by Charlene Burgi
A few weeks ago I wrote about using rain barrels in the garden and how to calculate the amount of rain water that could be collected off of your roof in a given year. What I failed to explore is if you collected all that water, would it be enough to sustain your irrigation for the year? Do you know roughly how many gallons of water it takes to irrigate your garden, or how large the tank would need to be to capture that amount of rainwater?
The rain barrel example I gave used a 1,000-square-foot roof, which could potentially collect 24,000 gallons of water, assuming 40 inches of rainfall for the year. While you might say that is a lot of water, I challenge you to figure out just how much water your garden uses.
There are several factors we will need to know before starting this process. First we need to know the evapotranspiration (ET) rate—that is, how much water is being lost from plants and soil into the air. In Marin, the average ET is 44 inches a year. Visualize installing water-tight walls to a height of 44 inches around your lawn and pouring 44 inches of water into the contained area! That’s what you would need to do to replace ET. Since most poorly designed irrigation systems use 40 percent more water than is needed, you might need to build the walls much higher. Got the picture? You may note that ET is lower close to the Bay and, conversely, much higher as you move north in Marin. The lower the ET, the less irrigation is needed.
Next we have a plant factor (PF), which represents the percentage of ET a particular plant requires. I will help by providing the average PF according to Water Use Classifications of Landscape Species (WUCOLS). (Note these are typical examples—if you really want to get into the nitty-gritty each plant species will have its own PF.) Your job is to measure the square footage of your lawn area, high-water-use plant area and low-water-use plant area. Finally, we have a conversion factor of .62 that is constant in our calculations.
Let’s start with the lawn. A lawn is at the top of the list for irrigation needs with a typical PF of .80. Remember your 44-inch wall? Multiply ET (44) by the lawn’s PF of .80. Then multiply that number by the square footage of your lawn, and finally by our conversion factor of .62 to get the gallons of water your lawn needs each year.
Now measure the low-water-use plant area and the high-water-use plant area, and write down those figures. High-water-use plants such as annual color, potted plants and shallow-rooted plants can be calculated using a plant factor (PF) of 70 percent of ET. Low-water-use plants such as lavender, sages and geraniums only require a PF of 25 percent of ET.
Here is the formula and an example to calculate the gallons of irrigation water needed. Let’s assume your lawn is 1,500 square feet (SF) with a PF of .80, the high-water-use plant area is 800 SF with a PF of .70, and the low-water plant area is 3,000 square feet with a PF of .25. We would then multiply our answers with the conversion factor (CF) of .62 to get the total gallons of water needed per year.
(ET x PF x SF x 0.62) = Gallons of Water
Given the above example, you would need 68,473 gallons of water a year to irrigate your garden! And that, my friends, is a lot of water, even without going into effective rainfalls, irrigation efficiencies and various other factors that come into the picture. For those of you techies who love this additional challenge, see the MAWA table.
As for me, a breath of fresh air is in order!