The Orange County Water District (OCWD; the District) thinks that a rainy day is wonderful. Unlike most of us who "save for a rainy day," OCWD saves, instead, for the dry days. OCWD is the agency tasked with the capture and placement (recharge) of water from a variety of sources into the Orange County Groundwater Basin, which provides drinking water to 2.4 million people in central and north Orange County.

Rain is the most important source. When it rains, water finds its way underground by a variety of paths such as:

1. Direct percolation through permeable surfaces such as the lawn in your backyard or unlined creek bottoms. This percolation is characterized as incidental recharge.
2. Stormwater captured behind Prado Dam and subsequently diverted into OCWD's network of manmade recharge facilities. This rainwater runoff is characterized as stormflow.

Rainfall-derived recharge to the basin comes in two types – incidental recharge and stormwater recharge.

Incidental Recharge

When rainwater falls within OCWD's area (about 229,000 acres) and the surrounding Santa Ana mountains and foothills, it is estimated that for every inch of rain, roughly 4,500 acre-feet (AF) (about 1.5 billion gallons) of water finds its way into the basin. This sum of rain amounts to about $4.5 million per inch, when compared to the price of water imported from the Colorado River or Northern California. The relationship between rainfall and incidental recharge is complicated and depends on surface soil types, geology and the frequency and distribution of the rain. It can’t be measured directly. However, statistical analysis confirms that the groundwater basin receives about 60,000 AF per year (about 19.5 billion gallons) of incidental recharge for an average rainfall year.

Stormwater Capture from the Santa Ana River and Santiago Creek

Thankfully, when a large storm arrives in Orange County, it usually extends into San Bernardino and Riverside counties as well. Surprisingly, it is that so-called "upper watershed" rainfall that counts most when considering stormwater capture for OCWD.

There are many reasons why the District's recharge operations are at the top of the basin in the cities of Anaheim and Orange. The primary reasons include the favorable coarse-grained geology near the hills and unfavorable geologic layering in the flat areas near the coast. In addition, filling a basin on its upper end has the benefit of allowing for gravity to move recharged water toward many lower areas of pumping south of Anaheim.

The rainfall in the upper watershed makes its way into the Santa Ana River and flows towards the Pacific Ocean. Fortunately for OCWD, the Prado Dam, located at the border of Riverside and Orange counties, stands in its way to control flooding. Prado Dam impounds stormwater and slows it down, allowing for OCWD to divert it and fill the District's 20-plus recharge lakes in Anaheim and Orange. OCWD has invested millions of dollars in land and facilities allowing it to capture most of this water. However, the system has limits and the U.S. Army Corps of Engineers dam operators work under a flood control priority that calls for significant releases from Prado Dam during big storm events. The high-flow releases during storms are frequently too fast for OCWD's system to capture, allowing for water to sweep past its diversions all the way to the Pacific Ocean. The stormwater recharge benefit from an inch of water translates into approximately 3,000 AF (about 1 billion gallons), with a value of approximately $3 million compared to imported water.

So, the next time you hear the weatherman forecast an inch of rain, know that a wonderful gift of about 7,500 AF (2.4 billion gallons) of water for nearly 2.4 million residents is on its way to OCWD's underground basin, and eventually to you. Mother Nature will have saved us $7.5 million that we didn't have to use to import water.