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

U.S. Geological Survey (USGS) | July 17th, 2003

Ground water is the main source of water in the Santa Clara–Calleguas ground-water basin that covers about 310 square miles in Ventura County, California. A steady incr

Ground water is the main source of water in the Santa Clara–Calleguas ground-water basin that covers about 310 square miles in Ventura County, California. A steady increase in the demand for surface- and ground-water resources since the late 1800s has resulted in streamflow depletion and ground-water overdraft. This steady increase in water use has resulted in seawater intrusion, inter-aquifer flow, land subsidence, and ground-water contamination.  The Santa Clara–Calleguas Basin consists of multiple aquifers that are grouped into upper- and lower-aquifer systems. The upper-aquifer system includes the Shallow, Oxnard, and Mugu aquifers. The lower-aquifer system includes the upper and lower Hueneme, Fox Canyon, and Grimes Canyon aquifers. The layered aquifer systems are each bounded below by regional unconformities that are overlain by extensive basal coarse-grained layers that are the major pathways for ground-water production from wells and related seawater intrusion. The aquifer systems are bounded below and along mountain fronts by consolidated bedrock that forms a relatively impermeable boundary to ground-water flow. Numerous faults act as additional exterior and interior boundaries to ground-water flow. The aquifer systems extend offshore where they crop out along the edge of the submarine shelf and within the coastal submarine canyons. Submarine canyons have dissected these regional aquifers, providing a hydraulic connection to the ocean through the submarine outcrops of the aquifer systems. Coastal landward flow (seawater intrusion) occurs within both the upper- and lower-aquifer systems. A numerical ground-water flow model of the Santa Clara–Calleguas Basin was developed by the U.S. Geological Survey to better define the geohydrologic framework of the regional ground-water flow system and to help analyze the major problems affecting water-resources management of a typical coastal aquifer system. Construction of the Santa Clara–Calleguas Basin model required the compilation of geographic, geologic, and hydrologic data and estimation of hydraulic properties and flows. The model was calibrated to historical surface-water and ground-water flow for the period 1891–1993. Sources of water to the regional ground-water flow system are natural and artificial recharge, coastal landward flow from the ocean (seawater intrusion), storage in the coarse-grained beds, and water from compaction of fine-grained beds (aquitards). Inflows used in the regional flow model simulation include streamflows routed through the major rivers and tributaries; infiltration of mountain-front runoff and infiltration of precipitation on bedrock outcrops and on valley floors; and artificial ground-water recharge of diverted streamflow, irrigation return flow, and treated sewage effluent.  Most natural recharge occurs through infiltration (losses) of streamflow within the major rivers and tributaries and the numerous arroyos that drain the mountain fronts of the basin. Total simulated natural recharge was about 114,100 acre-feet per year (acre-ft/yr) for 1984–93: 27,800 acre-ft/yr of mountain-front and bedrock recharge, 24,100 acre-ft/yr of valley-floor recharge, and 62,200 acre-ft/yr of net streamflow recharge. Artificial recharge (spreading of diverted streamflow, irrigation return, and sewage effluent) is a major source of ground-water replenishment. During the 1984–93 simulation period, the average rate of artificial recharge at the spreading grounds was about 54,400 acre-ft/yr, 13 percent less than the simulated natural recharge rate for streamflow infiltration within the major rivers and tributaries. Estimated recharge from infiltration of irrigation return flow on the valley floors averaged about 51,000 acre-ft/yr, and treated sewage effluent averaged about 9,000 acre-ft/yr. Artificial recharge as streamflow diversion to the spreading grounds has occurred since 1929, and treated-sewage effluent has been discharged to stream channels since 1930.  Under predevelopment conditions, the largest discharge from the ground-water system was outflow as coastal seaward flow and evapotranspiration. Pumpage of ground water from thousands of water-supply wells has diminished these outflows and is now the largest outflow from the ground-water flow system. The distribution of pumpage for 1984–93 indicates that most of the pumpage occurs in the Oxnard Plain subareas (37 percent) and in the upper Santa Clara River Valley subareas (37 percent). The total average simulated pumpage was about 247,000 acre-ft/yr (59 percent); of which about 146,000 acre-ft/yr was from the Fox Canyon Groundwater Management Agency (FGMA) subareas and 101,000 acre-ft/yr (41 percent) from the non-FGMA subareas. Of the total 1984–93 pumpage, 46 percent was contributed by natural recharge, 22 percent was contributed by artificial recharge from diverted streamflow, 20 percent was contributed by irrigation return flow, 4 percent was contributed from sewage-effluent infiltration, 6 percent was contributed from storage depletion, and 2 percent was contributed from coastal landward flow (seawater intrusion). Seawater intrusion was first suspected in 1931 when water levels were below sea level in a large part of the Oxnard Plain. The simulation of regional ground-water flow indicated that coastal landward flow (seawater intrusion) began in 1927 and continued to the end of the period of simulation (1993). During wet periods or periods of reduced demand for ground water, the direction of coastal flow in the upper-aquifer system reverses from landward to seaward. During the 1984–93 period, the simulated total net seaward flow was 9,500 acre-feet in the upper-aquifer system, which is considerably less than that simulated for predevelopment conditions. During the same period, total simulated landward flow in the lower-aquifer system was 64,200 acre-feet. Water-level declines in the basin have induced land subsidence that was first measured in 1939 and have resulted in as much as 2.7 feet land subsidence in the southern part of the Oxnard Plain. The model simulated a total of 3 feet of land subsidence in the southern part of the Oxnard Plain and as much as 5 feet in the Las Posas Valley subbasins. Model simulations indicate that most of the land subsidence occurred after the drought of the late 1920s and during the agricultural expansion of the 1950s and 1960s. The results also indicate that subsidence occurred primarily in the upper-aquifer system prior to 1959, but in the lower-aquifer system between 1959–93 owing to an increase in pumpage from the lower-aquifer system. The calibrated ground-water flow model was used to assess future ground-water conditions based on proposed water-supply projects in the existing management plan for the Santa Clara–Calleguas ground-water basin. All the projections of the proposed water-supply projects in the existing management plan have reduced pumpage in the FGMA areas which resulted in a reduction but not an elimination of storage depletion and related coastal landward flow (seawater intrusion) and subsidence, a reduction in streamflow recharge, and an increase in coastal seaward flow and underflow to adjacent subareas from the Oxnard Plain. A comparison of management simulations based on historical inflows and a spectral estimate of inflows shows increased coastal landward flow (seawater intrusion), storage depletion, and increased land subsidence due to a drought projected earlier in the spectral estimate of inflows than in the historical inflows. The spectral estimate probably provides a smoother and more realistic transition between historical and future climatic conditions. The model also was used to simulate potential alternative water-supply projects in the Santa Clara–Calleguas ground-water basin. These seven alternative water-supply projects were proposed to help manage the effects of increasing demand and variable supply on seawater intrusion, subsidence, increased withdrawal from storage, and vertical and lateral flow between subareas and aquifers systems. Stopping pumpage primarily in the lower-aquifer system in the South Oxnard Plain subarea had the largest effect on reducing coastal landward flow (seawater intrusion) of all the potential cases evaluated. Shifting pumpage from the lower- to the upper-aquifer system in the South Oxnard Plain subarea yielded the largest combined effect on coastal flow with a reduction of coastal landward flow in the lower-aquifer system and coastal seaward flow from the upper-aquifer system. A seawater-barrier injection project stopped coastal landward flow (seawater intrusion) in the upper-aquifer system but also resulted in large quantities of coastal seaward flow. The recharge of water in Happy Camp Canyon resulted in water-level rises that were above land surface (not feasible) in the East Las Posas Valley subarea but in no significant changes in hydrologic conditions in other parts of the basin. 

Simulation of ground-water/surface-water fow in the Santa Clara-Calleguas ground-water basin, Ventura County, California

U.S. Geological Survey (USGS) | July 17th, 2003

Groundwater is the main source of water in the Santa Clara–Calleguas ground-water basin that covers about 310 square miles in Ventura County, California. A steady incre

Groundwater is the main source of water in the Santa Clara–Calleguas ground-water basin that covers about 310 square miles in Ventura County, California. A steady increase in the demand for surface- and ground-water resources since the late 1800s has resulted in streamflow depletion and ground-water overdraft. This steady increase in water use has resulted in seawater intrusion, inter-aquifer flow, land subsidence, and ground-water contamination. The Santa Clara–Calleguas Basin consists of multiple aquifers that are grouped into upper- and lower-aquifer systems. The upper-aquifer system includes the Shallow, Oxnard, and Mugu aquifers. The lower-aquifer system includes the upper and lower Hueneme, Fox Canyon, and Grimes Canyon aquifers. The layered aquifer systems are each bounded below by regional unconformities that are overlain by extensive basal coarse-grained layers that are the major pathways for ground-water production from wells and related seawater intrusion. The aquifer systems are bounded below and along mountain fronts by consolidated bedrock that forms a relatively impermeable boundary to ground-water flow. Numerous faults act as additional exterior and interior boundaries to ground-water flow. The aquifer systems extend offshore where they crop out along the edge of the submarine shelf and within the coastal submarine canyons. Submarine canyons have dissected these regional aquifers, providing a hydraulic connection to the ocean through the submarine outcrops of the aquifer systems. Coastal landward flow (seawater intrusion) occurs within both the upper- and lower-aquifer systems. A numerical ground-water flow model of the Santa Clara–Calleguas Basin was developed by the U.S. Geological Survey to better define the geohydrologic framework of the regional ground-water flow system and to help analyze the major problems affecting water-resources management of a typical coastal aquifer system. Construction of the Santa Clara–Calleguas Basin model required the compilation of geographic, geologic, and hydrologic data and estimation of hydraulic properties and flows. The model was calibrated to historical surface-water and ground-water flow for the period 1891–1993. Sources of water to the regional ground-water flow system are natural and artificial recharge, coastal landward flow from the ocean (seawater intrusion), storage in the coarse-grained beds, and water from compaction of fine-grained beds (aquitards). Inflows used in the regional flow model simulation include streamflows routed through the major rivers and tributaries; infiltration of mountain-front runoff and infiltration of precipitation on bedrock outcrops and on valley floors; and artificial ground-water recharge of diverted streamflow, irrigation return flow, and treated sewage effluent. Most natural recharge occurs through infiltration (losses) of streamflow within the major rivers and tributaries and the numerous arroyos that drain the mountain fronts of the basin. Total simulated natural recharge was about 114,100 acre-feet per year (acre-ft/yr) for 1984–93: 27,800 acre-ft/yr of mountain-front and bedrock recharge, 24,100 acre-ft/yr of valley-floor recharge, and 62,200 acre-ft/yr of net streamflow recharge. Artificial recharge (spreading of diverted streamflow, irrigation return, and sewage effluent) is a major source of ground-water replenishment. During the 1984–93 simulation period, the average rate of artificial recharge at the spreading grounds was about 54,400 acre-ft/yr, 13 percent less than the simulated natural recharge rate for streamflow infiltration within the major rivers and tributaries. Estimated recharge from infiltration of irrigation return flow on the valley floors averaged about 51,000 acre-ft/yr, and treated sewage effluent averaged about 9,000 acre-ft/yr. Artificial recharge as streamflow diversion to the spreading grounds has occurred since 1929, and treated-sewage effluent has been discharged to stream channels since 1930. Under predevelopment conditions, the largest discharge from the ground-water system was outflow as coastal seaward flow and evapotranspiration. Pumpage of ground water from thousands of water-supply wells has diminished these outflows and is now the largest outflow from the ground-water flow system. The distribution of pumpage for 1984–93 indicates that most of the pumpage occurs in the Oxnard Plain subareas (37 percent) and in the upper Santa Clara River Valley subareas (37 percent). The total average simulated pumpage was about 247,000 acre-ft/yr (59 percent); of which about 146,000 acre-ft/yr was from the Fox Canyon Groundwater Management Agency (FGMA) subareas and 101,000 acre-ft/yr (41 percent) from the non-FGMA subareas. Of the total 1984–93 pumpage, 46 percent was contributed by natural recharge, 22 percent was contributed by artificial recharge from diverted streamflow, 20 percent was contributed by irrigation return flow, 4 percent was contributed from sewage-effluent infiltration, 6 percent was contributed from storage depletion, and 2 percent was contributed from coastal landward flow (seawater intrusion). Seawater intrusion was first suspected in 1931 when water levels were below sea level in a large part of the Oxnard Plain. The simulation of regional ground-water flow indicated that coastal landward flow (seawater intrusion) began in 1927 and continued to the end of the period of simulation (1993). During wet periods or periods of reduced demand for ground water, the direction of coastal flow in the upper-aquifer system reverses from landward to seaward. During the 1984–93 period, the simulated total net seaward flow was 9,500 acre-feet in the upper-aquifer system, which is considerably less than that simulated for predevelopment conditions. During the same period, total simulated landward flow in the lower-aquifer system was 64,200 acre-feet. Water-level declines in the basin have induced land subsidence that was first measured in 1939 and have resulted in as much as 2.7 feet land subsidence in the southern part of the Oxnard Plain. The model simulated a total of 3 feet of land subsidence in the southern part of the Oxnard Plain and as much as 5 feet in the Las Posas Valley subbasins. Model simulations indicate that most of the land subsidence occurred after the drought of the late 1920s and during the agricultural expansion of the 1950s and 1960s. The results also indicate that subsidence occurred primarily in the upper-aquifer system prior to 1959, but in the lower-aquifer system between 1959–93 owing to an increase in pumpage from the lower-aquifer system. The calibrated ground-water flow model was used to assess future ground-water conditions based on proposed water-supply projects in the existing management plan for the Santa Clara–Calleguas ground-water basin. All the projections of the proposed water-supply projects in the existing management plan have reduced pumpage in the FGMA areas which resulted in a reduction but not an elimination of storage depletion and related coastal landward flow (seawater intrusion) and subsidence, a reduction in streamflow recharge, and an increase in coastal seaward flow and underflow to adjacent subareas from the Oxnard Plain. A comparison of management simulations based on historical inflows and a spectral estimate of inflows shows increased coastal landward flow (seawater intrusion), storage depletion, and increased land subsidence due to a drought projected earlier in the spectral estimate of inflows than in the historical inflows. The spectral estimate probably provides a smoother and more realistic transition between historical and future climatic conditions. The model also was used to simulate potential alternative water-supply projects in the Santa Clara–Calleguas ground-water basin. These seven alternative water-supply projects were proposed to help manage the effects of increasing demand and variable supply on seawater intrusion, subsidence, increased withdrawal from storage, and vertical and lateral flow between subareas and aquifers systems. Stopping pumpage primarily in the lower-aquifer system in the South Oxnard Plain subarea had the largest effect on reducing coastal landward flow (seawater intrusion) of all the potential cases evaluated. Shifting pumpage from the lower- to the upper-aquifer system in the South Oxnard Plain subarea yielded the largest combined effect on coastal flow with a reduction of coastal landward flow in the lower-aquifer system and coastal seaward flow from the upper-aquifer system. A seawater-barrier injection project stopped coastal landward flow (seawater intrusion) in the upper-aquifer system but also resulted in large quantities of coastal seaward flow. The recharge of water in Happy Camp Canyon resulted in water-level rises that were above land surface (not feasible) in the East Las Posas Valley subarea but in no significant changes in hydrologic conditions in other parts of the basin.

Simulation of Groundwater and Surface-Water Resources of the Santa Rosa Plain Watershed, Sonoma County, California

U.S. Geological Survey (USGS) | April 25th, 2014

Water managers in the Santa Rosa Plain face the challenge of meeting increasing water demand with a combination of Russian River water, which has uncertainties in its fut

Water managers in the Santa Rosa Plain face the challenge of meeting increasing water demand with a combination of Russian River water, which has uncertainties in its future availability; local groundwater resources; and ongoing and expanding recycled water and water from other conservation programs. To address this challenge, the U.S. Geological Survey, in cooperation with the Sonoma County Water Agency, the cities of Cotati, Rohnert Park, Santa Rosa, and Sebastopol, the town of Windsor, the California American Water Company, and the County of Sonoma, undertook development of a fully coupled groundwater and surface-water model to better understand and to help manage the hydrologic resources in the Santa Rosa Plain watershed. The purpose of this report is to (1) describe the construction and calibration of the fully coupled groundwater and surface-water flow model for the Santa Rosa Plain watershed, referred to as the Santa Rosa Plain hydrologic model; (2) present results from simulation of the Santa Rosa Plain hydrologic model, including water budgets, recharge distributions, streamflow, and the effect of pumping on water-budget components; and (3) present the results from using the model to evaluate the potential hydrologic effects of climate change and variability without pumpage for water years 2011–99 and with projected pumpage for water years 2011–40.

Simulation of Water-Management Alternatives in the Mojave River Ground-Water Basin, California

U.S. Geological Survey (USGS) | July 1st, 2002

Siskiyou County Agricultural Economics Analysis Considering Groundwater Regulation

University of California, Merced | September 2nd, 2021

This economic analysis estimates potential impacts in gross revenues from changing cropping patterns in Siskiyou County’s three agricultural valleys namely Butte Valle

This economic analysis estimates potential impacts in gross revenues from changing cropping patterns in Siskiyou County’s three agricultural valleys namely Butte Valley (Butte), Scott River Valley (Scott), and Shasta Valley (Shasta). This analysis provides insight on economic costs of benefits of land and water use decisions, while identifying areas that may benefit from intervention and stakeholder processes.

Small Water Systems and Rural Communities Drought and Water Shortage Contingency Planning and Risk Assessment, Part 1

California Department of Water Resources (DWR) | March 25th, 2021

This report is submitted pursuant to California Water Code (CWC) Section 10609.42, which directs the California Department of Water Resources (DWR) to identify small wate

This report is submitted pursuant to California Water Code (CWC) Section 10609.42, which directs the California Department of Water Resources (DWR) to identify small water suppliers and rural communities that may be at risk of drought and water shortage vulnerability, and to propose recommendations and information in support of improving their drought preparedness. The report is published in two parts: Part 1 deals with drought and water shortage contingency planning recommendations, and Part 2 presents a methodology of drought and water shortage vulnerability assessment and risk scoring. Additionally, below are links for Part 1 appendices. Appendix 1: Literature Review Appendix 2: Proposed Water Shortage Contingency Plan Components for Small Water Systems Serving 1,000 to 2,999 Service Connections Appendix 3: Proposed Umbrella Water Shortage Contingency Plan Components Appendix 4: Indian Health Service Template for a Drought Contingency Plan for a Tribal Public Water System Appendix 5: County Drought Advisory Group General Funding and Financing Issues

Small Water Systems and Rural Communities Drought and Water Shortage Contingency Planning and Risk Assessment, Part 2

California Department of Water Resources (DWR) | March 25th, 2021

This report is submitted pursuant to California Water Code (CWC) Section 10609.42 which directs California Department of Water Resources (DWR) to identify small water sup

This report is submitted pursuant to California Water Code (CWC) Section 10609.42 which directs California Department of Water Resources (DWR) to identify small water suppliers and rural communities that may be at risk of drought and water shortage vulnerability and propose recommendations and information in support of improving the drought preparedness of small water suppliers and rural communities. The report is published in two parts: Part 1 dealing with drought and water shortage contingency planning recommendations, and Part 2 presenting a methodology and results of drought and water shortage vulnerability assessment and risk scoring. Additionally, below are links for Part 2 appendices. Appendix 1: Drought and Water Shortage Risk Scoring: California’s Small Water Supplier and Self-Supplied Communities Appendix 2: Drought and Water Shortage Risk Scoring: Risk Score Results of Small Water Suppliers Appendix 3: Drought and Water Shortage Risk Scoring: Risk Score Results of Self-Supplied Communities

Sniffing the Delta for Greenhouse Gases

San Francisco Estuary Partnership (SFEP) | June 16th, 2022

Global food production contributes an estimated 21% to 37% of the greenhouse gases that are warming the planet, including huge releases of carbon when vast aquatic ecosys

Global food production contributes an estimated 21% to 37% of the greenhouse gases that are warming the planet, including huge releases of carbon when vast aquatic ecosystems like the Delta are transformed for agriculture. [Dennis] Baldocchi and colleagues are now trying to figure out if bringing back tidal wetlands like the one at Dutch Slough can help reverse these processes.

Snow monitoring at strategic locations improves water supply forecasting more than basin-wide mapping

Nature Portfolio (Springer Nature) | August 15th, 2025

In the Western United States, water supply forecasting has traditionally relied on snow water equivalent measurements at ground-based stations due to their strong correla

In the Western United States, water supply forecasting has traditionally relied on snow water equivalent measurements at ground-based stations due to their strong correlations with streamflow volume during spring and summer. However, stations are sparse and sample a small area, prompting interest in spatially complete – but costly – basin-wide mapping from airborne surveys or future satellite missions. Here we show that adding strategic measurements at snow hotspots – localized areas with untapped information for predicting streamflow – consistently outperforms spatially complete surveys that provide basin-average snowpack, both in basins with and without existing stations. While both improve forecast skill, hotspot monitoring increases correlations with streamflow volume by 11-14% (median) across 390 basins, compared to 4% from basin-wide surveys. These findings hold across snowpack datasets, skill metrics, and statistical models. The greatest gains in water supply prediction come from leveraging existing stations and expanding snow measurements to the right places, rather than everywhere.

Snow Survey Procedure Manual

California Department of Water Resources (DWR) | October 1st, 2014

By any measure, California’s agriculture, hydroelectric production, domestic use, riparian health, and recreation all put tremendous demands on the state’s often limi

By any measure, California’s agriculture, hydroelectric production, domestic use, riparian health, and recreation all put tremendous demands on the state’s often limited water resource. Because of California’s Mediterranean climate, little significant precipitation occurs during the summer and autumn months. Accurate assessments of mid-winter precipitation, therefore, are a vital determinant of the state’s available water. During most years, maximum snow water equivalent1 (SWE) in the Sierra Nevada denotes the annual peak of surface water resource. Snow water equivalent is a key index not only for forecasting stream and river flow timing and amount, but for a wide variety of water management decisions targeted days and months into the future. The goal of the snow survey is to obtain an accurate measure of SWE at predetermined locations: snow courses. Snow courses typically have between five and ten measurement points spread out over one or more straight-line transects. Transects can be short, or several hundred meters in length. Some snow courses are coincident with recording or data-transmitting weather instrumentation; many others stand alone in very wild and remote locations. Most snow courses are measured once per month throughout the winter (accumulation) and spring (ablation) seasons, though some may be measured at different frequencies. Several California snow course records go back to the 1920s (some to 1910). Data from the measurements are used to develop—and continuously refine—indices of stream flow for the respective watersheds. A critical first step in characterizing the spatial and temporal distribution of California’s SWE is by measuring its 261 snow courses throughout the state. The aim of this document is to address snow survey procedure and equipment, review data collection, and discuss surveyor safety. While some subjects (avalanche safety, wilderness first aid) are, in their entirety, beyond the scope of this work, industry standards and procedures are presented. The author hopes this document will find utility as an office-based training manual for those new to snow surveys, as well as a field guide and reference to all who conduct snow surveys. As of this writing, many advances in the remote sensing (by ground-, aircraft- and spacecraft-based instrumentation) of snow cover are being developed. No doubt these developments in both procedure and instrumentation will continue to advance. However, there currently exists no practical, automated technique with which to measure SWE at the number of locations and at the point-accuracy of ground-based, manual measurements. Manual measurements remain the standard for calibrating and “ground-truthing” automated instrumentation, and have the advantage that they can be obtained independent of most surface and weather conditions. For the foreseeable future, snow surveys will continue to be a vital part of the water management structure in California.