What Happens to Rainwater in Southern California?

The saying, “April showers, brings May flowers,” may get you to start thinking about rainwater and where it goes after it falls from the sky. It is true that the springtime brings new blooms and the recently bare vegetation begin to show signs of life, giving evidence that spring has arrived. While the rain brings needed water to plants, what happens to rainwater and does it eventually become drinking water here is Southern California?


Taking a drive in your neighborhood, you may notice storm drains. In Riverside County, these storm drains are imprinted with text to remind people that, “Only rain belongs down the storm drain.” According the Riverside County Flood Control and Water Conservation District, many people do not realize that storm drains connect to local waterways. Whatever ends up in the storm drains flows directly, without treatment, to rivers, lakes and streams. Some of this water eventually ends up in the ocean. This is important to Southern Californians because a portion of drinking water comes from local water sources, including rainwater. In San Bernardino, the majority of the water supply comes from groundwater supply, which is replenished or “recharged” by snowmelt, water runoff and occasionally imported water.

“San Bernardino County Flood Control utilizes many of the basins at the base of the foothills for flood control. In addition to this primary function, these basins also percolate rain/snow melt run-off, recharging the groundwater.  This is where a majority of groundwater replenishment occurs,” shares Miguel Guerrero, P.E. director of water utility for San Bernardino Municipal Water Department.”


As rain and snow fall to the ground it seeps into the soil and can eventually enter underground aquifers where it becomes groundwater, which can later be pumped out, treated (cleaned) and used for drinking water. It is important to remember that water that enters storm drains can be released into local creeks and rivers, and can become a source for future drinking water.

In Orange County, the Orange County Water District works closely with the U.S. Army Corp of Engineers to maximize rainwater capture at Prado Dam. The Dam, located in the Chino Hills foothills, along the Santa Ana River and is used for flood control, as well as to capture rainwater.


“Water is slowly released from behind the Dam and then travels down the Santa Ana River to 20 recharge basins in Orange County,” states Greg Woodside, executive director of planning and natural resources for Orange County Water District. “Along the route, there are two inflatable dams that are filled to divert water to the recharge basins when the flows are low.”


In the Santa Ana River Watershed located here in Southern California, runoff flows from the San Bernardino Mountains to Huntington Beach. A watershed is an area of land that collects water anytime it rains or snows. With the help of gravity, this water will flow to an area where it is collected in rivers, tributaries and other water bodies and then typically returns to the soil for groundwater supplies.

The Santa Ana River runs through Riverside County. The County, which extends much further east of the Santa Ana River watershed, receives approximately 4,000,000 acre-feet of rainfall in an average year.  One acre-foot of water is equivalent to 325,851 gallons. Water runoff that is not captured is discharged to the ocean through the Santa Margarita River and the Santa Ana River.  Runoff in the County also drains naturally to the Salton Sea and the Colorado River in the County’s desert communities.  In minor portions of the County, stormwater runoff drains into other river systems that drain through Orange County. Some portions of that rainfall infiltrate into the ground, but only a portion of the infiltrated water benefits local groundwater basins.

“Local water agencies and the Riverside County Flood Control District work to artificially capture rainwater runoff in areas where it can be effectively stored in local groundwater reservoirs.  Projects along the San Jacinto River, Coldwater Creek, Bautista Creek and Noble Creek represent just a few examples of these stormwater recharge projects,” says Jason Uhley, acting general manager – chief engineer for Riverside County Flood Control and Water Conservation District. “Stormwater recharge definitely plays an important part in managing local water supplies and reducing reliance on imported water. The District has currently budgeted projects that are expected to capture an additional 10,000 acre-feet of stormwater on an average annual basis.”


Because rain is unpredictable and land requirements for capturing water can make stormwater capture expensive, in comparison to other supply options, stormwater capture alone cannot be depended upon to solve challenges with the availability of imported water. “Conservation, maximizing benefits and use of existing supplies can and must also play a part in providing sustainable supplies for our communities,” continues Uhley.

With storm water capture efforts varying from one region to the next, it is certain that a portion of water runoff can eventually replenish the local groundwater basins and reservoirs in Southern California. Some of the rain and runoff that travels through the storm drains can end up combining with future drinking water supplies. Specific stormwater capture is also a means to supplement water resources. Remember to protect our water resources, you never know if it might end up as drinking water.