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Simulation of Ground-Water Flow

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Simulation of Ground-Water Flow 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.
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