Gardening : Drip-System Formula Adds up to Savings

The instructions that come with the various drip or low-flow irrigation kits tell you how to install the system--how many emitters, how much porous piping or how many mini-sprinklers are required to water certain plants or a given area.

But how do you figure out how long to water with these systems that are so unlike sprinklers? Looking at the ground probably won’t help because true drip systems wet very little surface area though several feet may be soaked underground.

You can, of course, simply follow the recommendations that come with the particular kit you’ve installed, and this is a good idea at first. But you are working with someone’s best estimate and not with figures precise enough to save you substantial amounts of water--and savings of as much as 70% are possible.

For instance, look in the instruction book that comes with Raindrip systems and you’ll find a simple little chart that suggests how often and how long to water a variety of plants during three kinds of weather--cool, warm and hot.

During warm weather, it says to turn on the drip system every two days for five hours if you are watering shrubs 4 to 5 feet tall. Most kits include this kind of general guideline.

Much more precise irrigation is possible because of some recently compiled figures called ET (“evapotranspiration”) rates. ET tells us how much water a standard field of pasture grass loses to the atmosphere in a day. It measures both evaporation from the soil and plant surfaces and transpiration from the leaves.

This, of course, varies from day to day--plants obviously losing more in summer or during Santa Ana winds, less in winter or on overcast days--so to simplify things we work with averages.

ET rates also vary with location: The farther inland you live, the higher the rate--the greater the water loss--especially in summer. These figures are presented in Table A.

Note that these ET figures represent inches of water lost per day not for the whole month. And that they are based on a standard plant--pasture grass--so we need another table to work from, one that converts that ET figure for pasture grass into a figure that works for other kinds of plants. Table B gives us these “plant factors.” Note that most are fractions of the ET because most plants use less water than pasture grass.

Armed with these figures, we are ready to plug them into a formula that looks like this (and you thought you would never have any use for that algebra):

THE FORMULA

Gallons of water per plant, per day = ET x Plant factor x Plant area* x .623

divided by

Drip irrigation efficiency factor

“Plant area” is easy to figure. Simply multiply the diameter of the plant by itself (d x d), then by .785 (d x d x .785). Get out the tape measure and calculator and give it a try. A rose measures 3 feet across, so 3 x 3 x .785 = about 7 square feet.

One plant down and 200 to go you think? Well, you should figure out a few fruit trees and other large plants, but then you can come up with a few standard sizes for everything else, which will simplify things greatly.

Find the ET in Table A and the Plant Factor in Table B. Now do the math.

Once you have done all the multiplication in the formula, you divide the whole thing by a “drip efficiency factor.” In warm to hot, dry climates, this would be .85; in mild dry climates (near the coast), it is .90.

Totally lost? Let’s try an example.

If you live in West Los Angeles and want to know how many gallons of water a rose needs every day in July, here’s the math: .17 (the ET) x 1 (the plant factor--roses are thirsty plants) x 7 (the plant area) x .623 = .75. Now, divide that by .90 and you get about .8 gallons per day. How about that? You now know exactly how much water that rose needs on any given day during July.

If one emitter is taking care of this rose, and it is rated as delivering 1 gallon of water every hour (1 gph), it would have to be on for about 48 minutes each day to provide the water lost during that warm July day.

To figure this out, divide the gallons required by the plant, by the flow rate of the emitters (.8 gallons divided by 1 gallon per hour = .8 hours), then multiply that by 60 to get minutes (.8 x 60 = 48 minutes).

The formula here is:

Total gallons required divided by total flow of emitters = hours of irrigation.

You can water every day, but more likely you will be watering every few days so you will have to multiply this daily amount by the interval between irrigations. If you water the rose every three days, the water must be on for about 2 hours and 24 minutes (3 days x 48 minutes).

How often should you irrigate? Here are some guidelines on how often to turn on a drip system: For small shrubs and flowers, every 2 to 3 days; for large shrubs and small trees, every 3 to 4 days, and for medium to large trees, every 5 to 6 days. You can irrigate less often, but these suggested intervals maintain a fairly constant supply of moisture.

Two hours and 15 minutes is a long time, and that points up the big difference between sprinklers and drip systems. Sprinklers put a lot of water on real fast. Some of this water puddles and soaks in unevenly, or runs off. Drip systems put water on slowly but it all soaks in, right where it’s needed.

If you plan to automate, you should note that special timers are required for drip systems because many timers will not keep valves open for two hours. Special timers suitable for drip irrigation systems are coming onto the market.

While it is easiest to figure out how long to water with individual drip emitters, you can use the formula and tables to calculate how long to water with other systems.

For instance, if you are using mini-sprinklers to water a 5-by-16 foot bed of flowers that includes roses, figure out how much water the entire bed needs based on the needs of the biggest water user there (the roses).

Let’s do it for the month of November: .08 (November ET) x 80 square feet (area of flower bed) x 1 (plant factor for roses) x .623 = 3.98. Divide by .90 (efficiency factor) and you get 4.4 gallons used by the entire bed each day.

Now figure out how much water the mini-sprinklers are delivering to the entire bed. The particular brand we are using requires four mini-sprinklers that deliver 6 gallons an hour and two that deliver 12 gph, so all together they deliver 48 gallons an hour (4 x 6 gph + 2 x 12 gph)--this you learn from the kit instructions.

Now let’s figure how long to leave the mini-sprinklers on: total gallons required by the whole bed divided by the flow rate of the entire system (4.4 divided by 48 = .09 hours) and convert that to minutes (.09 x 60 = 5.4 minutes). If we water every three days we must let the system run for about 16 minutes (3 x 5.4 minutes = 16.2 minutes). We’ve done it again!

You can figure out other kinds of systems in the same fashion.

If you are building your own system from scratch this information can be found in the “Landscape Drip Irrigation Design Manual” booklet D-38829, available from Rain Bird Sales, Attention: Laura, 145 N. Grand Ave., Glendora, Calif., 91704. The cost is $5.)

It may take a little while for all this to soak in, if you will pardon the pun, but you can learn how to irrigate very accurately.

Begin with the instructions that come with a kit and then try being more precise as you become acquainted with how it all works.

HOW LONG TO WATER THE FORMULA Gallons of water per plant, per day =ET x Plant factor x Plant area* x .623 Drip irrigation efficiency factor DAILY ET (1)

Coastal Coastal Basin San Gabriel (Redondo, (Los Angeles Valley Long Beach) and Anaheim) (Pasadena) January .07 .07 .06 February .09 .09 .09 March .11 .12 .12 April .13 .16 .15 May .15 .18 .18 June .17 .19 .20 July .17 .20 .24 August .16 .19 .22 September .15 .17 .19 October .11 .13 .13 November .08 .09 .09 December .06 .05 .06

San Fernando Inland Valleys High Desert Valley (Riverside) (Lancaster) January .06 .06 .07 February .09 .10 .11 March .12 .13 .15 April .15 .14 .20 May .18 .20 .28 June .20 .24 .32 July .24 .26 .35 August .22 .24 .31 September .18 .20 .24 October .13 .13 .15 November .09 .09 .09 December .06 .06 .06

PLANT FACTORS (2) Plant: Factor Mature trees: .80 Vines & shrubs (over 4 ft.): .70 Small shrubs (under 4 ft.): 1.00 Arid climate (native plants): .35-.25 HOW LONG TO WATER Total gallons required x total flow of emitters = hours of irrigation. DRIP EFFICIENCY FACTOR Climate: Decimal Equivalent Warm to hot and dry: 0.85 Moderate: 0.90 DRIP FREQUENCY Plant Type: Frequency Small shrubs, vines: 2-3 Days Large shrubs, small trees: 3-4 Days Medium to large trees: 5-6 Days (1) ET: Evapotranspiration--This tells how much water a standard field of pasture grass loses to the atmosphere in a day. (2) Plant factors: Types of plants. *Plant area: Multiply the diameter of the plant by itself, then multiply by .785.