On-site Retention of Stormwater

November 27, 2004

By Jiordana Stark, Master Gardener

With the rainy season already here, it seems an appropriate time to consider where all of our precious rainwater goes. Down to the creeks, into the ground you say. Or do you hope? How much of an impact do our houses, lawns, driveways, sidewalks, parking lots, and shopping malls make on our stormwater runoff? Like a lot of us, I have always wanted to believe that somehow all of our rainwater safely reaches our creeks, pure as the rain, like a fairytale. Little did I realize that that may only happen in places where man has not yet interfered.

Under natural conditions (undeveloped by humans, and excepting desert and arctic regions) rainfall infiltrates slowly into the ground. Most raindrops do not initially hit bare ground. Instead, they are intercepted by plant materials, eventually making their way over trunks, branches, leaves, and stems, downward to the leaf cover below before moving into the soil and eventually, possibly, groundwater. The relatively small percentage of rainfall, typically 10%, which does become surface runoff usually travels at a slow meandering pace, allowing particles and sediment to settle out along the way, ridding the water of impurities before flowing into rivers and streams. Most of the other 90% may evaporate into the air, be absorbed by plants, or infiltrate to replenish groundwater supplies. The earth’s water continuously circulates from the atmosphere to the land, from there into groundwater and surface water, and then back to the atmosphere.

Human development can drastically alter this scenario. Impervious surfaces such as buildings, roads, parking lots, and sidewalks prevent rain from soaking into the ground. There is less vegetation to intercept, soak up and store water. Less soil surface is available for infiltration. As a result, stormwater runoff over the land surface greatly increases, even during small rainstorms. This alteration to the water cycle can significantly impact ground and surface water. Increased runoff volume and speed can contribute to flooding and erosion and destroy natural habitat.

Because less water may infiltrate into the ground, less groundwater recharge may occur. This may reduce stream base flows, with potentially harmful impact on fish and other aquatic organisms. Impervious surfaces retain heat, which may increase runoff temperatures during warm weather. This may in turn raise the temperature of receiving waters, and again, may negatively impact aquatic life. For example, what used to be 10% runoff has now become 55% runoff, and what used to be 50% infiltration and 40% evaporation have now become 15% infiltration and 30% evaporation.
Stormwater runoff is not typically clean water. In fact, it is often a major source of water pollution.

Various pollutants from human activities are washed off during storms into storm drains or directly into streams, rivers, and lakes. Sediment from construction sites, roadsides, and other disturbed areas; nutrients from over-fertilized lawns; leaky sewers and septic tanks (bacteria); oil and grease from motor vehicles and industrial areas; toxic and synthetic chemicals from pesticide applications and motor vehicles are but a few that can be and often are washed directly into our waterways, untreated.

There are a number of ways that each of us can have an impact on the quality and quantity of rain going down our drains. Traditional stormwater management was to “get rid of” runoff into gutters, drains, and pipes. All untreated water from impervious surfaces rapidly discharged into natural water bodies. Today, it is all about onsite retention and we can all help. Divert roof water to lawns or gardens where it can safely soak in. Collect roof runoff into barrels. Keep pesticides, oil, and other pollutants out of storm drains. Keep pesticide use to a minimum and use the least toxic alternatives. Use fertilizers wisely. Read and follow fertilizer and pesticide label directions. Maintain vegetation on steep slopes to keep soil in place. And most importantly, be aware and care for one of our most precious resources.

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request. Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM, the Arroyo Grande office at 473-7190 on Wednesdays from 9 AM to 1 PM, or the Paso Robles office at 237-3100 on Wednesdays from 9 AM to Noon.  The San Luis Obispo Master Gardener website is at http://groups.ucanr.org/slomg/. Questions can be e-mailed to mgsanluisobispo@ucdavis.edu.