Cuyama Valley, California: A Groundwater Basin Being Pumped Dry

In California's Cuyama Valley, in Santa Barbara County, groundwater is the only source of water for homes, farms and towns — and it is running down. Irrigated land grew from 13% of the valley in 1949 to 35%, and farm demand has outstripped natural recharge. With the Santa Barbara County Water Agency, the USGS studied how much water is left and what could keep it.

The overdraft

Cumulative change in groundwater storage, 1950–2010, by subregion, with wet and dry periods. USGS.

• Storage lost: about 2.1 million acre-feet since about 1949 — on average, enough to supply every Californian for 4 months. About 72% came from the Main zone.
• Water levels: down more than 300 feet in places since the 1940s, falling up to 7 feet a year in parts of the south Main zone and 1–2 feet a year across most of the basin.
• Irrigation uses about twice the average annual recharge.
• Subsidence: up to 0.2 foot of permanent sinking since 2000, and nearly 1.6 feet simulated historically around New Cuyama; another foot is projected if demand continues.

Wet years once refilled the basin, but more dry and average years and rising use have left deep unsaturated zones in the alluvial aquifers.

Three separate basins

The groundwater basin's three subregions, faults, and the model's line of section. USGS.

Faults divide the valley into the Main zone, the Sierra Madre Foothills and the Ventucopa Uplands. Little water crosses between them, so each responds on its own: heavy overdraft in the Main zone, some in the Foothills, and no permanent depletion in the Ventucopa Uplands, where water levels rise and fall with wet and dry years.

The model

The Cuyama Valley Hydrologic Model simulates surface water and groundwater across the valley for 61 water years (1950–2010), including pumping that was never metered. It was checked against observed water levels and subsidence.

• Engine: MODFLOW-OWHM, the One Water Hydrologic Model, tracks water across the land, streams and aquifers; a Basin Characteristics Model estimates recharge and runoff from the surrounding watersheds.
• Grid: 6,817 cells of 15.4 acres — about a typical land parcel — in 3 layers up to 4,710 feet thick.
• Geology: logs from 65 oil and gas wells and 153 water wells built a 3-D model of the aquifers.

The aquifer system along line A–A′: recent alluvium (up to about 630 feet thick), older alluvium (up to 1,350 feet) and the Morales Formation (up to 4,710 feet), with 1966 and 2008 water levels. Vertical exaggeration 26 times. USGS.

The 3-D geologic framework, built from well logs, cross sections and geologic maps. USGS.

Three new multiple-well monitoring sites, new stream gages on the Cuyama River and Santa Barbara Creek, and ground-deformation data now form a monitoring network for the valley.

Water quality

The groundwater is naturally poor: high in dissolved solids and sulfate, and very old, a sign of little recharge. It may be worsening slightly as nitrates are added and falling water levels mobilize sulfate, arsenic and chromium. The Ventucopa area, with local recharge, has somewhat better water.

Can the basin be sustained?

Natural recharge follows cycles of roughly 27, 22 and 13.5 years, which could frame management periods. In 2014 there was no management plan, no metering and no local definition of sustainability. A basin plan built on the model, and on those climate cycles, could reduce the deficit and extend the basin's life.

Sources

Based on "Cuyama Valley, California hydrologic study: an assessment of water availability," U.S. Geological Survey Fact Sheet 2014–3075, prepared with the Santa Barbara County Water Agency; a work of the United States government in the public domain. Figures and the study's summary points are taken from the fact sheet's PDF; the full model is described by Hanson and others (2014). The fact sheet gives the simulation period both as 61 water years, 1950–2010, and as 1949–2010.