- Author: Greg Overduin
Last week, the UC Cooperative Extension, the Department of Water Resources, local water districts, Ewing Irrigation and many other partners held the "Get Ahead or Get Parched" workshop at the Irvine Civic Center. The purpose was to familiarize landscape professionals in Orange County and the surrounding areas with best practices in conserving water, especially the water lost through preventable run-off.
Jim Borneman, from Ewing Irrigation, presented for the lion's share of the day. He outlined a simple, easy-to-follow approach to using less water while irrigating professional landscapes. The foundation of the concept was simple: match the precipitation rate of your irrigation method to the infiltration rate of the soil. If your precipitation rate is higher than the infiltration rate, you get runoff. Runoff means lost water and lost profits - almost literally money down the drain.
These sprinkler precipitation rates, by the way, are easily accessible from the catalog supplied by the manufacturer. It will show the precip in inches per hour based off a certain GPM, radius, PSI and layout. As long as your irrigation meets the recommended configuration outlined by the manufacturer, you can easily find your precip rate and apply it to your soil type. This is an excellent reason to carefully check and maintain your irrigation system to keep it up to spec. If your PSI or GPM is too low or too high, you're losing money.
Jim gave an example of a clay soil with an infiltration rate of 0.1" per hour and a spray sprinkler with a precipitation rate of 1.6" per hour. Based on these numbers, the maximum runtime before runoff occurs is four minutes. It's not unusual to walk by sites in clay soil where they keep their sprinklers on for ten, fifteen or twenty minutes, with water running off into the gutter.
Obviously, you can't keep your plants alive by just watering them for four minutes a day. Every plant takes up a certain amount of water during the day to provide for its biological functions. The amount of water used is determined by the species as well as the weather and soil conditions. These environmental factors (wind, temperature, humidity, solar radiation, etc.) are used to determine the evapotranspiration, or "ET". The important part is that irrigators use the ET multiplied by a "crop coefficient" (K) to give them the amount of water that a particular species has used on a particular day.
The ET data is freely available online thanks to the CIMIS project from the California Department of Water Resources. They placed 145 automated weather stations all over the state that measure the surrounding environmental factors and spit out the current ET at an hourly rate. We have one of these stations here at the South Coast REC that is widely used across the county. If you don't happen to live close to a research station or agricultural college, you can make good assumptions based off the station closest to you. But for this example, we'll use the Irvine station.
I ran a report asking for a "Monthly Avg Eto" for the Irvine station (#75) and received this data. This is the average ET for each month based off the previous years (since 1987, I believe, when the station came online). Obviously, in a dry year like we're in, the average will be higher than this, but for our purposes we'll use this.
In landscaping, the traditional crop coefficient (K) is 0.7. This number is based largely on cold season turfgrass. Jim suggests using a lower coefficient of 0.6 in a drought year to conserve water but still keep the grass alive. So the formula for how much water that needs to be replaced in an average landscape uses in an average August is:
Then, to break this down into days:
That means, if you're watering every three days, you will need to replace 0.3576" of water (0.1192" * 3) each time you irrigate. If we go back to the example of the clay soil with a 0.1" infiltration rate we can figure out how many cycles we need to program for the sprinkler. We take the amount of water we need to replace (0.3576") and divide by the infiltration rate of the soil (0.1") to ensure we don't have any runoff.
The final step is finding the total runtime for the irrigation. Jim uses the following formula:
Where ET_L is the water we need to replace, PR is the precipitation rate of the sprinkler and 60 is a constant. So, we get:
We can round this up to 14, then divide by the number of cycles we need to prevent runoff like so:
Voila! All you need are four cycles of three and a half minutes each every three days for a great looking lawn with no runoff. The only numbers you need are your monthly average ETo (from the CIMIS website), the precipitation rate of your sprinklers (from the catalog) and the infiltration rate of your soil (find your soil from the USGS website and use a table like this one for the infiltration rate). Plug those numbers into these simple formulas and you'll be doing water wise irrigation in no time!