (4) 43.2 (5)(6) 4.32 0.864 0.432 c Location of model grid, location general-head boundary cells with associated head altitude and conductance, locati Hydraulic characteristic, in feet per day -131 -49 a

P , Upper-aquifer system. -98 Oak Ridge (0) A Consolidated and unconsolidated deposits Shallow alluvium and unconsolidated deposits Shallow alluvium ° ° 34 22' 30" 34 07' 30" A Santa Clara-Calleguas ground-water basin Outside Santa Clara-Calleguas ground-water basin Figure 16. hydraulic characteristic, and location of evapotranspiration model cells percent riparian vegetation cover per cell in t California.

78 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California T 2 N T 4 N 45' 5MILES ° 118 KILOMETERS 5 Piru R18W Lake Camulos (5) Springville Extension (6) 0 0 Fairview (7) Somis (5) Little Simi Valley (5) Conejo NE2 (5) Simi (9) Oak Village (5) San Cayento San Pedro (5)

Berylwood (5) Round Mountain (5) River Camarillo (6) ° Sycamore Extension (8) 119 Sycamore Canyon (8) Central Las Posas Fault (7) Bailey (5)

Oak Village Extension (5) at Clara Santa

k

e

Fox Canyon (5) e

r

C

s

a

u

g

La Loma (5) e

l l

a Paula C Santa Mugu Slope 1 (6) Oil Wells (4) South Hueneme Canyon (5) Old Hueneme Canyon (5) Simi Extension (10) R22W

Foothill (5) -98 Club (1) Country 6 -6

Hueneme Canyon (5) -230 15' ° 119

n -33 a ce

(7) 0.086 (8) 0.043 (9) 0.0086 O (10) 0.00086 (11) 0.000086 entura (5) -66 ic V f ci Pa Pitas Point -492

-656 Hueneme Slope 1 (5) El Rio (3) McGrath (10) Oak Ridge (10) Boundary head altitude is 16.67Boundary feet conductance is 259 feet squared per day General-head boundary cells -131 -230 Hydraulic characteristic, in feet per day (1) 43,200 (2)(3) 86.4 (4) 43.2 (5)(6) 4.32 0.864 0.432 (5)

8 Oxnard Slope 1 (5) —Continued.

-9 Oxnard Slope 2 (5) Jamaica ° ° 34 22' 30" 34 07' 30" B Figure 16

Simulation of Ground-Water Flow 79 2 T 4 T N N 45' ° 118 at Wastewater Treatment Plants Piru Cells with recharge R18W Lake A Wastewater-Treatment Plant City of Fillmore City of Santa Paula Limoneira Assoc.-Olive Lawn Limoneira Assoc.-Limoneira Saticoy Sanitation District Moorpark-Ventura County #19 City of Thousand Oaks Camarillo Sanitation District Camarillo State Hospital/Camarosa River and selected streams Fault Model-grid boundary of flow region and subareas 1 2 3 4 5 6 7 8 9 6 and simulated rates of mountain-front recharge in 5MILES KILOMETERS

5 1 River 0 0 ° 7

119 at Clara Santa 8

9 B k

e

e

r

C

s

a

u

g EXPLANATION e

l l

2 a Subbasin names are given in figure 1 C (California Department of Water Resources, 1975) Ground-water subbasin boundary – Hydrologic Unit boundary Subdrainage basin boundary – 3 C 4 D 5 R22W 15' ° 0.016 0.031 0.092 119 Lower- Aquifer Piru Santa Paula El Rio Saticoy

A B C D n a ce Cells with Artifical Recharge 0.005 - 0.05 0.05 - 0.1 0.1 - 0.2 0.2 - 0.3 0.3 - 0.812 O Consolidated and unconsolidated deposits , Areal extent of model grid, location cells representing mountain-front, artificial, and treated wastewater recharge, c Shallow alluvium and unconsolidated deposits Shallow alluvium

Upper-Aquifer i A in cubic foot per second

f ci Cells with mountain-front recharge, a P Santa Clara-Calleguas ground-water basin Outside Santa Clara-Calleguas ground-water basin ° ° 22' 30" 07' 30" 34 34 A Figure 17. the ground-water flow model of Santa Clara–Calleguas basin, Ventura County, California.

80 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 119°30' 119°15' 119° 118°45' B EXPLANATION 1 Upper-Aquifer system T Subareas with valley-floor recharge 4 26 N 14 34° 2 Lower-Aquifer system 27 22' (Offshore areas include offshore 15 30" areas of upper layer) 28 11J5 Shoreline 16 8F1 Upper 25 Layer 13 23 31 24 11 17 2 12 T 18 1 2 10 22 N 32 13N2 29 30 3 5 4 6 21 19 9 15D2 Lower 33 7 34° Layer 8 Hydrologic Unit 07' 30" 20 boundary

34 010MILES Wells with flowmeter logs 010KILOMETERS (See figure A5.1 for selected examples) R22W R18W

PERCENTAGE OF ASSIGNED PERCENTAGE OF PERCENTAGE CHANGE UPPER-AQUIFER SUBAREA SEASONAL CLIMATIC PUMPAGE FOR WELLS SPANNING IN AGRICULTURAL LOWER-AQUIFER SUBAREA NUMBER NAME PERIOD PRECIPITATION BOTH MODEL LAYERS PUMPAGE FOR NUMBER NAME (WET/DRY) (UPPER/LOWER) WET/DRY PERIODS 1 Oxnard Plain Forebay (30/30) (100/0) 3 2 Oxnard Plain Forebay 3 Northwest Oxnard Plain (0/0) (90/10) 0 4 Northwest Oxnard Plain 5 Northeast Oxnard Plain (30/30) (90/10) 0 6 Northeast Oxnard Plain 7 South Oxnard Plain (0/0) (100/0) 0 8 South Oxnard Plain 14 Piru (30/15) (100/0) 2 26 Piru 15 Fillmore (30/15) (30/15) 11 27 Fillmore 16 Santa Paula (30/30) (70/30) 1 28 Santa Paula 17 Mound (0/0) (50/50) 4 31 Mound 18 Offshore Mound (0/0) (0/0) -- 32 Offshore Mound 19 Offshore North Oxnard Plain (0/0) (0/0) -- 33 Offshore North Oxnard Plain 20 Offshore South Oxnard Plain (0/0) (0/0) -- 34 Offshore South Oxnard Plain 21 South Pleasant Valley (15/5) (90/10) 10 9 South Pleasant Valley 22 Santa Rosa Valley (15/5) (100/0) 9 10 Santa Rosa Valley 23 South Las Posas Valley (30/5) (100/0) 9 11 South Las Posas Valley 24 West Las Posas Valley (30/5) (50/50) 9 12 West Las Posas Valley 25 East Las Posas Valley (30/5) (20/80) 9 13 East Las Posas Valley 29 North Pleasant Valley (10/0) (70/30) 10 30 North Pleasant Valley

Figure 17—Continued. B, modeled subareas for the upper-and lower-aquifer systems, poercentage of infiltration for seasonal precipitation during wet and dry climatic periods, location of wells with flowmeter logs, and the related percentage of pumpage assigned to wells spanning the upper andlower layers.

Simulation of Ground-Water Flow 81 The offshore boundary representing the density- The offshore Pitas Point and onshore Ventura, Foothill, dependent seawater interface was simplified with a Santa Paula, and San Cayento (thrust) Faults form the general-head boundary simulation which may limit the northern boundary of the ground-water flow system accuracy of the model for the simulation of some along the northern side of the Santa Clara River Valley small-scale features in the coastal areas. Since the subareas (fig. 16). The Oak Ridge Fault and South actual location of the boundary through time along the Mountain form the southern boundary of the entire coast is unknown, the location of the boundary ground-water flow system for the Mound (coastal) was held stationary at an average location for all subarea and the inland subareas of the Santa Clara simulations. A general-head boundary represents the River Valley, respectively. inflow or outflow of water in a model cell and is Internal faults are represented as a horizontal- represented by boundary head and conductance to flow flow barrier (Hsieh and Freckleton, 1993), across between the model cell and the boundary. Flow which the flow of water is proportional to a fault between the boundary and the aquifer is controlled by hydraulic characteristic determined during model the boundary conductance and by the head gradient, calibration. The hydraulic characteristic is defined as which is calculated by the model as the difference the transmissivity of the fault divided by the fault width between the aquifer head in the model cell and the for confined aquifers. All faults in the lower-aquifer specified boundary head. The boundary head that system and a subset of these faults in the upper-aquifer represents the equivalent freshwater head of seawater at system were simulated as flow barriers (fig. 16). The the depth of outcrop was estimated to be equivalent to most notable boundary occurs at the intersection of the 3.75 ft at 46 cells in the upper model layer (fig. 16A) Oak Ridge and Country Club (left-lateral reverse) and 16.67 ft at 65 cells in the lower model layer Faults (fig. 16A) where the springs at Saticoy seeped (fig. 16B). The equivalent freshwater head at the ground water to the surface under predevelopment upper-aquifer boundary was estimated by dividing the conditions. Ground-water level differences as great as depth to the submarine outcrop (150 ft below sea level) 100 ft are reported across the Country Club Fault by 40 (density ratio between saltwater and freshwater); (Turner, 1975); data collected in the spring of 1992 this outcrop was assumed to represent the basal suggest water-level differences of about 10 to 40 ft coarse-grained layer in the Oxnard aquifer. In a similar across this fault (Law/Crandall Inc., 1993). manner, the equivalent freshwater head for the lower- Other faults at the subbasin boundaries acting as aquifer boundary was estimated by dividing the depth potential barriers to ground-water flow in the lower- to the submarine outcrop (667 ft below sea level) by aquifer system include a previously unmapped fault 40; this outcrop was assumed to represent the basal (hereinafter referred to the “Central Las Posas Fault”), coarse-grained layer in the Hueneme aquifer that which separates the lower-aquifer system between the generally occurs at a depth from 400 to 800 ft below West and East Las Posas Valley subbasins, and the land surface. extension of the Springville Fault, which separates the Boundary conductances initially were based on South Las Posas Valley and North Pleasant Valley aquifer transmissivity and were modified during model subbasins (fig. 16B). The Camulos Fault, which forms calibration. An initial uniform conductance of the northeastern boundary of the Piru subbasin, also 4,320 ft2/d was derived from the assumed values used was included as a potential barrier to ground-water in the extension of a model by Reichard (1995). The flow in the lower-aquifer system because of the final distribution of conductances were 1,296 and 259 extension of the ground-water model to the flanks of ft2/d for the upper- and lower-aquifer systems, the mountain front. The Ventura Fault, which is aligned respectively (fig. 16 A,B). with the Pitas Point Fault (fig. 16) near the Faults are simulated as barriers to ground-water northwestern boundary in the Mound subbasin, also flow and as such provide peripheral and internal was included as a potential interior boundary to boundaries to the ground-water flow system. The ground-water flow in the lower-aquifer system peripheral faults, however, were not simulated as faults (fig. 16B). because they are coincident with no-flow boundaries.

82 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Offshore faults of Pliocene to Miocene (?) age, locations through the stream network to the outflow mapped by Green and others (1978) and Kennedy and locations, the model simulates streamflow infiltration others (1987), were included as barriers to to the ground-water flow system, ground-water ground-water flow in the lower-aquifer system discharge to the streams, streamflow diversions, and (fig. 16B). Some of these offshore faults (figs. 7, 9, and discharge of streamflow to the ocean. To simulate 16) are curvilinear and generally are subparallel to the streamflow routing, each cell containing a reach of submarine shelf; their northwest trend is typical of stream channel is assigned a segment number and a structures of the southern Coast Ranges Province. reach number within the segment. The network of Other offshore faults trend west to southwest and are streams and diversions contains 233 model cells subparallel to the axes of the anticlines, synclines, and (reaches) that are grouped into 30 segments (fig. 18A). submarine canyons (figs. 7, 9, and 16) typical of The segments are groups of model cells that are structures of the Transverse Ranges Province. The coincident with the stream channels and represent the northwest-trending faults included in the lower-aquifer major parts of the river systems, which are divided at system are an extension of the Sycamore Fault and the points of confluence (fig. 18B). Streamflow minor fault traces, hereinafter referred to as “Hueneme entering the headwater segment of each stream and slope 1,” “Mugu slope 1,” “Oxnard slope 1,” and major tributary (fig. 18B) is specified for every season “Oxnard slope 2” (fig. 16B). Offshore faults subparallel for the entire historical simulation period. The Santa to the fold structures include extensions of the Clara River and Calleguas Creek stream segments were McGrath-Jamaica, Bailey, and El Rio Faults, and linked at the confluence with their major tributaries and smaller faults coincident with the submarine canyons, are shown in figure 18B. The altitude of the stage of the hereinafter referred to as the “Hueneme Canyon,” “Old stream and streambed conductance for every reach of Hueneme Canyon,” and “South Hueneme Canyon” each segment and the altitudes of the top and base of (fig. 16B). the streambed are specified for each model cell. Estimates of the hydraulic characteristics of For this study, streamflow infiltration was faults were not available from aquifer tests or other calculated using measured and estimated streamflow field data. An initial uniform hydraulic characteristic of and the streamflow-routing program component of the 0.09 ft/d was used to simulate faults as horizontal-flow ground-water flow model. Streamflow routing required barriers in the lower-aquifer system. The final construction of streamflow records for the major rivers distribution was derived by fitting simulated and tributaries in the basin for January 1891 to the water-level changes near faults and water-level period of the continuous gaged streamflow record. differences across faults to measured data; the Streamflow was estimated using regression equations distribution ranges from 43,200 to 8.6 × 10 –5 ft/d with seasonal precipitation for wet and dry climatic (figs. 16A and B). On the basis of subsurface periods (described later in Appendix 4, tables A4.1– stratigraphy, mapping, and trenching (California A4.4). Precipitation data from three coastal, one Department of Water Resources, 1954; California State intermontane, and two mountain precipitation stations Water Resources Board, 1956; Weber and others, 1976; were normalized and then used to produce “wet-day” Jakes, 1979; Dahlen and others, 1990; Association of nonlinear regression estimates of seasonal streamflow Engineering Geologists, 1991; Dahlen, 1992), selected (Duell, 1992). Because precipitation data were faults were simulated to extend into the upper-aquifer available for coastal stations only for 1891–1905, an system of the model for this study (fig. 16B). These additional set of nonlinear relations was estimated for faults include Oil Wells, Country Club, Camarillo, Fox streamflow reconstruction for this early period of Canyon, Springville Extension, Oak Ridge, San Pedro, water-resources development. Correlations between and Bailey Faults. precipitation and streamflow were better for the wettest periods (wet winters) than for the driest periods (dry Streamflow Routing and Ground-Water/Surface-Water summers). Most of the natural streamflow occurs Interactions during wet winters. Between 51 and 84 percent of the variance in natural streamflow during wet winters was Streamflow was simulated using the streamflow- estimated using the nonlinear relations between routing package developed by Prudic (1989). As the precipitation and gaged streamflow data. numerical model routes the streamflow from the inflow

Simulation of Ground-Water Flow 83 2 T 4 T N N 1

10 MILES

ey 45' l

° 1

2

Cr

u r 118 Pi

1 iiVal Simi Number indicates 11 R18W Piru Lake 2 5 2 4 5 9 for model cell group 3 5 ) B oyo Simi 10 KILOMETERS 5

5 Arr

, Stream reaches and related gaging

idge ig Mtn 6 r 5 C A B 6 6

6 kR

and

3 a

s

Hopper O aks 25

O rnia. 7 25

Thou 77 25 7

25

7

r C 25 7

25 7 Pole 7 25 Number indicates stream segment. Two colors used to distinguish adjacent(see segments. table (figure 18 identification) diversion stream segment number 7 4 Model cell reaches within a stream segment – Streamflow-diversion cell – 8 25 8 8 9 25 8 0 0 29 28 25

5

9 rooLsPosas Las Arroyo 3 25 9 10 25 10 e 10

Cr 9

p 10 lls

25 i

Ses e 9 10 25 26 10

9 as H

25 Cr 26 10 s

llmor

10 11 o 26

Fi 26 11 25

26 Las P 11 25 25 26 26 11 25

26

26 ° 11 Conejo 25 26

11

25 tain 26 ver

119 i 25 27

11

25 25 R 25 27 11 25 25

25 25 27 11

for explanation) 28

Creek )

25 28 11

Honda B 13 B

28 rillo 11 11 30 South Moun Arroyo 14 30 30 28 12

28 ma

12 15 30 Hills

28 12 s 12 15

30

Ca illo 28 30 r

Cr 29

12

15 a

29

15

Wash 30 6 allegua 29 29 C 29

29

15 Cam 29 29

ula 29 15 30 29 29

Paula 15

a 30 29

ant 15 S h (see figure 18

Santa 30 Pa

30 30 30 15 Clara 30 30 30 29

15 Sloug 30 30 29

15 n 17 30

30 30

18 volo

eman Diversion sley Re

30 15 d nta 30 30 30

re

30 30 30 EXPLANATION

F 19 Sa ing 30 oy Bear

19 See figure 4 for completenumber. gaging Number station indicated is referenced to diagram (figure 18

30 n Selected streamflow gaging station – 21 Hydrologic Unit boundary Model-grid boundary of flow regionsubareas and 20

Satic 20 21

7 Grounds 20

Spread Cr 21 20 9 20 21

ounds

20

El Rio r Mountai , Schematic routing diagram of simulated streamflow network and streambed conductance for stream segments.

preading 20 21

G

S B Ellsworth 21 21

Saticoy io

Sulphur rd 21 21

Plain na

21

El R x R22W

21 O 22 24 22

24 Oxnard 24

22

ll

24 Barranca 15' e 24 °

22 22

8 24 Arund 22 24 119 24 23

Ventura

n

Ocea Consolidated and unconsolidated deposits

Pacific Shallow alluvium and unconsolidated deposits Shallow alluvium Streamflow network as simulated in the numerical model of Santa Clara–Calleguas ground-water basin, Ventura County, Califo Santa Clara-Calleguas ground-water basin Outside Santa Clara-Calleguas ground-water basin ° ° A 34 22' 30" 34 07' 30" Figure 18 stations used for inflow and calibrations of simulated streamflow.

84 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 1 /d (rounded) 2 Grounds ft Spreading /s /s 3 3 Diversion to Piru /s /s 3 3 200 ft 1 /s ≥ 00 00 00 50 ft 2

.06 5,200 3

.06/0 5,200/0 ft ≥ .153/0 13,200/0

ek .100/.06 8,600/5,200 .138/.06 11,900/5,200

e Streambed conductance .01/.003/0 860/260/0 Cr

Upper Santa Clara River Group Piru 2 Streamflow gaging station used to obtain stream inflow (see figure 17A) Stream segment junction Group of stream segments with same streambed conductance change No streambed conductance change (Coastal Plain Group) streambed conductance increased (x2.75) streambed conductance decreased (x2.75) 4 Release-Diversion Group stream inflow < 50 ft stream inflow 1 streambed conductance decreased (x2.75) streambed conductance increased (x2.75) Piru Creek stream inflow < 200 ft Piru Creek stream inflow (1-4)/(5-8) (1-18)/(19-30) 5

(1-4)/(5-8) oprCr Hopper /s /s Segment descriptions 3 3 6 Santa Clara River above Ventura wastewater treatment plant Santa Clara River to Pacific Ocean Arundell Barranca to Pacific Ocean Arroyo Simi/Arroyo Las Posas/Calleguas Creek above Conejo Creek Upper Conejo Creek (1-4)/(5-12) Lower Conejo Creek above Calleguas Creek Calleguas Creek above Camarillo Calleguas Creek to Pacific Ocean Honda-Beardsley to Revolon Slough-Pacific Ocean (1-9)/(10-12)/(13-36) 200 ft EXPLANATION (for schematic map) ≥

22. 23. 24. 25. 26. 27. 28. 29. 30. 7 No.

ek

RIVER oeCre Pole Specified-stream inflow Streamflow segment number (See table below) Artificial-recharge diversion (type 3) Outflow from streamflow network 8 Unregulated Tributary Group 15 /d for streambed conductance decreased (x2.75) streambed conductance increased (x2.75) 2 Sespe Creek stream inflow < 200 ft Sespe Creek stream inflow /d (rounded)

9 2

ft ep Cr Sespe 5432 /s and ft 2 9 10 /s 2 ft 00 00 00 .138 11,900 .138 11,900 .138 11,900 Streambed conductance .115/0 9,900/0 13 .138/.04 11,900/3,500 .138/.115 11,900/9,900 /s /s 11

3 3

10 Simi 6 royo

in downstream order) with

13 ft Ar CLARA at al Cr Paula Santa A ≥ Arroyo Simi Group

12 Cr 25

Spreading Grounds

14 Number in parentheses is model cell

Conejo Upper – Diversion to Santa Paula

streambed conductance increased (x1.25) streambed conductance decreased (x1.25) 26 15 osas stream inflow < 13 ft stream inflow

(1-9)/(10-11)

Freeman oyo

Diversion

Arr

17 P Las

7

ranca Lower

ar Segment descriptions

B 16 jo Cr rth

wo s Cone Ell Santa Paula Creek Diversion Santa Paula Creek above Santa Clara River Santa Clara River above Freeman Diversion (1-4) / (5-13) Santa Clara River- Freeman Diversion Santa Clara River above Saticoy Diversion Santa Clara River- Saticoy Diversion Santa Clara River above Ellsworth Barranca Ellsworth Barranca (1-4) / (5-8) Santa Clara River above Montalvo 19 18 27 20 21. Table explanation groups (numbered on map respective streambed conductance, in ft time-averaged predevelopment flow (steady-state) 13. 14. 15. 16. 17. 18. 19. 20.

No. NTA 28

21 SA

Saticoy Calleguas Diversion Hondo Arroyo 8 11 past 1/2 mile Camarillo Montalvo Wastewater Treatment Plant /d (rounded) 2 /s /s ft 3 3 22

Wash

Beardsley 0.23 ft ≥ /s 00 2 Arroyo Hondo Group 24 30 29 .138 11,900 .153 13,200 .115 9,900 .138.138 11,900 .138 11,900 .138 11,900 .138 11,900 .138 11,900 .153 11,900 13,200 ft 0.115 9,900 Streambed conductance Ventura Wastewater

Arundell Barranca Treatment Plant

Calleguas Creek

gh streambed conductance decreased (x1.25) streambed conductance increased (x1.25) stream inflow < 0.23 ft stream inflow 23

Slou

Revolon Segment descriptions Ocean Pacific Santa Paula Creek Santa Clara above Piru Piru Creek above Piru Diversion Piru Creek Diversion Piru Creek above Santa Clara River Santa Clara River above Hopper Creek Hopper Creek Santa Clara River above Pole Creek Pole Creek Santa Clara River above Sespe Creek Sespe Creek Santa Clara River above Santa Paula Creek —Continued. 2. 3. 4. 5. 6. 7. 8. 9. 1. 12. 11. No. 10. B Figure 18

Simulation of Ground-Water Flow 85 The streamflow network represents gaged inflow diversion were based on the UWCD’s reported total along the Santa Clara River and tributaries, the monthly diversions (Greg Middleton, United Water Calleguas Creek and tributaries, Arroyo Hondo, and Conservation District, written commun., 1993). Arundell Barranca. Measured and estimated seasonal Streamflow stages for all the reaches were streamflow was used to simulate streamflow from estimated from relations between stream stage and 11 inflow points on the Santa Clara River, Piru Creek, streamflow at the inflow-gaging stations. The stream Hopper Creek, Pole Creek, Sespe Creek, Santa Paula stage was held constant for all reaches in all segments Creek, Ellsworth Barranca, Arrundell Barranca, for all simulations. Stream stage was initially estimated Arroyo Hondo, Arroyo Simi, and Upper Conejo Creek using extrapolated gaged height at the estimated (fig. 18B). Seasonal inflow rates were specified as the predevelopment flow, which ranged from 0.3 to 4.5 ft total seasonal flow volume divided by the number of for the steady-state flow rate at the inflow-gaging days in the season for the period of record of each stations. However, stream stages were simplified and inflow site. For the period prior to historical records, finally held to a constant value of 2.5 ft above the top of nonlinear regressions of flow as a function of the streambed for all simulation periods and for all precipitation were used to estimate wet- and dry-period river reaches. The altitude of the top of the streambed seasonal flows for the Santa Clara River, Piru, Hopper, was estimated from the arithmetic average of land- Pole, Sespe, and Santa Paula Creeks and Arroyo Simi surface altitudes for the entire extent of the stream (Appendix 4, table A4.1–A4.4). Streamflow estimates channel in each reach, which was estimated from for Ellsworth and Arrundell Barrancas, Arroyo Hondo, digital altitude model data, 1:24,000-scale topographic and Conejo Creek were based on seasonal ratios of maps, and gaging-station altitudes. The altitude of the gaged runoff to precipitation (modified rational base of the streambed was assumed to be 10 ft below method) for Pole and Hopper Creeks. The modified the altitude of the top of the streambed for all the rational method was used for the period prior to the reaches for all time periods. period for which streamflow-gaging data are available As water flows down the channels of the Santa because there was no period of unregulated gaged Clara River and Calleguas Creek and their tributaries, streamflow that could be used to establish regression some of the water infiltrates through the streambed and relations between streamflow and precipitation. becomes ground-water recharge. In a few places, Streamflow between the segments is the simulated however, shallow ground water discharges to streams. streamflow routed from all upstream segments In the model, this vertical flow between the stream and connected to a given segment. The simulation of the aquifer is controlled by the streambed conductance predevelopment conditions used time-averaged and a vertical gradient that is driven by the difference streamflow estimates based on the geometric means between the specified stream stage and the simulated and median streamflow values for the gaged ground-water level. Stream stage for each stream reach streamflow (table 2) and the geometric-mean values of was specified and was not changed for the entire long-term runoff for ungaged tributaries. simulation time. Streambed conductance initially was The diversions at Piru, Santa Paula, and Saticoy estimated as the product of the assumed channel width, and at the Freeman Diversion, which provide surface channel length, and vertical hydraulic conductivity of water for irrigation and artificial recharge, were the streambed deposits divided by the streambed simulated as losses from the stream network thickness. Streambed conductance also can be (fig. 18 A,B). The streamflow-routing package of this estimated as the product of the streamflow and the model was altered to offer additional types of diversion fraction of streamflow loss divided by the streambed (Appendix 2). The modified diversion type used for all thickness. Although the actual stream channel width four simulated diversions is referred to as an “artificial and streambed thickness vary spatially and with flow recharge diversion” [type 3 (Appendix 2)]; it will within many of the model cells, the streambed accept all streamflow available up to the specified conductances were simplified into groups of segments amount of diversion. The seasonal amounts of with the same streambed conductance values (fig. 18B).

86 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Initial estimates of streambed conductance were conductance of zero, the upper Santa Clara River based on streamflow-loss estimates made in the early group, the release-diversion group, the unregulated 1930s (California Department of Public Works, 1934) tributary group, the Arroyo Simi group, and the Arroyo and in 1991 (Densmore and others, 1992); however, Hondo group (fig. 18B). Streambed conductances for these direct estimates of streamflow losses vary each group were increased and decreased from the widely—from 1 to 100 percent. Various mass-balance predevelopment values and were changed on the basis estimates for the Santa Clara River (Taylor and others, of threshold values of stream inflows (fig. 18B). 1977; Dal Pozzo, 1992; Law/Crandall Inc., 1993) also Results of model calibration indicate that the three have been made; these estimates also vary widely, groups of streambed conductances for the Santa Clara ranging from 0 to 100 percent, with an average loss of River system were increased when streamflows were about 22 percent. A water-balance approach yielded an greater than the flow threshold and decreased when estimate of streambed hydraulic conductivity of about they were less than the flow threshold by a factor of 2 ft/d for the Santa Paula subarea (Law/Crandall Inc., 2.75 with respect to conductances used to simulate 1993). The simulation of streamflow in the Santa Rosa time-averaged predevelopment conditions. The Arroyo Valley subarea model used vertical hydraulic Hondo and Arroyo Simi groups were increased when conductivities of 3 ft/d for Arroyo Conejo and Conejo streamflows were greater than the flow threshold and Creek and 1 ft/d for Arroyo Simi and an assumed decreased when streamflows were less than the flow streambed thickness of 1 ft (Johnson and Yoon, 1987). threshold by a factor of 1.25 with respect to The assumed width is 50 ft, and the assumed streambed conductances used to simulate time-averaged length was assumed to be the length of the cell predevelopment conditions. This change in (2,640 ft). Using values from Johnson and Yoon conductance is believed to reflect the change in channel (1987), estimated streambed conductance is width and is similar to the factors of 1.2 to 2.0 used for 13,200 ft2/d for Arroyo Simi and 39,600 ft2/d for the simulation of the streamflow routing of the Little Conejo Creek. Humboldt River, Nevada (Prudic and Herman, 1996). For the regional-scale model, the stream channel The final distribution of streambed conductances width initially was assumed to range from 50 to 200 ft, ranges from 0 to 13,200 ft2/d (fig. 18B) for time- the length of the reach was assumed to be the length of averaged predevelopment conditions. These final the cell (2,640 ft), and the streambed thickness was values are the product of model calibration for time- assumed to be 10 ft. The streambed conductances were averaged predevelopment (steady-state) conditions and then put into six groups: the coastal plain group for of comparisons of the streamflow hydrographs for which segments and reaches were set to a streambed historical downstream streamflow and diversions.

Simulation of Ground-Water Flow 87 Mountain-Front Recharge exceeded that amount. This value was determined from Natural recharge along the model boundaries, streamflow seepage measurements of low flows on mountain-front recharge, was simulated as a constant- Santa Paula Creek (Dal Pozo, 1992). flux inflow for each season (fig. 17A,B). Mountain- The estimated total time-averaged mountain- front recharge was simulated as a seasonally varying front recharge rate used for the steady-state simulation estimate of runoff specified as infiltration at the of predevelopment conditions was 12,500 acre-ft/yr. mountain front for 64 ungaged surface-water The constant rate of recharge for the steady-state subdrainage basins (California Department of Water simulation, which was based on the geometric-mean Resources, 1975, plate 2) that surround and drain into ratios, was used to estimate the time-averaged runoff the 12 ground-water subbasins of the Santa Clara– from each mountain-front subdrainage basin. The Calleguas Basin (figs. 1 and 17A). The average for total estimated total time-varying mountain-front recharge wet- and dry-seasonal precipitation was estimated for rate used for transient-state simulation of historical each ungaged subdrainage basin. The modified rational conditions ranged from 6,000 acre-ft/yr in 1923 to method was used to estimate the seasonal runoff for 80,600 acre-ft/yr in 1993. Mountain-front recharge was each of the 412 seasons in the simulated historical simulated as injection wells, with a constant rate of period January 1891–December 1993. The ratio of recharge per season, for 119 model cells in the runoff from Pole or Hopper Creeks to the total seasonal uppermost active layer that coincide with the stream precipitation for these two index subdrainage basins channels in the ungaged-tributary drainage basins ranged from 0 to 7, but most of the ratios were less than (fig. 17A). 0.25. These ratios were comparable to the fraction of Additional recharge as direct infiltration on the precipitation as ground-water recharge estimated from outcrops of the San Pedro Formation (fig. 7A) was detailed water-balance studies completed by Blaney estimated based on wet-period average winter (California Department of Public Works, 1934) for precipitation for 54 model cells that coincide with the water years 1928–32. Blaney estimated annual San Pedro Formation in the Fillmore, Santa Paula, and fractions of rainfall penetration ranging from 0.01 to Las Posas Valley subareas (figs. 7A and 17A). The 0.17 for dry years and from 0.06 to 0.34 for wet years. recharge rate representing deep infiltration over the Using the modified rational method, estimated ratios outcrops was estimated using the modified equation greater than 1 would result in a runoff total that is developed by the Santa Barbara County Water Agency greater than the average precipitation. On the basis of (1977): previous infiltration studies in the Santa Clara– Recharge = (Pwet - 17 inches)/1.55, Calleguas Basin (California Department of Public where Works, 1934; Taylor and others, 1977; Densmore and Recharge is average recharge rate, in inches per year, others, 1992), most fractions of runoff that infiltrate are and Pwet is wet-period total annual precipitation of less than 0.9. The ratios selected for estimating 20.75 in. for outcrops surrounding the Las Posas Valley recharge were from Pole Creek for winter and fall subareas and 21.25 in. for outcrops on the north side of seasons and from Hopper Creek for spring and summer the Santa Clara River Valley subareas. seasons. When any ratio exceeded 0.9, the ratio from This method assumes uniform temporal and the other index subarea was used. When both ratios areal distributions of rainfall without regard to the exceeded 0.9, the ratios were replaced with the intensity of individual storms. The resulting recharge geometric mean of ratios less than or equal to 0.9 for rate is reduced by the fraction of wet years (32 years) in that respective wet or dry climatic season. The the total period of historical simulation (103 years). estimated mountain-front recharge for each The resulting estimates for a constant average recharge subdrainage basin was then equally distributed to one were 470 acre-ft/yr for East Las Posas Valley subarea, or more cells that are coincident with the stream 740 acre-ft/yr for South Las Posas Valley subarea, channels at the model boundary (fig. 17A). The 400 acre-ft/yr for West Las Posas Valley subarea, resulting recharge estimates for an individual cell was 240 acre-ft/yr for Fillmore subarea, and 320 acre-ft/yr reduced to 3.4 ft3/s if the estimated recharge value for Santa Paula subarea. Thus, the long-term average recharge to the lower-aquifer system for a total bedrock recharge was about 2,200 acre-ft/yr (table 4).

88 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Valley-Floor Recharge (United Water Conservation District, 1986, plate 5a,b; Direct infiltration of precipitation on the valley Greg Middleton, United Water Conservation District, floors, hereinafter referred to as “valley-floor written commun., 1993). recharge,” was simulated using the model recharge Recharge of treated sewage effluent was package and was distributed equally to all cells in each simulated as constant-flux inflows using the valley floor of the Piru, Fillmore, Santa Paula, Las MODFLOW well package. This recharge was based on Posas Valley (East, West, and South), Pleasant Valley reported and extrapolated annual amounts of treated (North and South), Oxnard Plain Forebay, and Santa sewage discharge (California Department of Water Rosa Valley, and the Northeast Oxnard Plain subareas Resources, 1975; W.D. Jesena, California Regional (fig. 17B). The estimated total time-averaged recharge Water Quality Control Board, written commun., 1991; rate used for the steady-state simulation of E.G. Reichard, U.S. Geological Survey, written predevelopment conditions was 4,800 acre-ft/yr, which commun., 1993; Mitri Muna, Ventura County is based on the geometric-mean ratios of runoff to Waterworks, written commun., 1995) and was assigned precipitation at Pole and Hopper Creeks. The total to nine model cells (fig. 17A) at a rate reduced to time-varying valley-floor recharge used for the 74 percent (Farnsworth and others, 1982) of the transient-state simulation of historical conditions was reported or interpolated annual rate of discharge to varied seasonally using the same percentages of account for the free-water surface evaporation while in infiltration of irrigation based on model calibration percolation ponds and streambeds. The treated sewage (fig. 17B). The recharge rates ranged from 18,300 effluent represents discharge from the city of Fillmore acre-ft/yr for dry-year periods to 32,700 acre-ft/yr for during 1958–93, the city of Santa Paula during wet-year periods (table 4). 1937–93, the Limoneira Association at Olive Lawn Farm and Limoneira Farm during 1975–93, the Saticoy Sanitation District during 1960–93, the Camarillo Artificial Recharge Sanitation District during 1959–93, the city of Recharge of infiltration of diverted streamflow, Thousand Oaks during 1962–72, the Camarillo State discharge of treated sewage effluent, and irrigation Hospital during 1960–80, the Camarosa wastewater- return flow were simulated as a constant-flux inflow treatment plant during 1981–93, and the Moorpark- using the MODFLOW well package. No artificial Ventura County wastewater-treatment plant No. 19 recharge was applied to predevelopment (steady-state) during 1973–93. Additional sewage effluent discharged conditions. For developed (transient-state) conditions, from the city of Thousand Oaks Hill Canyon Plant is infiltration of diverted streamflow was applied for the represented as streamflow during 1973–93. Treated period 1928–93, infiltration of irrigation was applied sewage effluent from the percolation ponds near the for the period 1891–1993, and infiltration of treated Santa Clara River which was used by the city of Piru sewage effluent was applied for the period 1936–93. during 1975–93 was not included because of the small Recharge of diverted streamflow was simulated volumes of discharge (Charles Rogers, city of Piru, oral at the artificial-recharge spreading grounds (basins) commun., 1995). Total treated-sewage effluent that operated by the UWCD in the Piru and Santa Paula becomes ground-water recharge was applied at a subareas and in the Oxnard Plain Forebay subarea constant rate for all four seasons of every year; the rate (figs. 4 and 18A). The quantity of artificial recharge increased from 20 acre-ft/yr in 1936 to 9,000 acre-ft/yr simulated in the model (fig. 11A) was based on in 1993 (fig. 11A). reported annual and seasonal amounts of recharge

Simulation of Ground-Water Flow 89 Irrigation return flow was estimated as a upper- and lower-aquifer systems through failed and percentage of the total applied water and included abandoned wells. Because the initial water-chemistry ground-water and surface-water components for many data indicate a potentially small effect and because of the subareas. This recharge was simulated as a water-level hydrographs indicate a potentially constant-flux inflow using the MODFLOW well complicated relation, this element was not included in package for the uppermost layer of the model. the current regional simulation. Any potential leakage Irrigation return flow was estimated for each of the through intraborehole flow or failed wells was included land-use periods and held constant for the same periods collectively and simulated in the irrigation-return-flow used to estimate ground-water pumpage (fig. 11A,B). component. The irrigation return flow was applied over a 245-day growing period prior to 1927 and applied uniformly for Natural Discharge the entire year for the remainder of the simulation Natural discharge is simulated as seaward period. It was applied over the entire year because coastal flow through submarine outcrops and as infiltration through the unsaturated zone tends to evapotranspiration (ET) along the flood plains of the extend the period of infiltration. The 1969 land-use Santa Clara River and Calleguas Creek. The coastal map was used to estimate the distribution of irrigation flow of water to the ocean was determined through return flow for the period of reported pumpage, model simulation and calibration; it is described in the 1973–93. The assumed infiltration ranged from 5 to “Model Boundaries” section. 30 percent of applied irrigation water for all subareas ET by riparian vegetation (phreatophytes) and and was varied for wet- and dry-year periods (fig. 17B). evaporation from bare soil were simulated at The percentage of irrigation return flow was estimated 306 model cells of layer 1 (upper-aquifer system) during model calibration. Irrigation return flow ranged (fig. 16A) using the MODFLOW evapotranspiration from less than a few hundred acre-feet per season for package. Using previous estimates (California the Mound and North Peasant Valley subareas to about Department of Public Works, 1934), a maximum ET 1,400 acre-ft per season for the Santa Paula subarea rate of 2.4 ft/yr was assumed when the water table is at (fig. 11). Total irrigation return flow ranged from land surface, and ET was assumed to decrease linearly 14,600 acre-ft/yr for the 1890s to 51,500 acre-ft/yr for to zero when the water table reaches a depth of 10 ft or the drought period 1987–91. more below land surface. The ET rate was multiplied by the ratio of riparian vegetation area to total model- Other Sources of Recharge cell area to account for the riparian vegetation density Other sources of recharge include flow of water in each model cell. The weighting factor is the number along some fault zones from older (Miocene age) of acres of riparian vegetation, estimated from the marine sedimentary rocks and brines related to oil 1912, 1927, 1932, and 1950 land-use maps, for each deposits. Some of these potential sources of water may cell divided by the total number of acres (160) in a yield water of poor quality or water of different model cell. The composite ET rates and the model cells chemical composition. Water-chemistry data indicate with the potential for ET in 1912, 1927, 1932, and that the amount of leakage from the deeper, older 1950 (Conejo Creek area) were used for the formations in the South Oxnard Plain subarea and the predevelopment and historical simulation for 1891– South Pleasant Valley subarea probably is small 1926. The ET surface remained the same for the (Izbicki, 1991, 1996a); therefore, it was not included in remainder of the simulation periods, but the ET rates the current regional simulations. were updated to reflect changing ET acreage. Thus, Another source of potential recharge is leakage acreage for riparian vegetation was updated using the of the semiperched water to the upper-aquifer system. 1932 acreage for 1927–46 and the 1950 acreage for the Leakage of semiperched ground water may enter the remainder of the simulation period.

90 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Pumpage Indirect estimates of agricultural pumpage were The simulation of ground-water withdrawal from compiled for five land-use periods that span from 1912 wells as pumpage required a compilation of historical to 1977 (Koczot, 1996). The compilation was based on estimates that include indirect estimates of agricultural land-use maps for 1912, 1932, 1950, and 1969 and on a pumpage based on land use (1891–1977), reported mosaic of areal photos from 1927 (Predmore and municipal pumpage (1914–77), and metered others, 1997). The distribution of estimated agricultural agricultural and municipal pumpage (1978–93) pumpage was based on well locations reported in 1987 reported to and compiled by the UWCD and the and on percentages of pumpage within each subarea. FGMA. Estimated pumpage ranged from 34,800 acre- The estimates of agricultural pumpage were distributed ft for the drought years of the 1920s to a maximum over time on the basis of major changes in crop types pumpage of 301,400 acre-ft for the 1990 drought year. and climatic periods. Because the growing periods of Estimated pumpage is shown in figure 11B for the the various crop types spanned an 8-month period, period of simulation. The annual and biannual pumpage was estimated and distributed using a pumpage estimates were temporally distributed for 245-day growing season (Koczot, 1996) spanning model input to the seasonal intervals on a well-by-well March through October for the period 1912–26. The basis. The initial vertical distribution of pumpage growing season was extended to 275 days, spanning between aquifer systems was based on well from March through November for the period construction (Predmore and others, 1997) and wellbore 1927–77. The extension of the growing period was flowmeter studies completed as part of the RASA based on inspection of water-level hydrographs and the studies (table 5). For wells completed only in the wider variety of truck and orchard crops introduced upper-aquifer system, all water was derived from the during this period. The magnitude of pumpage was upper model layer, and for wells completed only in the reduced during wet climatic periods and increased lower-aquifer system, all water was derived from the during dry climatic periods. The percentage change in lower model layer. For wells that were completed in agricultural pumpage was based on the ratios of both the upper- and lower-aquifer systems, a wet-year to average-annual reported pumpage for each percentage of total well pumpage was assigned to the subarea and dry-year to average-annual reported upper and lower layers on the basis of wellbore pumpage (fig. 17B). The reported municipal pumpage flowmeter data, slug tests, and model calibration (fig. for the cities of Ventura, Camarillo, and Oxnard and the 17B). Pumpage from wells with no construction data Channel Island Community Services District; pumpage was distributed using these same assumed percentages for the fish hatchery in the southern end of the Piru of pumpage. The distribution of pumpage from the subarea; and pumpage of artificial recharge in the upper- and lower-aquifer systems, estimated from the Oxnard Plain Forebay were estimated independently land-use map for agricultural pumpage, also used these and combined with agricultural pumpage for input to same percentages for all the subareas. the ground-water flow model for the period of simulation prior to 1983 (fig. 11B).

Simulation of Ground-Water Flow 91 Regional management of ground-water resources these parameters are used in the model and represent was implemented by the State of California in 1983 the hydraulic properties which are the spatial averages with the creation of the Fox Canyon Groundwater over individual model cells. They generally are held Management Agency (FGMA) for controlling seawater constant through time. Except for fault hydraulic intrusion. The FGMA jurisdiction covers part of the characteristics, vertical conductances of the streambed, Santa Clara–Calleguas Basin and includes the Oxnard subsidence parameters, and areas where model layers Plain, Oxnard Plain Forebay, Pleasant Valley, and Las were extended, the initial estimates for all the model Posas Valley subareas (figure 26 presented later in the parameters were derived largely from the spatial section “Analysis of Ground-Water Flow”). Reported estimates used in previous ground-water flow models pumpage was compiled from the technical files of the of the basin (California Department of Water FGMA and the UWCD for the period July 1979– Resources, 1974a,b, 1975; Johnson and Yoon, 1987; December 1993. These data generally consist of CH2M HILL, 1993; Reichard, 1995). semiannual totals of user-reported agricultural, nonagricultural (municipal, industrial, and domestic), Transmissivity and total pumpage. Agricultural pumpage was distributed based on a 275-day growing period and the Transmissivity is the product of the hydraulic nonagricultural pumpage was distributed equally over conductivity and saturated thickness of the aquifers; seasonal periods of the flow model. Pumpage for 1980, therefore, transmissivity values may be affected by which was based on water-level hydrographs and on changes in saturated thickness. Transmissivity climate data, was used for the period 1978 through throughout much of the modeled area is associated 1980. When only total pumpage was reported, that with the basal coarse-grained layers of the aquifers that pumpage was assumed to be for agricultural use. Early remain saturated; many parts of the aquifers are pumpage data were incomplete for the Las Posas confined or show water-level changes that are a Valley, the eastern part of the Pleasant Valley, and the relatively small percentage of the saturated thickness. Santa Rosa Valley subareas. For these areas, 1984 Because the effective saturated thickness is relatively FGMA-reported pumpage was used to represent constant over most of the model area, this model uses pumpage for 1978 through 1983. Total reported annual constant transmissivities for the entire period of pumpage ranged from as little as 850 acre-ft in the simulation. Transmissivities estimated from specific- South Las Posas Valley subarea during 1992 to as much capacity tests were used to simulate ground-water flow as 107,300 acre-ft in the Oxnard Plain and Oxnard using the Theissan-Weber Polygon model (California Plain Forebay subareas during 1990. Department of Water Resources, 1975). Estimates for the upper-aquifer system range from 650 ft2/d along the northern edge of the Santa Paula subarea to more Hydraulic Properties than 53,000 ft2/d in the northern Oxnard Plain and Estimates of transmissivities and storage 67,000 ft2/d north of the Mugu submarine canyon coefficients for both model layers and estimates of (California Department of Water Resources, 1975, coefficients of vertical leakance between layers are pl. 8). Estimates for the lower-aquifer system range required to simulate the flow of ground water. from about 1,300 ft2/d near Moorpark to 53,000 ft2/d Estimates of the horizontal conductance of faults are north of Port Hueneme (California Department of required to simulate potential barriers to ground-water Water Resources, 1975, pl. 8). The coastal estimates flow, and the vertical conductance of streambeds is from the Theissan-Weber Polygon model were required to simulate the flow of water between shallow extended as constant values to the adjacent offshore ground water and streamflow. The average values for regions by Reichard (1995, fig. 10).

92 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California The current model modified these estimated upper model layer of the previous models for all the transmissivities and used additional estimates beyond subareas (figs. 17B and 19A). The transmissivities of the areal extent of the previous models for the upper- the coarse-grained deposits of the East Las Posas aquifer system (layer 1) in the Las Posas Valley, Valley subarea were estimated using a geometric-mean Pleasant Valley, and Santa Rosa Valley subareas and for hydraulic conductivity of 0.3 ft/d, which was based on the lower layer in the Santa Clara River Valley subareas slug-test values that range from 0.21 to 0.47 ft/d in (fig. 17B and 19A). The estimated transmissivities for monitoring wells completed in this subarea. the upper-aquifer system (layer 1) ranged from 1.3 ft2/d Transmissivities of the coarse-grained deposits of the for the Las Posas Valley subarea to about 73,800 ft2/d West Las Posas Valley subarea were estimated using a for the Oxnard Plain Forebay (fig. 19A); the estimated geometric-mean hydraulic conductivity of 0.19 ft/d, transmissivities for the lower-aquifer system (layer 2) which was based on slug-test values that range from ranged from about 38 to 26,500 ft2/d. A constant 0.14 to 0.27 ft/d in monitoring wells completed near transmissivity of about 4,700 ft2/d was assigned to the Arroyo Hondo. Transmissivities of the coarse-grained lower-aquifer system (layer 2) for the offshore part of deposits of the South Las Posas Valley subarea were the Mound subarea on the basis of the estimated estimated using a geometric-mean hydraulic thicknesses and the hydraulic conductivities used conductivity of 1.58 ft/d, which was based on slug-test onshore (fig. 19A). values that range from 0.48 to 3.49 ft/d in monitoring The final estimates of transmissivities in the wells completed in the subarea. The transmissivities of calibrated model for both model layers were refined for the coarse-grained deposits of the Santa Rosa Valley each subarea using the sum of transmissivities for the subarea (fig. 20) were based on two sets of hydraulic aggregate thicknesses of the coarse-grained and fine- conductivities: A reported value of 80 ft/d for the grained deposits in each model cell (fig. 20). The Saugus Formation (Johnson and Yoon, 1987) was used transmissivity of the coarse-grained deposits was to represent the upper and lower aquifers on the west determined as the product of the thickness of the side of the San Pedro Fault; reported values of 150 and coarse-grained deposits (estimated from resistivity 120 ft/d for the alluvium and the Santa Margarita logs) and a geometric-mean hydraulic conductivity Formation, respectively, were used for the east side of (estimated from slug tests). The transmissivity of the the San Pedro Fault (Johnson and Yoon, 1987). fine-grained deposits is the product of the thickness of Transmissivity for the Pleasant Valley subarea was fine-grained deposits and an assumed hydraulic estimated using a geometric-mean hydraulic conductivity of 0.1 ft/d. conductivity of 8.8 ft/d for the coarse-grained deposits, Some of the transmissivities from previous which is based on slug-test values that range from 0.13 regional models for the upper-aquifer system were to 11.8 ft/d in monitoring wells in this subarea. reestimated using estimates of a geometric-mean Estimates of hydraulic conductivities for the hydraulic conductivity from the slug tests and the lower-aquifer system (layer 2) deposits range from 1 to aggregate thicknesses of the coarse- and fine-grained 8 ft/d for monitoring wells completed in the northern deposits (fig. 20A). Transmissivity estimates were part of the Oxnard Plain subarea and from 5.5 to 44 ft/d made using a hydraulic conductivity of 35.1 ft/d for the for monitoring wells completed in the Piru and Santa coarse-grained deposits in the Piru and Santa Paula Paula subbasins (E.G. Reichard, U.S. Geological subareas; these values were based on slug-test values Survey, written commun., 1992). Transmissivities for that range from 18 to 88 ft/d in monitoring wells the coarse-grained deposits within layer 2 of the Santa completed in these subareas (E.G. Reichard, U.S. Clara River Valley subareas were estimated using a Geological Survey, written commun., 1995). geometric-mean hydraulic conductivity of 15.4 ft/d. Transmissivities for the upper-aquifer systems (layer 1) of the Las Posas Valley, Santa Rosa Valley, and Pleasant Valley subareas were needed to extend the

Simulation of Ground-Water Flow 93 4 2 T T N N 45' 10 MILES ° 118 R18W Piru Layer 2 Lake 10 KILOMETERS 0 0 , Distribution of transmissivity

A

iver aR

° Clar

119

Santa

k

e

e

r

C

s

a ounty, California. u

g

e l

l a C EXPLANATION Fault Model-grid boundary of flow region and subareas Hydrologic Unit boundary River and selected streams R22W Consolidated and unconsolidated deposits Shallow alluvium and unconsolidated deposits Shallow alluvium 15' ° 119 Santa Clara-Calleguas ground-water basin Outside Santa Clara-Calleguas ground-water basin 15,000 26,474 5,000 10,000 ≤ ≤ ≤ ≤ 1,000 ≤

T n

≤ a ce

38 1,000 < T 5,000 < T 10,000 < T 15,000 < T O c fi ci per day a ransmissivity (T), in feet squared otal range 38 - 26,474 P T T ° ° 07' 30" 22' 30" 34 34 4 2 4 2 T T T T N N N N 45' 45' 10 MILES 10 MILES ° ° and 2 , Storage coefficient for model layers 1 and 2. 118 118 layer 1 Between B Layer 1 R18W Piru Piru Lake Lake R18W 10 KILOMETERS 10 KILOMETERS

0 0 0 0

iver iver

R

aR a

r r a

° °

aCl

119 t 119

n

a

at Cla Santa S

k k

e e

e e

r r

C C

s s

a a

u u

g g

e e l l

l l a a

C C R22W R22W 15' 15' ° ° 119 119 45,000 73,767 5,000 15,000 ≤ ≤ 30 50 303 ≤ ≤ 15 5 1,000 ≤ ≤ ≤ ≤ n ≤

≤ n a a

e e Distribution of hydraulic properties in layers 1 and 2 the model Santa Clara–Calleguas ground-water basin, Ventura C T c e1-303 c

O VC ≤ O ≤ per day 5

94 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 2 T 4 T N N 2 T 4 T N N 45' 45' 10 MILES 10 MILES ° ° 118 118 Layer 2 Layer 2 R18W R18W Piru Piru Lake Lake 10 KILOMETERS 10 KILOMETERS

0 0 0 0

iver iver

R aR

° °

Clar a

119 119

Sant Clara Santa

k k

e e

e e

r r

C C

s s

a a

u u

g g

e e l l

l l a a

C C R22W R22W 15' 15' ° ° 0.005 119 119 0.0001 ) ≤ ≤ 0.01 0.03 0.05 0.097 ke 0.0002 0.0003 0.0007 0.07 S ≤ ke ≤ ≤ ≤ ≤ ≤ ≤ ≤ S ≤ ke ≤ ke ke ke

n n a a ce ce 0.00015 0.005 < S 0.01 < S 0.03 < S 0.05 < S

0.0000005 0.0001 < S 0.0002 < S 0.0003 < S 0.0007 < S O O c c

i Coefficient (S i

otal range 0.0000005 - 0.07 f f ci otal range 0.00015 - 0.097 ci Storage Coefficient (S) T Elastic Skeletal Storage Pa T Pa ° ° ° ° 22' 30" 07' 30" 34 22' 30" 34 07' 30" 34 34 2 2 T T 4 4 T T N N N N 45' 45' 10 MILES 10 MILES ° ° 118 118 R18W R18W Piru Piru Lake Lake Layer 1 Layer 1 10 KILOMETERS 10 KILOMETERS

0 0 0 0

r

ive

River

aR

r ra

a

la

° ° l

C

aC

t

119 119 ta

n

an a

S S

k k

e e

e e

r r

C C

s s

a a

u u

g g

e e l l

l l a a

C C R22W R22W 15' 15' ° ° 119 119 0.01 ) 0.001 0.003 0.005 0.01 ≤ ≤ ke 0.0146 0.05 0.1 0.19 ≤ ≤ ≤ S 0.15 ≤ ke ≤ ≤ ≤ ≤ ke ke ke S ≤ n n

a ke a

e ≤ e c c O O 0.00002 0.01 < S 0.05 < S 0.1 < S 0.15 < S

c c —Continued.

i 0.0001 0.001 < S 0.003 < S 0.005 < S 0.01 < S i if if ac ac

otal range 0.00002 - 0.19 P P Coefficient (S otal range 0.0001 - 0.0146 Storage Coefficient (S) T Elastic Skeletal Storage T ° ° ° ° 22' 30" 07' 30" 22' 30" 07' 30" 34 34 34 34 B Figure 19

Simulation of Ground-Water Flow 95 119°15' 119° 118°45' Lake A Thickness of coarsed-grained Piru deposits (b) T Total range 5.5 - 589 feet 4 ≤ ≤ 34° 5.5 b 100 N ≤ er 22' 100

T 2 N

ek P e a r c C i s 34° fi a c u g e 07' l l O a 30" ce C 0 10 MILES an 0 10 KILOMETERS

R22W R18W

119°15' 119° 118°45' Lake Thickness of fine-grained Piru deposits (b) T Total range2-406 feet 4 2 ≤ b ≤ 25 34° N 25 < b ≤ 50 22' ra River 50 < b ≤ 100 Cla 30" Santa 100 < b ≤ 200 200 < b ≤ 406

T 2 N

P ek a e c r i C fi s 34° c a u g e 07' l O l c a 30" e C an 0 10 MILES 0 10 KILOMETERS

R22W R18W EXPLANATION Santa Clara-Calleguas ground-water basin Hydrologic Unit boundary Fault Shallow alluvium and unconsolidated deposits River and selected streams Outside Santa Clara-Calleguas ground-water basin Shallow alluvium Model-grid boundary of flow Consolidated and unconsolidated deposits region and subareas

Figure 20. Distribution of estimated total thickness of coarse-grained and fine-grained interbeds used to estimate hydraulic properties and storage properties for the model of the Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Upper-aquifer system (model layer 1). B, Lower- aquifer system model (layer 2).

96 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 119°15' 119° 118°45' B Lake Thickness of coarsed-grained Piru deposits (b) T Total range 0.3 - 803 feet 4 0.3 ≤ b ≤ 100 34° N ≤ 22' 100

T 2 N

ek re C s 34° P a a u g c e 07' i l f l ic a 30" O C 0 10 MILES ce an 0 10 KILOMETERS

R22W R18W 119°15' 119° 118°45' Lake Thickness of fine-grained Piru deposits (b) T Total range 0.5 - 462 feet 4 0.5 ≤ b ≤ 25 34° N 25

T 2 N

ek re C s 34° P a u a g c e 07' l if l ic a 30" O C 0 10 MILES ce an 0 10 KILOMETERS

R22W R18W

Figure 20—Continued.

Simulation of Ground-Water Flow 97 Storage Properties storage coefficient of the coarse-grained deposits was The hydraulic properties used to simulate the estimated from the difference between an estimated changes in storage of water within the aquifer systems aquifer specific storage and the specific storage consist of three components (Hanson, 1989). The first representing the compressibility of water (Hanson, two components are specific yield and the elastic 1989). Specific storage is the ratio of the storage storage coefficient of the aquifer system, and the third coefficient to the thickness of the sediments, in this component is the inelastic storage coefficient, which case the aggregate thickness of the coarse-grained governs the irreversible release of water from the deposits. Reported values for aquifer specific storage inelastic compaction of the fine-grained deposits. The determined from local aquifer tests in the upper- and lower-aquifer systems range from 1.2 × 10–6 to 2 × specific yield and the elastic storage coefficients –6 –1 represent and govern the reversible release and uptake 10 ft (Neuman and Witherspoon, 1972; Hanson and Nishikawa, 1996). An initial elastic specific storage of of water from storage. The elastic and inelastic storage –6 –1 coefficient represents the sum of storage owing to the 3 × 10 ft was assumed from other reported values compressibility of water and to the compressibility of for alluvial sediments (Ireland and others, 1984; the matrix or the skeleton of the aquifer system. Hanson, 1989). The aquifer elastic skeletal storage Storage owing to the compressibility of water coefficient was estimated as the product of the aquifer was estimated as the product of the compressibility and skeletal specific storage and the aggregate cell-by-cell the specific weight of water, the porosity, and the total thickness of the coarse-grained deposits for each model thicknesses of the coarse- and fine-grained deposits in layer (fig. 20). In a similar manner, the elastic skeletal the aquifer (fig. 20). The assumed porosities were 35 storage coefficient of the fine-grained deposits was and 25 percent for fine- and coarse-grained deposits, estimated from the difference between a specific respectively; they were estimated from transport storage for the fine-grained deposits and the specific modeling of seawater intrusion along the Hueneme storage representing the compressibility of water submarine canyon (Tracy Nishikawa, U.S. Geological (Hanson, 1989). The elastic storage coefficient for the fine-grained deposits was estimated as the product of Survey, written commun., 1994) and range from 1.8 × the elastic skeletal specific storage of the fine-grained 10–5 to 2.5 × 10–4 for the upper-aquifer system (layer deposits and the aggregate cell-by-cell thickness of 1) and from less than 1 × 10–6 to 4.5 × 10–4 for the lower-aquifer system (layer 2). The ranges were fine-grained deposits for each model layer (fig. 20). specified within MODFLOW as the aquifer-system The composite aquifer-system elastic skeletal storage storage coefficients. coefficient was the sum of the elastic skeletal storage The upper-aquifer system (layer 1) was coefficients for the coarse-grained and fine-grained simulated as unconfined in the Santa Clara Valley, the deposits for each cell in each model layer (fig. 19B). Las Posas Valley, parts of the Santa Rosa Valley The third component of storage, owing to the subareas, the Oxnard Plain Forebay subarea, and the inelastic compaction of the fine-grained deposits, was Northeast Oxnard Plain subareas (fig. 19B). In the estimated as the product of the inelastic specific storage remainder of the Oxnard Plain and the Mound and the aggregate cell-by-cell thickness of the fine- grained deposits for each model layer (fig. 20). An subareas, the upper-aquifer system was simulated as –4 –1 confined. Storage coefficients, estimated from specific initial inelastic skeletal specific storage of 2 × 10 ft yields from previous models, range from 0.01 to 0.19 was based on the estimates from a consolidation test in the Santa Clara River subareas; the estimate was performed on the cores of fine-grained deposits from 0.12 along Conejo Creek in the Santa Rosa Valley the Oxnard and Mugu aquifers (California Department subarea. The storage coefficients (specific yields) were of Water Resources, 1971, figs. VI–12 and VI–13) and assumed to range from 0.02 to 0.19 in the Las Posas aquifer-test analyses (Neuman and Witherspoon, 1972; Neuman and Gardner, 1989); these estimated range Valley subareas (fig. 19B). –4 –4 –1 The elastic and inelastic skeletal storage from 1.3 × 10 to 4.3 × 10 ft . coefficients were simulated using the interbed storage package (Leake and Prudic, 1991). The elastic skeletal

98 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California The transition from elastic to inelastic storage is hydraulic conductivity of 0.05 ft/d to simulate flow controlled by the preconsolidation stress—the across the aquitards separating the Fox Canyon and maximum previous load that has been put on each Grimes Canyon aquifers in the Las Posas Valley sedimentary layer. The preconsolidation-stress subareas. Published values of vertical hydraulic threshold, expressed in terms of equivalent hydraulic conductivity range from 0.01 to 1 × 10–4 ft/d for the head, can range from 50 ft of water-level decline in Oxnard Plain subarea (California Department of Water some well-sorted, fine-grained deposits that have had Resources, 1975; Neuman and Gardner, 1989) and minimal sedimentary loading or lithification to more from 24. to 6 × 10–4 ft/d for the Pleasant Valley subarea than 150 ft of water-level decline in some lithified, (Hanson and Nishikawa, 1996). compressed, poorly sorted, or coarse-grained deposits The initial estimates of vertical leakance were (Holzer, 1981). The transition from elastic to inelastic from previous ground-water flow models. For the storage was estimated to be 150 ft of water-level extensions of the two model layers, the initial values decline from predevelopment conditions throughout used were 1 × 10–6 (ft/d)/ft for the Mound, the Santa the lower-aquifer system and 100 ft of water-level Clara River Valley, the Pleasant Valley, and the Santa decline throughout the upper-aquifer system, with the Rosa Valley subareas and for the offshore regions, and exception of 50 ft of water-level decline in the 1 × 10–5 (ft/d)/ft for the Las Posas Valley subareas. upper-aquifer system in the South Oxnard Plain These are largely assumed values. The final distribution subarea. These estimates were based, in part, on of vertical leakance was based on fitting simulated consolidation tests (California Department of Water head differences to those measured at multiple-well Resources, 1971), water-level hydrographs (figs. 13 completion sites (fig. 15). All vertical leakance values and 14), subsidence trajectories (fig. 9C), and were held constant for the period of simulation. lithologic data (Densmore, 1996).

Vertical Leakance Model Calibration Vertical leakance controls vertical flow between Calibration of the transient-state simulations was the upper- and lower-aquifer systems. Vertical leakance done for 1891–1993 and was based on matching water was calculated for this model (fig. 19A) as the levels (fig. 13, 14, 15, and 21) and streamflows (fig. estimated vertical hydraulic conductivity divided by 22). Predevelopment conditions (steady-state) were the combined half-thicknesses of each adjacent model used as the initial conditions for the transient-state layer for the estimated fine-grained deposits (fig. 20) in calibration. The long period of transient simulation was the upper- and lower-aquifer systems (McDonald and required because features of development, such as Harbaugh, 1988, eq. 5). Estimates of vertical leakance coastal landward flow (seawater intrusion) and of flow between the upper and lower aquifers used in subsidence, are dependent on the initial state of the previous regional models range from less than 9 × 10–6 aquifer systems. to 0.002 (ft/d)/ft for the Oxnard Plain subarea (California Department of Water Resources, 1975; Calibration Summary Reichard, 1995, fig. 12). A subregional model Calibration was achieved through trial-and-error developed for the Santa Rosa Valley subarea (Johnson adjustments to recharge, hydraulic properties, and and Yoon, 1987) yielded estimates that range from 1.5 pumpage to achieve a good fit within each subarea over × 10–3 (ft/d)/ft between the alluvium and the the historical period of record. These adjustments were underlying Santa Margarita Formation to 3 × 10–5 made as systematically as possible, starting with (ft/d)/ft between the Santa Margarita and Saugus recharge and streamflow, then hydraulic properties, and Formations and the underlying Conejo Volcanics. A finally indirect agricultural pumpage estimates. subregional model developed for Las Posas Valley Calibration and model development began using the (CH2M HILL, 1993) used a uniform value of vertical extended model developed by Reichard (1995).

Simulation of Ground-Water Flow 99 800 One-to-one correlation line 1927 Composite measured water-level altitude 1932 Composite measured water-level altitude 1950 Water-level altitude 1991 Water-level altitude 600 1993 Water-level altitude

400

200

0 UPPER-AQUIFER SYSTEM (MODEL LAYER 1) A -200 -200 0 200 400 600 800 800 One-to-one correlation line 1950 Water-level altitude 1991 Water-level altitude 1993 Water-level altitude 600

400 SIMULATED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL

200

0 LOWER-AQUIFER SYSTEM (MODEL LAYER 2) B

-200 -2000 200 400 600 800

MEASURED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL

Figure 21. Relation between measured and simulated water-level altitudes for selected years for the transient simulation of developed conditions (1927, 1932, 1950, 1991, and 1993) in the Santa Clara–Calleguas ground-water basin, Ventura County, California. A, Upper-aquifer system (model layer 1). B, Lower-aquifer system (model layer 2). C, Oxnard Plain.

100 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 200 One-to-one correlation line 1993 Upper water-level altitude

150

100

50

0

-50 OXNARD PLAIN (MODEL LAYER 1) C -100 -100-50 0 50 100 150 200

SIMULATED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL MEASURED WATER-LEVEL ALTITUDE, IN FEET ABOVE MEAN SEA LEVEL

Figure 21—Continued.

Predevelopment Initial Conditions storage is not required to simulate steady-state Calibrating the model was an iterative process conditions. Initial recharge was based on the long-term between the steady-state and transient-state seasonal geometric-mean ratios of runoff to wet-period simulations. The steady-state simulation provided winter precipitation. Streamflows were simulated as initial conditions for the transient-state calibration. median streamflows. The composite ET rates and the After each transient-state calibration, the updated model cells with the potential for ET for the years model parameters were used to simulate updated 1912, 1927, 1932, and 1950 (Conejo Creek area) were steady-state conditions prior to additional calibration. used for the predevelopment simulation (fig. 23). The The steady-state conditions were dependent on initial hydraulic properties were based on Reichard’s recharge (streamflow, mountain-front recharge, and (1995) values and were adjusted during transient-state valley-floor recharge) and discharge (streamflow and calibration. Few data were available for comparison of ET) from the aquifer system, transmissivity, vertical steady-state conditions. However, the simulated initial leakance between layers, fault hydraulic characteristic, conditions are considered adequate if water levels are and general-head boundary conductance. Because 40 to 50 ft above sea level near the coast along the water levels are constant under steady-state conditions, Oxnard Plain subareas. This requirement was based on a report of early hydraulic conditions (Freeman, 1968).

Simulation of Ground-Water Flow 101 SANTA PAULA CREEK PIRU CREEK DIVERSION (1931 – 1993) DIVERSION (1930 – 1941) SANTA CLARA RIVER DIVERSION–SATICOY [ USGS-11113910] [UWCD] [UWCD] 1,000 AND AT FREEMAN [UWCD] 1,000 1,000 100 100 100 10 10 10 1 1 1 0.1 0.1 0.1 1920 1935 1950 1965 1980 1995 1920 1935 1950 1920 1935 1950 1965 1980 1995 119° 118°45'

Lake Piru T4N

34°22'30" River

Santa Clara

T2N

R18W P a c ek if re i C c s a STREAMFLOW u Oc g CALLEGUAS CREEK ABOVE HWY. 101 e l e l [VCFCD-806] a 34°7'30" a 05MILES n C 10,000 STREAMFLOW R22W 05KILOMETERS SANTA CLARA RIVER AT MONTALVO 1,000 [USGS-11114000] 10,000 STREAMFLOW 100 CALLEGUAS CREEK AT CAMARILLO 1,000 [VCFCD-805] 10 10,000 100 1 1,000 0.1 10 100

1 1890 1905 1920 1935 1950 1965 1980 1995 10 0.1 1

1890 1905 1920 1935 1950 1965 1980 1995 0.1 1890 1905 1920 1935 1950 1965 1980 1995

EXPLANATION Santa Clara-Calleguas ground-water basin Streamflow- and diversion-gaging station - Name on graph title indicates operating agency Shallow alluvium and unconsolidated deposits USGS - U.S. Geological Survey VCFCD - Ventura County Flood Control District Outside Santa Clara-Calleguas ground-water basin UWCD - United Water Conservation District SANTA PAULA CREEK Shallow alluvium DIVERSION (1930 – 1941) [UWCD] Consolidated and unconsolidated deposits 1,000 Streamflow and diversion hydrographs with colored Hydrologic Unit boundary 100 curves showing flow rate for the gaging station number of the same color. Blue and gray curves Ground-water subbasin boundary– 10 are reported and measured streamflow or diversion Subbasin names are given in figure 1 rates. Red curves are simulated streamflow or 1 River and selected streams PER SECOND diversion rates. Gaging station number appears in brackets of graph title. Diversion time period 0.1 FLOW RATE, IN CUBIC FEET appears in parenthesis following graph title. 1920 1935 1950 TIME, IN YEARS

Figure 22. Measured and simulated seasonal streamflows or diversion rates for the Santa Clara–Calleguas ground-water basin, Ventura County, California.

102 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 4 2 T T N N Number is 45' ° 118 1932 1950 1993 1891 (Predevelopment) Piru R18W Lake Cells Not Simulating ET Cells Simulating ET Evapotranspiration (ET) Model Cells Fault Model-grid boundary of flow region and subareas Ventura Flood Control District Evaporation Station - station number 227 f the Santa Clara-Calleguas ground-water 169 169 171 5MILES

ILOMETERS River 5K ° 0 0

119 Clara

Santa

k Subbasin names are given in figure 1 e

e

r

C Ground-water subbasin boundary – Hydrologic Unit boundary River and selected streams s

a

u

g EXPLANATION

e l

l a C 239 R22W 15' ° 119 4

n a ce Consolidated and unconsolidated deposits O Shallow alluvium and unconsolidated deposits Shallow alluvium ic if Model cells with simulated evapotranspiration in 1891 (predevelopment), 1932, 1950, and 1993 the upper layer of model o ac Santa Clara-Calleguas ground-water basin P Outside Santa Clara-Calleguas ground-water basin ° ° 22' 30" 07' 30" 34 34 Figure 23. basin, Ventura County, California.

Simulation of Ground-Water Flow 103 Transient-State Calibration Parameters the total reported diversions at Piru, Santa Paula, Transient-state conditions were dependent on Saticoy, and Freeman (fig. 22). Segments of the recharge (streamflow, mountain-front recharge, valley- streamflow network in the coastal plain (segments 22, floor recharge, and artificial recharge) to and discharge 23, 29, and part of 30) (fig. 18B) are not in direct (pumpage, streamflow, and ET) from the aquifer connection with the upper-aquifer system and therefore system and on transmissivity, storage, vertical leakance were assigned a streambed conductance of zero. This between layers, fault hydraulic characteristics, and allowed the simulated water levels for predevelopment general-head boundary conductance. Because of the conditions and the recovery periods for development large head differences within some parts of the aquifer conditions to rebound to the measured water levels (figs. 13 and 14). Streamflow was increased from about systems, water-level maps were used for comparisons 3 but are considered less reliable than time-series data. 1.5 to 14 ft /s for Arroyo Simi to account for treated- Estimates of spatial fit were made for selected times of sewage effluent discharged between 1964 and 1993. On the transient simulation (fig. 21). Calibration was the basis of streamflow data from the hydrographs for primarily based on temporal comparisons, instead of Calleguas Creek at Camarillo (fig. 22, VCFCD station 805), the initial discharge (1964–79) was estimated to spatial comparisons, using long-term water-level 3 3 hydrographs (figs. 13, 14, and 15), streamflow start at 1.5 ft /s and increase linearly to 10 ft /s. hydrographs (fig. 22) and time-series of bench-mark The hydraulic properties estimated by Reichard land-surface altitudes (subsidence trajectories) (fig. 9). (1995) were adjusted during model calibration; they Recharge was adjusted to reduce the include transmissivity, storage properties, and vertical overestimation of mountain-front recharge, valley-floor leakance. The initial estimates were described earlier infiltration, and streamflow infiltration. The modified (see section on “Hydraulic Properties”). The only rational method of estimating infiltration tended to change to the storage properties was the transformation overestimate the water available during the wettest of Reichard’s (1995) initial estimates to cell-by-cell seasons; therefore, the upper limit of runoff available estimates, as was described earlier. Additional for mountain-front recharge was limited to less than calibration also was done for fault hydraulic 90 percent of average precipitation. characteristics and offshore general-head boundary Simulated streamflow infiltration initially was conductance. These properties were adjusted for the too large when floodflows or intermittent flows were period of reported pumpage largely on the basis of the spread over an entire season, and it did not reflect the water levels in the hydrographs. observed and measured changes in streamflow during Transmissivity values were reduced by a factor low-flow and high-flow conditions. The flow- of 0.55 for the lower layer of the Port Hueneme area dependent changes in streambed conductance are and were increased by a factor of 1.5 for the lower believed to be related mostly to changes in channel layer of the East Las Posas Valley subarea (figs.17B width. Grouping and varying streambed conductance and 19A) compared with the values used by Reichard with flow were critical for accurately depicting water- (1995). The decrease in transmissivities in the lower level declines and recoveries in wells during wet and layer brought the values closer to those in the transport dry periods (figs. 13 and 14). Grouping and varying model of the Port Hueneme area (Nishikawa, 1997) streambed conductance for the dry periods helped to and to those estimated from aquifer tests completed in simulate a more accurate depiction of the conveyance the East Las Posas Valley area (CH2M HILL, 1992). (delivery) of controlled releases from Lake Piru that are The transmissivities of the aquifer layer underlying the routed down the Santa Clara River and are simulated as major streams and tributaries also were increased during model calibration.

104 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Adjustments in vertical leakance were made on ground-water levels in the subareas and on the the basis of water-level differences at multiple-well magnitude and changes of pumpage for the 1983–93 observation sites and, for some areas, on the basis of period of reported pumpage. Changes to land-use data from the hydrographs of selected production estimates of historical pumpage include elimination of wells. Recall that the vertical leakances were calculated pumpage from the Santa Clara River Valley subareas as the estimated vertical hydraulic conductivity divided and the Oxnard Plain Forebay subarea for 1891–1918 by the combined half-thicknesses of the estimated fine- so that the first significant ground-water pumpage grained deposits in the upper- and lower-aquifer began with the dry period of 1919–36. The 1950 and systems (McDonald and Harbaugh, 1988, eq. 51). 1969 estimates of the land-use-based pumpage also had Cell-by-cell estimates for the West Las Posas Valley to be modified for selected subareas. The changes in subarea were based on a vertical hydraulic conductivity the 1950 estimate of agricultural pumpage applied over of 0.0005 ft/d. Cell-by-cell estimates for the Forebay the period 1946–61 ranged from a 34-percent reduction region of the Oxnard Plain were based on a vertical in pumpage for the Mound subbasin to an approximate hydraulic conductivity of 0.001 ft/d for all but five cells 300-percent increase for the Piru subbasin; the changes in the Saticoy area, for which a value of 0.01 ft/d was in the 1969 estimate applied over the period 1962–77 used. The final distribution of vertical leakances ranged ranged from a 34-percent reduction for the Mound from 1 × 10–7 to 3.03 × 10–5 (ft/d)/ft (fig. 19A). subarea to an approximate 100-percent increase in the Cell-by-cell estimates initially were made for all the North and South Pleasant Valley and the Piru subareas. subareas, but the estimates did not improve model fit These changes brought the estimated historical for the East and South Las Posas Valley subareas. For agricultural pumpage into alignment with the reported these two subareas, estimates were not based on the agricultural pumpage (fig. 11B) and improved the thickness of the fine-grained deposits; the final alignment between the measured and simulated calibrated vertical leakances align with the underlying ground-water levels and the land-use changes in syncline-anticline structures within the lower-aquifer various subareas for these two periods. Pumpage was system (figs. 9 and 20). reduced to 40 percent of the 1932 estimate for the years Although pumpage was the largest stress in the 1935–45, which span the post-Great Depression and model, some uncertainty remained about the accuracy World War II period, as well as a severe drought that of the land-use estimates of pumpage. Some was followed by one of the wettest periods on record adjustments in the magnitude and distribution of the (fig. 2). This reduction was the only way to achieve the pumpage estimated from land use were made during record water-level recoveries that have been equaled the calibration of the flow model in order to have the only during predevelopment conditions and more model enter the final 10-years of reported pumpage at recently during 1993. These adjustments did not affect the correct water-level altitudes. These changes were calibration of hydraulic properties or recharge during largely based on the measured temporal variations in the period of reported pumpage.

Simulation of Ground-Water Flow 105 The percentage of pumpage between layers was distribution of potential ET, based on riparian changed during model calibration. The final vertical vegetation, and the spatial distribution of simulated ET distribution of pumpage between the model layers for for predevelopment and developed conditions in 1932, wells spanning both model layers is summarized in 1950, and 1993 were also compared (fig. 23). figure 7B for all the subareas. Measured and simulated subsidence for selected bench The general-head boundaries that initially were marks (fig. 24) were used to compare the potential placed at the submarine outcrops were moved landward effects of water-level declines on simulated subsidence to better represent the average location of the in the South Oxnard Plain subarea. And, finally, freshwater-saltwater interface. The values of the selected comparisons of ground-water flows were used boundary heads were aligned with the top of the basal to confirm that flows within the model (fig. 25) were coarse-grained layers in the Oxnard and Hueneme conceptually consistent with the framework provided aquifers for the upper- and lower-aquifer systems, by geohydrologic and geochemical analyses. respectively. The boundary conductances were grouped The best and primary comparison period is the into several coastal subreaches with different values, 10-year period of reported pumpage, 1984–93, which grouping the conductances, however, did not improve represents one dry period and parts of two wet periods model fit. The final configuration consisted of a single (fig. 2A). Within this period is a 4-year period value for each model layer, which was the simplest (1990–93) for which measured water levels and approach without additional data and was adequate for water-level differences between aquifer systems matching water levels along the coast. The flows at the measured at the multiple-well monitoring sites general-head boundaries were monitored to verify that (fig. 15) can be compared with model results. simulated outflow was occurring during wet periods The model generally matched the measured when recovery of water levels exceeded the specified water-level, streamflow, and bench-mark data for the heads of the seawater at the general-head boundary. To calibration period (figs. 12, 13–15, 21; 22, and 24, be consistent, the model should simulate coastal respectively). Comparisons of the simulated and landward flow (seawater intrusion) during the major measured water levels estimated from land-use maps droughts when water levels decline below the heads of have some uncertainty because the measured ground- the denser seawater. The current model is consistent water levels reflect a wide variety of screened intervals with the concept of the wet-period outflow, as shown in wells, and the “synoptic” measured water levels by the outflows of 1984–93 (figure 25B in the section reflect water levels measured over spans of several “Transient-State Model Comparisons”), and with the months over a season (fig. 12A,B). The model slightly concept of coastal landward flow (seawater intrusion) overestimates historical water-level altitudes for the during droughts, such as the drought of 1987–91. early period of development (fig. 12A). The correlation diagram on figure 12A shows no systematic Transient-State Model Comparisons discrepancies between measured and simulated water levels in the upper-aquifer system. Measured minus Calibration and goodness-of-fit of the transient- simulated water levels have a mean error (ME) for the state model were determined by comparing simulated upper aquifer system of –22.8 ft and a root mean values with measured values for ground-water levels, square error (RMSE) of 35.2 ft for 1927 (number of streamflow, and land subsidence. The simulated water comparison wells: N = 169), and a ME of 7.29 ft and a levels were compared with water-level maps for 1932 RMSE of 42.2 ft for 1932 (N = 354). A comparison of and 1993 (fig. 12) and correlated with the water-level the measured and simulated water levels for 1932 (fig. data for 1927, 1932, 1950, 1991, and 1993 (fig. 21) and 12A) indicate similar patterns. Water-level differences the water-level hydrographs of selected production between simulated and measured data range from less wells (figs. 13 and 14) and multiple-well observation than 5 ft near the coast to about 40 ft in the Forebay, sites (fig. 15). A comparison of simulated streamflow and they are less than 20 to 40 ft in the Santa Clara was made for the downstream gaging stations and the River Valley and Pleasant Valley subareas. streamflow-diversion sites (fig. 22). The spatial

106 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California 2 T 4 T N N Layer 1 1980 2000 (1959-93) 45' Subsidence ° Total Simulated 118

Measured

YEAR

Subsidence

5 2

. .25 0 0 R18W Contour interval varies County, California, and Piru Lake Layer 2 (1923-93) Subsidence Total Simulated Z901

1920 1940 1960

0.0 0.5 1.0 1.5 2.0 2.5 3.0

USDNE NFEET IN SUBSIDENCE, 1 0 OR COMPACTION . 00.1 00.1.1

Total simulated subsidence from withdrawalground of water (1891-1993) - Bench mark Model-grid boundary of flow region and subareas

1

.

0 0.1

5

1 1

2

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0.25 0 MILES 5

. 1 1

0

1 0.5 0.5 . 5

00.1 River

KILOMETERS

5 3 3

.

0 0.5 5

1 1

° 1

.

0.1 0

119 5 00.5.

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at Clara Santa 2

1 1

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EXPLANATION

2 2

1 Gound-water subbasin boundary– Hydrologic Unit boundary River and selected streams

. 1 0 0.1

Point Mugu

5

2 .

0.25 0 0.0 5 .25.2 0

0.5

1

5 1

5

00.5. 5

2

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1 34 1 1 0.10. 22'30" Port

° n a Hueneme 34 ce

O

1 1

5 . 0 0.5

Layer 2 5 25 2 . 0 0. 1980 2000 1980 2000 (1939-93) Layer 1 Subsidence Layer 2 Total Simulated ic Layer 1 cif Pa YEAR YEAR (1923-93) Consolidated and unconsolidated deposits Subsidence Shallow alluvium and unconsolidated deposits Shallow alluvium (1939-78) Measured Subsidence Total Simulated Simulated compaction owing to withdrawal of ground water, 1891–1993, in the Santa Clara–Calleguas ground-water basin, Ventura (1939-93) Santa Clara-Calleguas ground-water basin Outside Santa Clara-Calleguas ground-water basin (1923-93) Subsidence Measured Subsidence Subsidence Total Simulated Total Simulated E584 TIDAL3 1920 1940 1960

1920 1940 1960

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 USDNE NFEET IN SUBSIDENCE, FEET IN SUBSIDENCE,

OPCINOR COMPACTION OR COMPACTION Figure 24. locations of selected bench marks and related measured simulated bench-mark trajectories.

Simulation of Ground-Water Flow 107 119°15' 119° 118°45' A Lake Difference in water-level altitude (∆h) Piru Upper-Aquifer from 1984 - 93, in feet 400 T Total Range -92 to 48 feet 4 System ≤∆ ≤ N -92 h -30 Rise 15 34° 14 600 -30 < ∆h ≤ 0 22' River 400 200 500 30" 0<∆h ≤ 5 Santa Clara300 5<∆h ≤ 30 Decline ∆ ≤ 600 30 < h 48 16 125 30 1 0 00 150 700 25 159 125 600 1,702 23 555858 4,2024,202 24 500 T 17 400 0 1

2 2 877 40 18 10 92 45245 100 0 22 N 0 29 10 0 13,613,692 40 16, 16, 20 420 3 20 5 2,021 21 2,166 22,818 ,818 10 19 746 ek 0 re P C a s 34° c 10 7 a i u f g i 0 e 0 07' c l l 0 10 MILES O 29 a 30" 20 C ce 0 10 KILOMETERS an

R22W R18W 119°15' 119° 118°45' Lake Difference in water-level altitude (∆h) Piru from 1984 - 93, in feet Lower-Aquifer T Total Range -195 to 251 feet 4 30 ≤∆ ≤ N System -195 h -50 Rise 0 26 34° -50 < ∆h ≤ 0 27 40 River 50 22' 600 ∆ ≤ 0

0< h 5 0 30" nta Clara 5<∆h ≤ 50 Decline Sa 200 ∆ ≤ 100 50 < h 251 28 60 40 100 20 138 13 700 130 600 0 11 0 101099 0 T 12 0 31 0 0 0 383 20 500 2 2 30 40 32 6 0 10 N 0 30

949 ,296

5,29 5 0 40 04 1,869 40 2,8042,8 4 6 22,354,354 9 100 40 43 94 ek 33 29 5945 re 40 20 C s a 34° P 8 u a g c e l 07' if l 0 10 MILES ic a 30" C O 0 10 KILOMETERS c 34 e 3,477 an

R22W R18W EXPLANATION Santa Clara-Calleguas ground-water basin Hydrologic Unit boundary Subarea number – Shallow alluvium and unconsolidated deposits River and selected streams See figure 17B for subarea names 22 Upper-aquifer system Outside Santa Clara-Calleguas ground-water basin Model-grid boundary of flow region and subareas 10 Lower-aquifer system Shallow alluvium 100 Water-level altitude 949 Mean simulated ground-water Consolidated and unconsolidated deposits (Simulated December, 1993) – underflow and coastal flow – in feet. Contour interval varies in acre-feet per year

Figure 25. A, Simulated water-altitudes (December 1992), decline in ground-water levels from 1984 to 1994, and mean ground-water flow in the Santa Clara–Calleguas ground-water basin, Ventura County, California. B, Cumulative changes in ground-water storage and ground-water flow for selected subareas during 1984–93. C, Hydrologic budgets for predevelopment conditions. D, Hydrologic budgets for 1984–93 period.

108 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California B 50 50 100 2,000 wet dry 0 0 50 1,000 FLOW SEAWARD -50 -50 0 0

-100 -100 -50 -1,000 FLOW

-150 -150 -100 -2,000 LANDWARD 50 50 100 2,000 wet dry 0 0 50 1,000 FLOW SEAWARD -50 -50 0 0

-100 -100 -50 -1,000 FLOW LANDWARD -150 -150 -100 -2,000 50 50 100 2,000

0 0 50 1,000 wet dry FLOW CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET SEAWARD SEASONAL FLOW TO OCEAN, IN ACRE-FEET PER SEASON -50 -50 0 CUMULATIVE FLOW TO OCEAN, IN THOUSANDS OF ACRE FEET 0

-100 -100 -50 -1,000 FLOW

-150 -150 -100 -2,000 LANDWARD CUMULATIVE STORAGE, IN THOUSANDS OF ACRE FEET 50 1984 1988 1992 1996 1984 1988 1992 1996 YEAR YEAR 0 EXPLANATION -50 Subarea number – B Upper-aquifer system -100 See figure 17 for subarea names Upper-aquifer system -150 Lower-aquifer system Lower-aquifer system 1984 1988 1992 1996 YEAR

Figure 25—Continued.

Simulation of Ground-Water Flow 109 C Predevelopment Conditions

Valley-Floor Recharge Mountain-Front and Bedrock Recharge Net Streamflow Recharge 4,770 acre-feet per year 14,600 acre-feet per year 14,300 acre-feet per year

Mound 1% Santa Rosa 3% Mound 0% Santa Rosa 2% Mound Pleasant Valley Santa Rosa 0% 5% Oxnard Plain 5% Oxnard Plain Pleasant 2% 15% Pleasant Las Posas Valley Valley 4% 16% 8% Las Oxnard Plain Las Posas Santa Clara 40% Santa Posas Santa Clara 20% 62% 30% Clara 37% 50%

Total Recharge Total Net Recharge 59,900 acre-feet per year 33,650 acre-feet per year

Bedrock V.F.Rech. 8% recharge V.F.Rech. 14% 4% Net Streamflow 43% M.F. Rech. 21% Streamflow Infiltration 67% Bedrock M.F. Rech. Recharge 6% 37%

Evapotranspiration Total Discharge 14,800 acre-feet per year 59,900 acre-feet per year

Mound 0% Santa Rosa 0% Coastal Outflow Oxnard Plain South Pleasant ET Oxnard Plain 23% Valley 13% 25% 21% Coastal Las Outflow Posas Mound 5% Santa Clara 8% 61% Stream Baseflow 44%

ET = Evapotranspiration V.F. Rech. = Valley-Floor Recharge M.F. Rech. = Mountain-Front and Bedrock Recharge

Figure 25—Continued.

110 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California D Historical Period 1984-93

Valley-Floor Recharge Mountain-Front and Bedrock Recharge Net Streamflow Recharge 24,100 acre-feet per year 27,760 acre-feet per year 62,200 acre-feet per year

Pleasant Santa Rosa Oxnard Plain W.L.P(1) E.L.P.(1) 3% 3% Valley 7% Santa Rosa 1% 2% S.L.P.(1) 5% 0% N.P.V.(1) 6% Mound Mound Oxnard Oxnard Plain 5% S.R.V(1) 0% Plain 22% Forebay(1) 5% Pleasant 6% Las Valley S.P.V.(1) 9% Posas 8% Piru(1) 26% 26% Mound(1) Santa Clara Las Posas 4% 58% Santa Clara 23% Santa Paula(1) Fillmore(1) 22% 44% 15%

Total Recharge Coastal Inflow Subsidence 227,700 acre-feet per year 6,450 acre-feet per year 4,100 acre-feet per year Mound Pleasant Mound(2) 2% 2% Mound 10% Valley Santa Rosa NW Oxnard 10% Valley Plain(2) 3% S. Oxnard 43% Pleasant Valley Las Posas Plain(2) Valley 54% 7% Oxnard Plain 12% Oxnard Plain Las Posas 47% 36% S. Oxnard Valley Plain(1) 34% Santa Clara 37% Santa Clara 2% 1% River Valley River Valley Pumpage Evapotranspiration Downward Flow 247,000 acre-feet per year 1,060 acre-feet per year 67,010 acre-feet per year Pleasant Mound Pleasant Mound 0% Mound 1% 3% Santa Rosa Santa Rosa Valley Valley 9% Santa Rosa Valley 1% Valley 2% 9% Valley Pleasant Valley Las Posas Oxnard Plain 1% 22% Valley 11% Oxnard Plain Las Posas 34% 3% Valley Oxnard 13% Plain Las Posas 37% Valley 34% Santa Clara Santa Clara River Valley River Valley Santa Clara 37% 76% 7% River Valley Total Inflow By Component Total Outflow By Component S.P.V.(1) = South Pleasant Valley 364,600 acre-feet per year 364,600 acre-feet per year (Upper-aquifer system) Coastal Flow Subsidence N.P.V.(1) = North Pleasant Valley 2% 2.5% (Upper-aquifer system) Subsidence V.F.Rch Coastal Flow 3% 6% M.F. Rch Storage 0.2% S.R.V.(1) = Santa Rosa Valley (Upper-aquifer system) Storage 8% 26.9% 30% Stream Inflow S.L.P.(1) = South Los Posas Valley Stream 20% (Upper-aquifer system) Baseflow Wastewater W.L.P.(1) = West Los Posas Valley 2.4% Effluent 2% (Upper-aquifer system) Irrigation ET E.L.P.(1) = East Los Posas Valley Return Artificial Recharge 14% 0.3% Pumpage (Upper-aquifer system) 15% 67.7% S. Oxnard Plain(1) = South Oxnard Plain (Upper-aquifer system) ET = Evapotranspiration S. Oxnard Plain(2) = South Oxnard Plain V.F. Rech. = Valley-Floor Recharge (Lower-aquifer system) M.F. Rech. = Mountain-Front and Bedrock Recharge NW Oxnard Plain(2) = Northwest Oxnard Plain (Lower-aquifer system) Mound(2) = Mound (Lower-aquifer system)

Figure 25—Continued.

Simulation of Ground-Water Flow 111 The simulated water levels were lower than the Measured water levels for the 1992–93 measured water levels for 1950, the first period of wet-period recovered; the simulated water levels were substantial ground-water development in both aquifer lower than the measured water levels for the systems [ME and RMSE are 9.95 and 52.7 ft, upper-aquifer system [ME and RMSE are 9.68 ft and respectively, for the upper-aquifer system (N = 297) 20.5 ft, respectively (N = 161) (fig. 21A)] and higher (fig. 21A), and 8.39 and 39.3 ft, respectively, for the than the measured water levels in the lower-aquifer lower-aquifer system (N = 31) (fig. 21B)]. The system [ME and RMSE are –42.3 ft and 66.9 ft, simulated water levels for the 1987–91 drought were respectively (N = 94) (fig. 21B)]. When the comparison lower than the measured water levels in the upper- was restricted to the upper-aquifer system of the aquifer system [ME and RMSE are 1.96 ft and 26.1 ft, Oxnard Plain for spring 1993, the simulated water respectively, (N = 130) (fig. 21A)] and higher than the levels were only slightly lower than the measured water measured water levels in the lower-aquifer system [ME levels [ME and RMSE are 1.61 ft and 10.7 ft, and RMSE are –89.8 ft and 110.4 ft, respectively respectively (N = 90) (fig. 21C)]. (N = 101) (fig. 21B)]. The differences between the In general, the long-term water-level measured and simulated water levels in the lower- hydrographs (figs. 13 and 14) indicate that the match aquifer system are, in part, due to the many wells used between measured and simulated water-level altitudes for the calibration which are completed solely in the is good for the entire period of simulation, especially Fox Canyon or Grimes Canyon aquifer in parts of the those for the Oxnard Plain subbasin. However, some Pleasant Valley and Las Posas Valley subareas. These hydrographs show large discrepancies between the aquifers were not simulated as separate aquifer layers simulated and measured water levels; examples of in the current model and therefore the simulation these discrepancies can be seen on the hydrographs of represents the average water level for the entire lower wells along Beardsley Wash, such as well 2N/21W- aquifer system. The Fox Canyon and Grimes Canyon 16J1 in the West Las Posas Valley subarea and wells aquifers are relatively low-permeability aquifers; along the Santa Clara River, wells 2N/22W-2C1 and pumpage from these aquifers resulted in large 3N/22W-36K2 in the Santa Paula subarea, well water-level declines. The overlying Hueneme aquifer is 2N/22W-9J1 in the Mound subarea, and well 3N/19W- relatively more permeable; pumpage from this aquifer 29E2 in the East Las Posas Valley subarea (fig. 14). A resulted in smaller water-level declines. Measured comparison of the short-term hydrographs for the water levels for the multiple-well monitoring sites RASA multiple-well monitoring sites shows good indicate water-level differences within the agreement between the simulated and measured water lower-aquifer system of as much as 75 ft between the levels (fig. 15). The simulated water-level differences Hueneme aquifer and the Fox Canyon and Grimes between the upper and lower layers closely match the aquifers (fig. 15). The model was calibrated to the measured seasonal and multiple-year patterns of water- Hueneme aquifer and would have required additional level differences (fig. 15). This indicates that the layers to simulate the water-level differences for the collective estimates of vertical leakance, vertical lower aquifers. Some water-level measurements also distribution of pumpage, and recharge are reasonable. may have been affected by pumping, which resulted in Water-level differences between wells across measured water levels being lower than the simulated faults were calibrated by adjusting fault hydraulic levels. Another reason for the water-level differences characteristics; for example, the water-level differences may be that instantaneous water-level measurements between well 2N/20W-23K1 (fig. 13) and well were compared with simulated water levels controlled 2N/20W-23R1 (fig.14) across the San Pedro (Bailey) by average seasonal pumpage. Fault in the Santa Rosa Valley subarea.

112 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California Seasonal water-level variations in the Simulated subsidence generally matches total upper-aquifer system are controlled largely by measured subsidence in the South Oxnard Plain streamflow infiltration and related streambed subarea (fig. 24). The time-series comparisons of conductance; these factors, when combined with subsidence from bench-mark measurements are similar seasonally and climatically variable pumpage, resulted in trend but underestimate subsidence at BM Z 901 in water-level fluctuations of tens to a hundred feet in near Point Mugu and overestimate subsidence at BM wells in the Santa Clara River Valley subareas [wells TIDAL 3 near Port Hueneme (fig. 24). The extent of 4N/19W-25K2, 30R1; 22N/22W-11A1,2 (fig. 14)]. subsidence generally is not well known for areas Water-level fluctuations in the Oxnard Plain Forebay outside the South Oxnard Plain subarea but may be subareas include the effects of artificial recharge and overestimated for parts of the Pleasant and Las Posas pumping back artificially recharged water [wells Valley subareas. Field inspections throughout West and 2N/22W-12R1, 22R1 (fig. 14); wells 2N/22W-23B3–7, East Las Posas subareas did not reveal any surface 2N/21W-7L3–6 (fig. 15)]. expressions of land subsidence that would be expected Simulated streamflows for Montalvo and for the for the amount of simulated subsidence. This Piru, Santa Paula, Saticoy, and Freeman diversions overestimation may be caused by overestimation of closely match measured streamflow along the Santa inelastic skeletal specific storage, overestimation of the Clara River system. Simulated streamflows also match aggregate thickness of fine-grained material that is many of the historical high flow events (figs. 2B and actually subject to loading from water-level declines, 22); however, they overestimate low streamflow and a lack of separate model layers within the lower- conditions (less than 10 ft3/s) for some dry-year aquifer system for the Pleasant and Las Posas Valley periods at Montalvo on the Santa Clara River (fig. 22). subareas. A detailed land survey or Interferometric The simulations underestimated the diversions for Synthetic Aperture Radar (InSAR) imagery analysis some dry-year periods when flows were less than 2 to would be needed to resolve this issue. 10 ft3/s at Saticoy and less than 2 ft3/s at the Santa Paula and Piru diversions (fig. 22). Simulated Model Uncertainty, Sensitivity, and Limitations streamflows for Camarillo and above Highway 101 in Numerical models of ground-water flow are Calleguas Creek match measured streamflow; the useful tools for assessing the response of an aquifer simulated streamflow is intermittent in character after system to changing natural and human-induced the onset of ground-water development in the late stresses. Regional-scale models are especially useful 1920s (fig. 22). for assessing many of the components in the Simulation results indicate that land subsidence hydrologic cycle and the collective effect of ground- started as early as the 1920s and continued through water development in separate subareas of a regional 1984–93, the period when water levels declined below ground-water system. Models, however, are only an the water levels of the 1950s and 1960s. Results also approximation of actual systems and typically are indicate that preconsolidation may vary considerably based on average and estimated conditions. The and that subsidence occurred primarily during dry-year reliability or certainty with which a model can simulate periods when seasonal and multiple-year water-level aquifer response is directly related to the accuracy of declines exceeded past declines in the South Oxnard the input data, the amount of detail that can be Plain, Las Posas Valley, and Pleasant Valley subareas simulated at the scale of the model, and the model (figs. 24 and 25B). Subsidence started in the upper- discretization of time and space. Hence, the regional aquifer system in the South Oxnard Plain subarea models can be useful for simulating subregional and during the early period of development (1939–60) regional performance of a flow system and for (fig. 24). Subsidence has continued, in part, because of providing boundary information for more detailed the development of the lower-aquifer system, which local-scale models even though the results of the has contributed most of the subsidence in recent regional model for a local scale may not be appropriate decades (1959–93) (fig. 24). for site-specific problems such as the performance at a particular well.

Simulation of Ground-Water Flow 113 The certainty of a model is inversely related to affect the comparison between measured and simulated the duration, magnitude, and distribution of simulated streamflows. Based on gaging-station ratings, inflows and outflows. Thus, better time-varying inaccuracy in streamflow measurements can range estimates of pumpage, recharge, irrigation return flow, from 5 to 20 percent. For high flows, this inaccuracy streamflow, and coastal landward flow (seawater may result in an uncertainty of hundreds to thousands intrusion) could improve simulation of historical of acre-feet in potential recharge for some wet years. development. Additionally, the trial-and-error Other sources of uncertainty include estimates of calibration process is inexact, and this problem is precipitation, which may have estimation errors compounded by uncertainty of the variables and by (kriging errors) ranging from 5 to 10 percent which can sensitivity of the aquifer-parameter and boundary- result in thousands of acre-feet of uncertainty for wet- condition estimates. Uncertainty in model attributes year seasons; estimates of irrigation return flow, which results in a broader range of possible aquifer-parameter may have estimation errors ranging from 10 to and boundary-condition estimates used to constrain 20 percent owing to the uncertainty and the variability calibration of the ground-water flow model. of the estimates of applied water and irrigation Uncertainty in water levels in wells, streamflows, and efficiency (Koczot, 1996); and errors in the assignment altitudes of bench marks used for model comparison of percentages of pumpage for wells completed across during calibration can affect the degree of fit achieved. both aquifer systems, which may range from 10 to 20 Sensitivity to changes in model parameters and percent. boundary conditions during calibration also can affect Additional uncertainties also may exist with the degree of fit and the possible range of values used respect to boundary conditions such as the average to simulate historical ground-water flow. location of the seawater front, which is represented by An exhaustive analysis of the uncertainty and the general-head boundary cells; horizontal-flow sensitivity of every model parameter and boundary barriers, some of which may be of inferred extent; and condition is beyond the purpose and scope of this the conductance of some faults. The importance of report. However, a summary can yield insight into the some faults remains uncertain; for example, faults capabilities and limitations of the model, and specific whose traces generally are parallel to the hydraulic insight into its performance with respect to ground- gradient, such as the Oak Ridge and McGrath Faults in water management. The combination of the uncertainty the upper-aquifer system, or faults that are adjacent to a in the model-input and comparison data and the spatial contrast in transmissivity, such as the Country sensitivity of the model to changes in model input yield Club Fault. Considerable testing of these boundaries a qualitative measure of the importance of various was done during model calibration; the resulting model attributes. For example, uncertainties in the estimates for boundary locations and conductance measurement of streamflows may contribute to satisfy the conceptual framework and the measured uncertainties in the simulation of streamflows and comparison data.

114 Simulation of Ground-Water/Surface-Water Flow in the Santa Clara–Calleguas Ground-Water Basin, Ventura County, California