Southern California is flooding. Watering our lawns makes it worse.
Those sprinklers in Southern California backyards can make the difference between high water and flooding
As I type this sentence into my laptop, the rains continue. Everything for miles around is drenched. Here in Southern California, we were deluged with an unprecedented storm last week, with some parts of the county receiving over three inches of rain in under six hours. And this week’s atmospheric river continues to drench and soak us.
Just about anywhere anytime, that much rain will bring floods. Impervious surfaces, like buildings, homes, streets, and parking lots, make flooding worse. Rain falls on these surfaces and can’t percolate into the ground. It just flows immediately into gutters and streams, which quickly flood and overflow.
In Southern California we unwittingly make this a little bit worse. And our lawns are the culprits. We import water from Northern California and the Colorado River, most of which we use to water the grass and landscaping in our front and back yards. With the first late fall or early winter rains, that extra water in the ground prevents rainwater from soaking in and can make the difference between high water and flooding.
For a project in grad school, I looked at how much additional water winds up in the San Diego River because of lawn watering. During this week’s Noachian rains, I thought now might be a good time to revisit this project. But before we dive into the science and arithmetic, let’s back up a little.
The San Diego urban area receives about nine-and-a-half inches of rain per year, with most of that precipitation falling from November through March. Almost no rain falls during summer. The summer and early fall months are so dry that by the time the winter rains come in November, the soil moisture content for the western part of San Diego County, under normal conditions, is zero.1
In contrast, the lawns in the San Diego River watershed have higher moisture content. The Lawn Watering Guide for California, published by the University of California, suggests regular weekly watering all year long. Homeowners following this guide water their lawns the equivalent of 42 inches of precipitation a year.2 This is in addition to the nine to ten inches of precipitation the lawns naturally receive. While many residents probably vary from this schedule, it’s assumed that they water their lawns enough to raise the moisture content well above normal conditions.
I was interested in the effects this extra moisture had on one river, so I looked at the lawn watering in only one watershed. Once I ran the hydrology formula and generalized it to this watershed, I came up with an additional 3.5 million cubic feet of water runoff from the lawns of suburban San Diego due to a heavy storm dropping two inches of rain. This is probably conservative estimate.
That 3.5 million cubic feet turns out to be over 26 million gallons. This sounds like a lot, but let’s put it in perspective: It is roughly the amount of water in 40 Olympic-sized pools.* If that much more water winds up in big rivers, like the Mississippi or Colorado, no one knows the difference. On the other hand, these calculations were done for the San Diego River, which, at some points close to its mouth with the Pacific, is perhaps about 12 to 15 feet wide. An extra 40 Olympic pools of extra water in a river of that size will make a BIG difference. Further, much of the urban development through Mission Valley, the last five miles of the river where most flooding occurs, lies close to the river and is inundated whenever there is a flood. An additional 26 million gallons of water during a storm will exacerbate any flooding in this area.
Plants native to the area are adapted to the six months of summer drought. They don’t need watering. They are often suggested to homeowners in Southern California as a way to conserve water. As we can see, this type of landscaping can remediate flooding as well.
The sciencey part
The U.S. Department of Agriculture provides a formula for finding how much runoff occurs when it rains. Factors affecting runoff are soil type and the amount of moisture already in the soil. Soils of Lakeside, Santee and El Cajon, the most urbanized areas of the San Diego River watershed, are about 89% sandy loam. Soils of this type have a moderate infiltration rate. The formula to find precipitation runoff is:
Q = (P – 0.2S)2/P + 0.8S
Q = runoff amount
P = precipitation in inches
S = Water retention (This S number is based on the type of soil, in this case, sandy loam, and what is known as the Antecedent Soil Moisture Condition.)
It’s hydrology and kinda complicated. Plugging in the numbers for the formula for a storm of two inches of rain on this type of soil, we get 0.06 inch of runoff under the natural dry conditions and 0.54 inch runoff if the landscaping is watered.
To find the total runoff volume, I estimated the total area of all the lawns in Santee, El Cajon, and Lakeside. The city of El Cajon has 12,936 freestanding houses; Santee has 10,503 freestanding houses.*3 The number of houses for Lakeside was unavailable, as the town is unincorporated. The pattern of development and density in Lakeside is nonetheless similar to Santee. Lakeside has a population of 19,879 and Santee, a population of 54,709. Using these numbers gives us the following ratio of persons to households:
19,879/54,709 = 0.3634
0.3634 x 10,503 = 3,816.4
Thus, we estimate that Lakeside contains 3,816 freestanding houses. Adding these numbers gives the total number of freestanding houses in this urban/suburban portion of the San Diego River watershed:
12,936 +10,503 + 3,816 = 27,255
Data for the size of the lawns for these houses does not exist. At the time I performed this graduate school project, I lived in the San Diego River watershed in a house built in 1960. Using Google Maps and comparing the view of the property where I live and suburbs in El Cajon, Santee, and Lakeside, it looked as though my lawn was a typical size for suburban San Diego. I measured the size of the lawn around my house, arriving at the figure of 3,200 square feet.** Multiplying this figure with the number of houses gave an estimate of the area of all the lawns:
3,200 ft2 x 27,255 = 87,216,000 ft2
We can now compare the difference in total runoff.
87,216,000ft2 x 0.005ft = 436,080ft3
87,216,000ft2 x 0.045ft = 3,924,720ft3
3,924,720ft3 – 436,080ft3 = 3,488,640ft3
I admit that I don’t have a bunch of data to back up the estimates that I made. Even still, it is probably a fair guess that watering our lawns in Southern California raises the floodwaters a bit. What do you think? Any hydrologists out there? Please comment.
*I had not double checked the arithmetic when I figured this comparison, originally writing that 26 million gallons filled four Olympic pools. Thanks to George Liddle for catching the error!
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* El Cajon has 3,977 condominiums; Santee, 3,617 condominiums. While many of these structures are two unit buildings with yards, many more are multiple apartment-style units. Including them in the count would have skewed the results.
** McMansions that fill half-acre lots or larger make up some of the newer developments in Lakeside, so this figure is probably a conservative estimate.
[iv] USGS National Water Information System web site http://waterdata.usgs.gov/usa/nwis/dvstat/?site_no=11023000&por_11023000_1=2207517,00060,1
Miller, Philip C. and Poole, Dennis K. 1979. Patterns of Water Use by Shrubs in Southern California. Forest Science. Vol 25, no. 1, pp 84-98.
Lawn Watering Guide For California. 2001. Pub. 8044, University of California, Agriculture and Natural Resources.
2008 statistics from the San Diego County Assessor’s Office