Tuesday, July 22, 2008
Most drip systems are up and running by now. Yet, do you know where to start to get the proper amount of water on your flowers and/or vegetables? I'm working on a revised edition of my 1995 classic (so I'm told) Drip Irrigation, For Every Landscape and All climates. Here's an excerpt:
Daily irrigation doesn’t mean using countless gallons of extra water. In fact, with infrequent irrigation, it takes a certain amount of water just to rehydrate the soil before the plant can even make use of the moisture. Oddly enough, infrequent waterings can use more water than the same planting would receive with frequent, even daily, irrigation.
Years ago, for example, I planted a drought-resistant landscape, with plants such as lavender, santolina, rockroses, and rosemary, for a neighbor. The day after planting, the timer was set to irrigate each zone for 15 minutes. After the risk of transplant shock was over, the irrigation lines were turned on each day for only eight minutes. The plants flourished, even though each one-half-gph emitter was distributing the paltry amount of seven tablespoons of water-per-emitter each day. Contrast this with a nearby garden with a similar soil and lavender plants arbitrarily watered only twice a month for four hours. This amounts to two gallons per emitter for the two-week period, or just more than 18 tablespoons of water per day—more than twice the water used in the flourishing landscape.
No matter how you use drip irrigation, frequently or every once in a while, it will always be more efficient than any sprinkler you’re currently using. All sprinklers, except the most modern of micro- or mini-sprinklers, apply water faster than many silty and clayey soils can absorb it. This leads to anaerobic puddling and runoff, especially on steep slopes. Every sprinkler is vulnerable to wind- and sun-induced losses, with as much as 25% of the water wasted. In general, sprinklers are rated at an overall efficiency of 75% to 80%, compared with drip irrigation’s 90%. [Furrow irrigation can have an efficiency rating as low as 50%.]
How Long to Water
There are two general approaches to watering, the empirical and the more analytical, which uses the evapotranspiration rate as a guideline. Each works, but the ET-rate-based approach can be far more accurate and water conserving.
The most immediate, or empirical, way to understand your soil’s response to drip irrigation, and to determine how long to leave the system on, involves digging. Even after doing your experiment with the milk jug, you should test for the drip system’s underground pattern of moisture. Turn on the drip system for an hour, then turn off the hose and dig a number of small holes in the flower bed to see how deep and to what width the water has soaked in. Then turn the system on for another hour, to equal a test total of two hours, and check to see how much farther the water moves. Do this for several more intervals of time and observe vague changes in the wet spot. This test will reveal the shortest length of irrigation time to produce the widest wet spot, based on which test hole revealed the widest spread of the wet spot during the elapsed time. Without doing this test, you’ll just be guessing in the dark.
Another approach involves using the ET figures for your local climate. Your local Cooperative Extension office should be able to tell you either the current week’s ET rate or the month’s average rate; both are expressed in inches per day or month. If they don’t know, fire them. (Click on the chart to get a bigger version.)
WARNING: Math Ahead.
The chart above shows the daily water use for ten different ET rates. Remember, the amount of water needed to replace the ET losses depends upon the amount of soil covered [like a shadow] by the planting’s foliage. If the plants are young, the ET rate is less, corresponding to the smaller area of coverage. With a mature flower border, the coverage is complete and all you need to determine is the total square footage of the border. For example, a five- by 20-foot border [100 square feet] uses 18.7 gallons of water per day during a hot day when the ET rate is equivalent to nine inches of water per month. If you still prefer to water once a week, multiply the daily ET rate by seven to determine the total amount for the weekly watering.
To determine the length of each day’s watering, take the total amount of water the flower border requires and divide by the total flow of the drip irrigation system. Consider a theoratical five- by 20-foot border with a daily ET rate of 18.7 gallons. Since the total length of in-line emitter tubing in the bed is 84 feet [this figure is reached by adding together one header four feet long and four 20-foot laterals], the sum flow of the system is 52 gph [84 one-half-gph emitters times their actual flow of 0.62 gph]. Thus, dividing the daily water need of 18.7 gallons by 52 gph yields 0.36 hour, or 22 minutes per day 0.36 of an hour times 60 minutes. If you want to water once per week, then multiply 22 minutes per day times seven days to get a weekly watering time of 154 minutes, or nearly three hours.
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