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San Luis Obispo County, California


Groundwater Studies

Paso Robles

The information and data on water supply, water demand, hydrogeology and recharge in the RCS are taken from the following technical studies completed since 2002:

Groundwater Supply and Demand

The Fugro 2002 and 2005 studies established the annual supply of groundwater available basin wide at 97,700 AFY (see perennial yield above). The RCS contains an approximate water demand by sector for several years: 

Groundwater User





Net Agriculture

49,683 afy

56,551 afy

58,680 afy












Small Community





Small Commercial











Groundwater Basin Adjudication

Groundwater basin adjudication occurs when a groundwater user or a class of users files an action asking a court to establish groundwater rights. Generally this occurs when a groundwater basin is in overdraft . In basins where a lawsuit is brought to adjudicate the basin, the groundwater rights of all the overliers and appropriators are determined by the court. The court also decides: 1) who the extractors are; 2) how much groundwater those well owners can extract; and 3) who the Watermaster will be to ensure that the basin is managed in accordance with the court's decree. The Watermaster must report periodically to the court. There are 22 adjudicated groundwater basins in California. The California Department of Water Resources (DWR) has a listing of adjudicated groundwater basins on their website .

In San Luis Obispo County, there are two groundwater basins already in adjudication:

  • Los Osos Groundwater Basin
  • Santa Maria Groundwater Basin

The Santa Maria adjudication is now going on its 12th year and is still in court appeals. Total costs for all parties are over $11 million. In this adjudication, the Nipomo Community Services District along with three other water suppliers have been required by the court to bring in 2500 acre feet of new water supplies. The water suppliers have proposed a $25 million pipeline from Santa Maria in order to meet this court imposed requirement.

As this local case shows, groundwater adjudication takes water decisions away from groundwater users and vests those decisions in the courts.

Groundwater Monitoring

The County maintains a groundwater monitoring network throughout the groundwater basin. These monitoring points consist of public and private wells. Property owners have volunteered their wells to be part of this measurement program. Twice each year, the County measures the distance from the land surface to the water table. Additional wells are needed to cover the large area of the basin. The County has mapped the monitoring “gaps” on a map . If you own property with a well in these areas, please consider volunteering your well for this important effort.

Groundwater Levels

These measurements over the years have produced a map  showing how much groundwater levels have dropped. During the years 1997 to 2009, groundwater levels dropped more than 70 feet in portions of the basin. Falling groundwater levels are an important factor in considering groundwater supply and demand balance.

Groundwater Management Plan

Basin stakeholders such as water providers, property owners and agricultural interest have banded tighter to prepare a Groundwater Management Plan (GMP ). Implementation of the Plan will be a voluntary effort by property owners to stop the continued drop in groundwater levels and to maintain levels where they are now.