AQUA TERRA Consultants
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Ventura County, CA Flood Control, Watershed Modeling, and Watershed Management Studies

Client: Ventura County Watershed Protection District, Public Works Agency, Ventura, CA

The Ventura County Watershed Protection District (VCWPD), as owner and operator of the receiving watercourses throughout the County, is responsible for operation and maintenance of flood control facilities and, under the county-wide NPDES permit, administration of the stormwater quality program. To improve and expand its technical procedures, VCWPD is exploring the use of continuous watershed simulation as a tool for fulfilling its flood control, water resource management, and water quality management obligations. Since June 2000 AQUA TERRA has been assisting the Ventura County Watershed Protection District in its efforts to improve and expand its watershed assessment procedures, through the use of continuous watershed simulation as a tool for fulfilling its water resource management and water quality management obligations. This assistance has occurred through a series of individual projects described below

Arroyo Simi Pilot Study

This project was a pilot study to evaluate the use of the U.S. EPA Hydrologic Simulation Program -FORTRAN (HSPF) as a management tool for comprehensive watershed assessment within the climatic, physiographic, and topographic conditions of Ventura County.

In the initial phase of the pilot study, HSPF was set up on the Arroyo Simi watershed, and a preliminary hydrology calibration and validation was performed on an eleven-year period of observed flows at two sites within the watershed. The model results were presented in a Final Report in 2001, and a presentation was made to Ventura County FCD, along with a series of recommendations for future refinement of the Arroyo Simi model calibration.

HSPF-VCRAT Comparison Study

In a follow-up effort to the Arroyo Simi model, the ‘preliminary’ calibration of the Arroyo Simi was used to compare HSPF model predictions for flood peaks and volumes for various flood frequencies, to those generated by the VCRAT method, a modified rational formula approach previously used by VCWPD. Given that the HSPF model calibration was not complete, the objective was not to assess how close the two procedures agree, but rather to evaluate and compare the types of flood assessment information generated by each approach. The flood frequency comparisons of HSPF, VCRAT and the historic data show reasonable agreement (see probability plot on following page). We also explored and demonstrated the additional types of information available from the HSPF watershed model that would further assist VCWPD in its entire range of flood control, watershed and water quality management activities and responsibilities, including water quality considerations.

USEP and Hydromodification

To address water quality issues, VCWPD has developed a Stormwater Quality Urban Impact Mitigation Plan (SQUIMP) at the request of the Los Angeles Regional Water Quality Control Board. The SQUIMP addresses storm water pollution from new development and redevelopment in the private sector. One listed requirement of the SQUIMP (as defined in NPDES Permit No CAS004002) is that ““The discharger shall control the post-development peak storm water runoff discharge rates to maintain or reduce pre-development downstream erosion, and to protect stream habitat.”

To respond to this requirement, VCWPD developed, in conjunction with the LARWQCB, the Urban Stream Erosion Prevention Model (USEP) work plan to address the issues of urbanization impacts on stream erosion and habitat alteration. This effort extended the initial HSPF model setup on the Arroyo Simi to include the specific habitat sites monitored in USEP, finalized the preliminary hydrology calibration/validation, and used the final model to assess stream erosive conditions under current, natural and alternative scenarios for mitigation of urbanization impacts. The objective was to develop and demonstrate procedures within the Arroyo Simi watershed that might be applicable throughout the County. The approach was to analyze the model-generated information on flow and bed shear stress to determine how often (i.e., percent of time) channel scour/erosion conditions occur. Specifically, the calculated shear stress timeseries values were analyzed to determine how often the values exceeded critical values for channel scour (tau ratios of 1.5 and 2.5, see figure); this analysis established the potential for channel scour conditions at any point within the watershed.

Both current/baseline and scenario results were analyzed in a similar fashion, so that urbanization impacts could be identified and evaluated. The methodology assessed how often (i.e. what percent of time) scour/deposition conditions occurred under each scenario, with the differences from natural conditions representing the urbanization impacts, and the differences for detention scenarios representing mitigation impacts. Duration (cumulative frequency) curves were used to analyze results for flow rate, velocity, and shear stress; shear stress values were compared to ‘critical shear values’ for scour and deposition to assess the frequency, duration, and ‘percent of time’ for scour/deposition conditions. Also, flood statistics (e.g. 100-year, 50-year, 25-year, 10-year peak flows) were generated for each scenario using accepted standard statistical procedures for flood analysis (e.g. Log Pearson III), to assess scenario impacts on flood events along with changes in the time duration of erosive conditions.

In January 2005 the California Regional Water Quality Control Board (Los Angeles Region), recognizing the value of the approach, adopted Resolution No. 2005-002 to provide a regional policy for hydromodification analyses:

....The Regional Board and local agencies have undertaken or sponsored hydro- modification field assessments and studies to develop peak flow design criteria to minimize or eliminate adverse impacts form urbanization for watercourses. These studies include the ‘Urbanization and Channel Stability Assessment in the Arroyo Simi Watershed of Ventura County, CA…. The results will be used to develop objective criteria to reduce or eliminate the adverse impacts of hydromodification in the Los Angeles Region from new development and redevelopment.

Calleguas Creek Watershed Study

The Arroyo Simi work efforts were extended downstream in this study to develop a comprehensive watershed hydrologic model of the entire Calleguas Creek Watershed, from its origins in the Arroyo Simi to its discharge to Mugu Lagoon and the Pacific Ocean. This effort was jointly funded by the Calleguas Creek Watershed Management Plan and the Ventura County Watershed Protection District (VCWPD). HSPF was set up and calibrated to available flow records for recent hydrologic conditions, and customized to include consideration of localized groundwater pumping impacts and lawn/landscape and agricultural irrigation practices on surface water flow levels.

The Calleguas Creek Watershed covers an area of 340 sq. mi. and is surrounded to the north, east, and south by largely undeveloped hills and canyons, while the main stem flows through flat valleys consisting of a mixture of urban and agricultural land. The goal was to develop a watershed-wide hydrologic assessment tool. The watershed is subject to flooding and erosion, resulting in sediment deposition downstream in Mugu Lagoon. Topographic, soils, land use, and agricultural cropping information were used to develop the model segmentation and input, and detailed streamflow data were selected to allow calibration over a 9 year period (WY 1994 – WY 2002) and validation over a separate 6 year period (WY 1988 – WY1993). Both quantitative and qualitative comparisons were performed to support the model performance evaluation effort.

Santa Clara River Study and HSPF Model

Santa Clara River Watershed Location, Municipalities and Major Waterbodies

The objective of this study was to develop a comprehensive watershed hydrologic model of the Santa Clara River Watershed for use as a tool for watershed planning, resource assessment, and ultimately, water quality management purposes. The study was a joint effort of the Ventura County Watershed Protection District (VCWPD), Los Angeles County Department of Public Works (LACDPW), and U.S. Army Corp of Engineers Los Angeles District. The modeling package selected for the study was HSPF.

Two previous studies provided the foundation for this effort: a pilot study of the Arroyo Simi Watershed in the headwaters of Calleguas Creek, funded by VCWPD; and the ensuing Calleguas Creek Watershed study, jointly funded by VCWPD and the Calleguas Creek Watershed Management Plan. In those studies, HSPF was set up and calibrated to available flow records for recent hydrologic conditions, and customized to include consideration of localized groundwater pumping impacts and lawn/landscape irrigation practices on surface water flow levels. The Calleguas model also included consideration of diversions and deep groundwater recharge losses through the streambed.

The Santa Clara River main stem flows east-to-west from the San Gabriel Mountains of central Los Angeles County to its mouth at the Pacific Ocean near the towns of Ventura and Oxnard. After descending from the mountainous headwaters, the river passes through the northern Los Angeles suburb of Santa Clarita, across the Los Angeles/Ventura County line, then transitions to the mostly agricultural valley with a series of small towns, before discharging to the ocean.

The major tributaries flow from the north and include Bouquet Canyon, San Francisquito Canyon, Castaic, Piru, and Sespe Creeks. There are four large reservoirs within the watershed. Although the Santa Clara River watershed remains primarily in a natural physical state, the flow regime within the watershed is highly engineered to optimize water delivery schedules and aquifer recharge. Bouquet Reservoir is operated by the Los Angeles City Department of Water and Power and provides safety storage downstream from the San Andreas Fault for the water transported through the Los Angeles Aqueduct, as well as water from peak hydroelectric power generation.

Pyramid and Castaic Reservoirs are part of the California State Water Project (SWP) system, and are operated by the California Department of Water Resources (CADWR). Pyramid is located on Piru Creek while Castaic is located on Castaic Creek, but the two are hydraulically connected. SWP water is sent through a pPower plant into Pyramid Lake, through a tunnel into the Castaic Power plant, and then into Castaic Lake, terminus of the SWP. Piru Reservoir is run by the United Water Conservation District and is located on Piru Creek, below Pyramid. UWCD’s primary operational goals are groundwater recharge, recreation, and power generation.

The watershed drainage area is about 1646 square miles, 90% of which consists of mountains ranging up to 8800 feet. Los Padres and Angeles National Forests, home to most of the major northern tributaries, comprise 47% of the watershed area. The remaining 10% of the drainage area lies on the valley floor and coastal plain with the main stem of the Santa Clara River. The watershed is surrounded to the north, east, and south by largely undeveloped hills and canyons. The watershed is subject to severe flooding and erosion.

One of the goals of this project was to provide the capability to perform long-term simulations in order to assess the impacts of alternative conditions – Baseline, Natural (pre-development) Condition, Alternative Future Conditions (e.g., land use, facilities, reservoir operations) -- on flood frequencies. A long-term data base of 46 years of model input data (precipitation, evaporation, diversions, POTWs, etc.) with the most critical being precipitation and evaporation was developed. The model was used to generate storm event hydrographs for selected return intervals with synthetic input rainfall hyetographs for the corresponding rainfall return period developed from available rain gage data. This was performed in association with VCWPD and LACDPW for selected tributary and mainstem sites.

Land Use & Segmentation

The model was segmented into 209 subwatersheds and 192 stream reaches, based on a number of factors, including:

  • locations of the rain gages,
  • Thiessen network boundaries,
  • isohyetal contours,
  • drainage boundaries
  • slope and elevation,
  • stream gage locations
  • locations of debris basins,
  • TMDL impaired waters boundaries.

Each segment was further subdivided into individual model segments (pervious and impervious land segments) based on the following land use/cover categories:

  • forest/woodland
  • shrubland
  • open/grass
  • agriculture
  • low density residential
  • medium density residential
  • high density residential
  • commercial/ industrial
  • impervious (directly connected)


The reservoirs were modeled by simulating the natural runoff inputs, and defining daily interbasin transfers (i.e., imports) and reservoir releases based on observed data compiled by CADWR and USGS. Most of the imported water is ultimately used for water supply, and does not flow downstream. During calibration, the natural inflows were calibrated to maintain observed storages or water levels in the lakes. For the natural scenario, the reservoirs were removed and replaced with free-flowing reaches. For the Baseline Scenario – prior to the compilation of transfers and outflows by CADWR – a reservoir operation rule was used to compute the outflows based on simulated natural inflows.

Rainfall and Evaporation Data

The rainfall data was developed from an initial raw database of more than 100 stations operated by Ventura and Los Angeles counties and the National Weather Service. The rainfall data development and correction task required a significant effort due to missing and incorrect data, and the need to disaggregate daily data to hourly. The final database consists of 50 stations of hourly or 15-minute data. Ultimately, 35 of these were selected for modeling. These stations were assigned to model segments using Thiessen analysis and isohyetal information.

Evaporation data in the form of PET were developed from 27 stations in and near the watershed. The available stations were primarily monthly totals, which were distributed to daily values using the long term daily station at Lake Cachuma; they were further disaggregated to hourly values with a distribution function based on the pattern of daylight at the latitude of the watershed. Missing data were filled from nearby stations, and the database was extended back to 1956 for long term runs. The final PET dataset consisted of 12 stations distributed across the watershed as well as possible, with some gaps in the far eastern portion of Los Angeles County and the central portion of Ventura County.


Due to the high elevations in the upper watershed, snow accumulation and melt was modeled for several high elevation model segments in the upper Sespe and Piru creek watersheds. In these areas, significant snowfall, in the range of 5 to 10 inches per year often occurs, and may remain on the ground for up to two months. Since the model uses a degree-day snow method to simulate snow, hourly air temperature time series were required. A database of air temperature was developed, using observed data and adjustments for elevation, where necessary.


The Santa Clara River watershed includes significant areas of both agricultural and developed residential land, so the model considers both urban and agricultural irrigation applications for a complete water balance accounting. The approach to include irrigation applications was based on the assumption that amounts were applied to satisfy monthly crop and lawn evapotranspiration (ET) demands that exceed available rainfall. ET demands were computed based on the landscape coefficient method described the CADWR manual Water Use Classifications of Landscape Species. Daily reference ET (ETo) is given by month for each climate zone in the state. The Santa Clara River watershed is spread across a range of ETo zones, transitioning from the South Coast Marine to inland desert climates. Weighted crop coefficients were applied, and the amounts were adjusted to account for inefficiencies in application. The final daily application amounts in the various watershed areas were adjusted to account for the rainfall totals for each day in that area of the watershed.

Diversions and Point Sources

Several diversions and point sources were included in the model. Six diversions, including two sizable ones were defined; they are primarily used for groundwater recharge and agricultural irrigation. Most irrigation is derived from deep groundwater, and was not explicitly accounted for as diversions. Of the nine WWTP’s in the watershed, only one discharges directly to the river. The remaining facilities discharge to percolation ponds, which can overflow in wet weather; however, they were not explicitly included.


The HSPF model represents groundwater as both a shallow, active groundwater storage that can contribute directly to streamflow (baseflow), and a deep, inactive storage that represents deep aquifers that do not contribute to streamflow. The flux into the deep, inactive storage is represented as deep recharge. Both of these groundwater components are evaluated as part of the model calibration process and the water balance assessment (see Section 4.0 for calibration discussion). Thus, any process that involves a transfer between surface water and active or deep, inactive groundwater must be considered by the model.

In the SCR watershed, groundwater provides much of the water for human use through pumping from an extensive network of alluvial aquifers and the Saugus Formation in the river valley, thereby transforming deep, inactive groundwater into surface water. Most of the extracted groundwater (> 90%) is used for agricultural irrigation. In this model, it was assumed that most agricultural irrigation water was derived from deep aquifers and/or local channel losses recharging the alluvial aquifers. The model explicitly includes deep recharge and irrigation applications. These quantities were compared to available annual estimates for these fluxes in order to assess the accuracy of the modeled water balance.

Another groundwater-surface water interaction issue is the presence of recharge and discharge zones along the SCR channel. Areas of rising groundwater and recharge are observed at a number of locations in the watershed as shown in the figure. The significant literature documenting recharge and discharge zones of the Santa Clara River was used as the basis for modeling channel gains and losses in various reaches in the model.

Calibration and Validation

Calibration and validation periods were based on available meteorologic, Streamflow, and land use data. The data support model simulations spanning WY 1987 through 2005, thus the calibration and validation periods were: WY 1997-2005 for calibration, and WY 1987-1996 for validation. The calibration period was selected as the later time span because it covers a wider range of wet and dry years, included the most extensive coverage for POTW and diversion data, and provides a starting point for future conditions. Land use coverages were available for 2001 and 1993, and so these coverages were used for the calibration and validation periods, respectively.

The approach to calibration and validation of the watershed initially focused on the relatively natural, undeveloped areas in the upper portions of the watershed in both counties in order to provide the best estimate of hydrologic parameters without the complicating issues of irrigation, water regulations, importations, and channel losses. The major calibration/validation sites and watersheds are shown below. The Sespe Creek, Piru Creek, Santa Paula Creek, and Upper Santa Clara River (near Lang) were the initial sites. The next round of calibration sites included moving further downstream to the Highway 99 gage, and then the Hopper and Pole creek tributaries in Ventura County. Modeling of the major reservoirs was performed next as they provide major contributions to the SCR mainstem. Finally, the calibration proceeded to the SCR mainstem sites at the County Line and the SCR outlet gages. Calibration of these downstream and mainstem areas also included consideration and adjustment of channel losses and surface-groundwater interactions, in addition to irrigation applications in the major developed portions of the watershed.

Model parameterization was initially derived from the prior Calleguas and Arroyo Simi HSPF applications, with subsequent adjustments as part of the calibration process. Parameter adjustments focused primarily on lower zone storage and infiltration parameters, as a function of soils, land use, and slope conditions, to obtain reasonable overall water balances. These values were assigned based on spatially varying soil conditions across the watershed. C and D soils have lower storages and infiltration values than A and B soils. Adjustments to the interflow and baseflow parameters were made to improve agreements in the flow duration curves, daily time series, and storm events. Urban parameters were set to generate more surface runoff than the natural land uses. The groundwater recession and groundwater ET-related parameters are usually watershed specific as they are a function of local GW and riparian conditions; thus they were calibrated to local conditions in each watershed. Upland areas, generally with higher slopes, were set to generate more runoff than the valley areas.

Simulations of Baseline and Natural Conditions and Flood Analyses

The long term (45 year) database of meteorologic data and other inputs was used to make long term simulations of the baseline/existing conditions and natural conditions. For the existing run, the most recent land use dataset was used, and the reservoirs for the time prior to the validation period was modeled using a set of operation rules based on knowledge of the system. In the natural run, urban and agricultural land was converted to the natural categories, and the reservoirs, irrigation, point sources and diversions were removed from the model.

The calibrated model was also used as the basis for generating design storm peaks and hydrographs for use in a hydraulic modeling study performed by the clients. The approach involved identifying a storm where saturation levels were very high across the watershed and then applying balanced design storm hyetographs for the 100-yr storm for each rain gage used in the HSPF model. The gaged tributaries with long-term records were used as calibration points in the modeling. The calibration was done by adjusting the rainfall factors applied to the rain data for each subarea and associated reach at the calibration points to establish corresponding rainfall factors that could then be applied to ungaged tributaries. The model was then run with the appropriate rainfall distributions at 5-min timesteps for the storm of interest to provide 100-year design storm peaks at the ungaged tributaries. The 100-year peaks were converted to other return intervals of interest by using multipliers developed from flow frequency analyses of long-term Ventura County and Los Angeles County stream gages.

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