The Cuyama Groundwater basin encompasses 230 square miles and supports all land use in the valley15.  The Cuyama River is the principal source of recharge for the basin6 and flows only a short distance below Ozena throughout the year37.   However, during winter storms, flows may reach the State Highway 166 bridge near the town of Cuyama, roughly 25 miles from the river’s headwaters. The river only completes the whole length of its course to Twitchell Dam following rare winter flood events37.

In general, groundwater recharge is roughly proportional to the amount of rainfall.  Thus, the bulk of recharge in the basin occurs during and after storm events37.  Exact figures for recharge of the groundwater basin are unavailable, but studies have estimated average yearly recharge to be anywhere between 8,000 acre-feet15 and 13,000 acre-feet33.  These recharge estimates are based on the amount and distribution of rain, gauging station information from the Cuyama River, and other factors affecting infiltration, such as type of vegetation and soil moisture content.  Unfortunately, much of this information is limited and incomplete.

The roughly 23,000 acres of irrigated agriculture and ranching rely exclusively on groundwater resources.  Significant groundwater withdrawal for agriculture began in 1938 and has progressively increased since that time.  Over 95% of groundwater use is applied towards agriculture15.

The Cuyama Valley faces serious hydrologic impacts due to groundwater supply deficits, high evapotranspiration rates, and low annual rainfall.  The USGS has been monitoring groundwater wells in the Cuyama Valley since 1938.  To date, 371 groundwater wells have been monitored in the region, but the majority of these wells are no longer monitored due to well decommissioning and degeneration.  Currently, seventeen wells remain actively monitored by the USGS.  These monitoring sites are located in the central portion of the valley, where most agricultural pumping takes place.  

Groundwater levels have declined over 300 feet over the past six decades in the southeastern section of the basin.  The variation in depth to groundwater across the agricultural zone is primarily attributed to the complexities of the sub-surface geology (described in detail in the Geology section of this website) and not due to localized differences in withdrawal.  Faults near the intersection of Branch Canyon Creek and the Cuyama River create an upwelling of groundwater in a localized region.  Natural springs were historically found in the upwelling area, but groundwater depths are no longer shallow enough to foster spring formation.  Groundwater level decline near Ventucopa is less severe since the area of permeable soil deposits is smaller relative to the rest of the basin; therefore, groundwater recharge seems to be more significant.  As a result, groundwater levels are more variable and do not experience significant trends of decline.

In 1980, the Cuyama groundwater basin was identified by the California Department of Water Resources as one of the eleven basins in “critical condition of overdraft,” which is defined as a “continuation of present water management practices [that] would probably result in significant adverse overdraft-related environmental, social or economic impact”7.  Although the groundwater basin is experiencing serious hydrologic impacts due to unsustainable groundwater pumping practices, a groundwater management plan for the basin does not exist. Sections 10750-10756 of the California Water Code, commonly referred to as AB 3030, “provide a systematic procedure for an existing local agency to develop a groundwater management plan”5.  Groundwater management may be developed by a local agency following a local vote, or mandated at the county level7.  Since the Cuyama groundwater basin lies within four counties (Kern, San Luis Obispo, Santa Barbara and Ventura) any future efforts for a county groundwater management plan will likely be difficult.

A 1992 study prepared by Santa Barbara County contains the most current water supply and demand figures for the Cuyama groundwater basin2.  The County of Santa Barbara and the USGS are launching a study entitled Geohydrology and Water Availability of the Cuyama Valley, California, which is expected to be completed by the year 2012.  Its main objectives are to “refine the geohydrologic framework of the Valley, quantify the hydrologic budget of the region, and develop hydrologic modeling tools to evaluate and manage the groundwater resources”17.  A more detailed description of the basin and its storage will allow for a more accurate evaluation of groundwater resources.

There have been several attempts to develop an overall groundwater budget for the Cuyama Valley, but much of the information required to produce an accurate groundwater budget is limited and incomplete.  Data from existing sources and previous estimates were compiled in attempt to produce a more accurate estimate of the current groundwater budget. Refer to the final report for more details on the data and calculations used to develop this groundwater budget.

The following factors were considered in the development of the groundwater budget for a typical year:

Groundwater Underflow
This is the downstream flow of groundwater.  Calculating an exact value for underflow was beyond the technical scope of this project.  Therefore, the most recent and thorough estimate of 500 acre-feet per year (AFY) of underflow created in a 1966 survey was utilized33.

Baseflow
This is the amount of groundwater that seeps into a stream channel.  As of 1970, there have been no observations of water table intersecting the surface33.  Therefore the baseflow for the Cuyama Valley is assumed to be zero.

Floodflow
This is the amount of stream discharge during a flood.  Although a USGS stream gauging station is located near the town of New Cuyama, flow has not been monitored since 1972, rendering the calculation of a current estimate impossible.  Therefore, the 9,000 AFY floodflow estimate made by Singer and Swarzenski in 1970 was used33.

Vegetative Use
This is the magnitude of water loss due to evapotranspiration by native vegetation that is sustained by the groundwater aquifer.  Vegetative use in Cuyama was first calculated by Upson and Worts in 195137, but the area of riparian vegetation that relies on groundwater has significantly declined; therefore an updated estimation was made.  An aerial photograph from the 2005 National Agriculture Imagery Program (NAIP) was used to quantify and classify the amount of vegetation along the portion of the river where water-loving vegetation was historically found.  Vegetated areas were measured and classified into three categories: swamp (tulles, cattails, and grass); dense trees, grass, and brush; and sparse grass, brush, and a few trees.  Because field studies on evapotranspiration in the Cuyama Valley were not available, per-acre coefficients calibrated for the upper Salinas Valley were applied37.  The estimated total water consumed by current natural vegetation was calculated to be 1,440 AFY.

Net Agricultural Pumpage
This is the amount of water used for irrigation, which was determined by multiplying crop acreages with specific water requirements.  We assumed pistachios, vineyards, and orchards use drip irrigation systems; therefore a return flow percentage was not applied for those crops.  In some years soils are flushed out to remove salt build up but this was not taken into account in the overall groundwater budget for a given year.  For the remaining crops a 30% return flow was applied, which was based on California Department of Water Resources field surveys on applied water rates and irrigation efficiency rates for agriculture in Santa Barbara County30. 

Net Municipal-Industrial Use
Municipal and industrial water use comprise a relatively small amount of water; therefore, they were calculated as a collective factor in the overall groundwater budget.  The gross per capita demand for the Cuyama Valley was estimated at 215 gallons a day per person.  This per capita estimate, applied to the total population in the valley (1,350), generates municipal and industrial water use.  A 40% return flow (Santa Barbara County Water Agency & Boyle Engineering Corporation 2003) was then applied to obtain net water use.  The net amount was rounded up to 200 AFY for the groundwater budget calculation.

Total Runoff
Precipitation runoff from the mountains sustains flow in the Cuyama River.  Because reliable data on precipitation runoff within the study area is not available, a calculation made in 1970 was used.  The long-term average precipitation runoff made available for groundwater recharge was estimated at 22,000 acre-feet per year33.  To account for any losses, a value of 21,000 acre-feet per year was used.

Direct recharge from precipitation on the valley floor was not taken into account in this water balance since the value is relatively small and would not have a significant impact on the overall water budget calculation.

Based on these assumptions and calculations, we estimate the current groundwater budget of the Cuyama Valley to be in a deficit of 30,532 acre-feet per year. The trends of critical groundwater level decline across the valley seem to support such a number. This deficit exceeds the current estimated rate of recharge by almost a factor of three.