You are a rain barrel stud, but you're beginning to feel a slight unease. Your sense of equanimity is a little disturbed because you've been reading this blog. You figured out that tap water costs next to nothing, so the $500 you spent on rain barrels is beginning to chafe in your tender areas - not only is there no benefit during the majority of the year when we have no rain in our Mediterranean climate, but the barrels aren't the most aesthetic or space-efficient addition to your yard. You want to do the right thing for the environment and a casual survey of your yard during the last rain storm suggested that next to the quantity of rain running off your driveway and out to the street, your rain barrels were looking a little...paltry.
Many homeowners in suburbia already have an aesthetic* and affordable solution to keep rainwater on their property without the use of rain barrels or even new and exciting green ideas like planting with California native plants. It's called lawn. Though fast falling from fashion in many circles, your lawn is a great way to keep water on your property. This is a slightly heretical thought among the green crowd, but for most homeowners in southern California today, their lawn is the best choice to keep water on their property. This is a pragmatic observation based on our collective infatuation with lawn, our unwillingness to give it up, and the fact that it may take up the majority of a home's landscape. If you're offended by the idea of a lawn doing something useful, then just assume I misspelled garden as you read along.**
The bottom line is that whether you have a lawn or not, the same basic principles apply and they revolve around knowing your soil and keeping it healthy, because healthy soil infiltrates water and stores it for future use far more effectively than rain barrels. How much more effectively? I'm glad you asked. For detailed information I refer you back to my previous post called Soils primer, which I will quote here.
Suppose you have a lawn that's 40 feet x 15 feet. This is a modest sized area but it is also a convenient 600 square feet. A previous post shows that 0.15" of rain falling on 600 sq ft of roof will exactly fill a 55 gallon rain barrel, so we're making an apples to apples comparison of lawn versus rain barrel: If that 600 square foot lawn can store 0.15" of rain, then it will be doing as well as a rain barrel collecting water from a 600 square foot roof.
Since you've been reading this blog and following along at home, let's imagine that you've attended to your soil health by tilling it if compacted, adding 5% organic matter by weight to the first 12" and topping bare soil with a mulch. Don't worry about topping your lawn with mulch, the grass will act like one, with the added benefit that turf grass has a multitude of fine roots that help infiltrate water. Water retention and water availabilities will then go up by a factor of 1.37 and infiltration rate will go up by a factor of 3.0 from the minimum values indicated in the soil texture table below based on what I wrote in Soils primer. We'll be assuming these soil improvement / conditioning factors going forward.
(Remember that available water is the measure of water that plants have available to their roots, and the wilting point of the plants is when available water goes to zero. Total water is just that - all the water that can be stored, but a large part of that water is not accessible to plant roots.)
For the sake of argument, let's say we have a one foot depth of soil to work with. This might be close to true if your top foot of soil is well-conditioned, but you have compaction starting one foot down. However, for most people this will not be their case - their soil will drain their water table, at least by a tenuous connection. Engineers call these sorts of assumptions worst case bounding assumptions - meaning that you can expect your soils performance to be at least as good as that predicted using these bounding assumptions.
If you water your lawn during our dry season (many people do not) then you won't start with perfectly dry soil before a rainstorm: You'll start with some fraction of your available water since you will maintain your watering schedule up until the first rain so that your lawn does not wilt or brown. We'll choose a situation where 50% of available water has been depleted and is therefore available to refill in a rainstorm. At 50% available water in sandy, well-conditioned, soil you'll have 0.5*1.37*(1.2-0.9) = ~0.20 inches per foot of soil water remaining and ~0.20 inches per foot of soil water holding capacity. What was the amount of water I could hold in a ran barrel from a 600 square foot roof? Oh yeah, 0.15 inches. Certainly 0.20 inches is better than any old rain barrel! In fact it's 33% better in this example.
Under these assumptions the advantage to storing water in soil only gets better the more clay-like the soil becomes. However, the infiltration rate - the rate at which surface water can work its way into the soil - goes down dramatically as soil becomes more clay-like. If you have clay soil and the improvements noted above you can expect a worst case infiltration rate of 3*0.01 = 0.03" inches per hour. That's a low rate that will lead to runnoff. Many of our storms easily exceed that.*** So in the case of high rates of rainfall the rain barrel will be superior - at least until the first 0.15" has been captured. Nonetheless, if I had a choice to spend $500 on rain barrels**** or $500 on my lawn, the more effective and aesthetic expenditure would appear to be on the lawn, though you will have to maintain the soils condition.
Soils with low infiltration rates contribute to runoff more easily than those with higher infiltration rates and one way to mitigate that is to create a shallow depression or a berm that will trap runoff and prevent it from..running off. This modification can be part of your newly justifiable $500 lawn improvement program.
*aesthetic - remember, it's in the eye of the beholder.
**There's plenty of information on converting lawn to native plants out there already, so if he keeps reading our "rain barrel stud" may yet turn into a "naturals birder".
***rainfall rate - his is why we like slow steady rainfall on our fire-crisped hills: no mulch or growth makes infiltration even more difficult than usual.
****Susan Carpenter of the LA Times spent that much for two rain barrels and received a third for free from LA County.