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Irrigation System Evaluation

Irrigation System Evaluation

Visually look for signs of over or under irrigation and for causes of problems. Turn on the sprinklers and look for problems which are possible causes of dry or soggy spots. Items to look for are plugged sprinklers or emitters, misaligned or tipped sprinklers, improperly operating sprinklers, improperly adjusted sprinklers, sunken sprinklers, or imporperly spaced sprinklers or emitters.

Determining Application Rates

Spray irrigation is primarily used for the irrigation of turfgrass areas. Recommended application rates are based on inches per application or inches per week. For this reason, the application rate of an overhead spray system must be determined for proper irrigation scheduling. The easiest way to determine the application rate in inches is to use the catch can test (figure 8). A number of straight sided cans (canned vegetables, tuna, etc.) should be placed randomly in the lawn area. The greater the number of cans, the greater the test accuracy. Usually 5 or 6 cans per 500 square feet of lawn area is adequate. After running the sprinklers for a predetermined length of time, measure the depth of water in each can using a ruler. Determine the average application rate by adding the inches, or fraction of inches, in each can and divide by the number of cans. If this average is more than the recommended rate of application in inches, the length of watering should be reduced accordingly. If the average is less than the recommended rate then the length of watering should be increased accordingly.

In the case of drip irrigation, application rates cannot be measured in inches. To determine the run time of drip irrigation systems, the depth of wetting must be determined and adjusted based on the rooting depth of the plants being irrigated. See Irrigation Scheduling.

Irrigation Scheduling

Water scheduling is the process of determining how to apply water to plants. It includes knowing where to apply the water, how much water to apply, and how frequently it should be applied.

Soil Type and Plant Material

The first consideration in determining a water schedule for garden and landscape plants is the soil type. Soil can range from a sand to a heavy clay. Sand is easy to dig, but does not hold water well. Water penetrates deeply, but not very wide. Clay soils are difficult to dig, especially when dry, but hold water very well. These soils will dry more than sand and can crack. Soaker hoses and drip emitters need to be spaced more closely on sand than on clay (figure 9).

Compacted soils or shallow soils over caliche or bedrock can also cause problems. Water, air, and roots cannot penetrate compacted soils very well. Compacted soils should be tilled. Shallow soils (less than two feet deep) cannot hold very much water and are easy to flood. You should water a shallow soil more frequently than a deep soil. Conduct a soil probe test to learn how deep and wide your soil will wet. If possible, water from a single outlet (a single bubbler, soaker hose, or drip emitter) for a set length of time, say thirty minutes for the bubbler up to two hours for the soaker hose or drip emitter. Wait a short while after you turn the water off, then push a soil probe (Figure 9b) into the wet soil at several places. A soil probe is a 1/4 to 3/8 inch diameter metal rod, 2 to 3 feet long, with one end sharpened to a point, and the other bent to form a handle. The probe should easily push through the wet soil and stop when it reaches dry soil. Use this technique to learn how far to the side and how deep a single outlet has wet your soil. You may need to repeat the test for different times to wet the soil as deep as two feet. If your soil has rocks or gravel that makes using the soil probe difficult, you can always dig holes to see how far the water penetrated. It’s best to wait 18 to 24 hours to dig in wet soil. Use this information to decide how far apart to space bubblers, soaker hoses, or drip emitters.

The type, size, and density of plants in the landscape also affect water requirement. Saguaros and roses do not require the same amount of water, for example. A dense landscape, with many plants in a small area, will require more frequent irrigations than a sparse landscape. Exposure is also important. A rose in full sun on the west side of your home will require more water than one on the north side.

Where to Irrigate

Plants absorb water from the soil through roots. In a natural setting, most of the plant’s roots spread to 1.5 to 4 times the width of the canopy and are within the top two to three feet of soil. This is called the root zone. Most of the water used by a plant comes from outside the "canopy drip line" (figure 10). Shallow or compacted soils can affect root distribution, as can improper irrigation. Often it is not feasible to water the entire root zone, but we should duplicate the natural conditions as much as possible. One approach is to water at least half of the root zone. This entails watering from near the trunk out to and beyond the ends of the branches. It is important to wet the same area of soil to the same depth every time you water to maintain a healthy, well distributed root system.

Placement of Drip Emitters

Drip systems are the most efficient and accurate method of applying water when properly designed, installed, and operated. Poorly designed, installed, or operated systems can lead to many problems. One of the most common is too few emitters that are poorly spaced. All too often only a single emitter is placed at the base of a newly planted tree or shrub. In clay soils a single emtter typically wets a 5' in diameter area, on sandy soils only an area 2 feet in diameter (figure 9). Given the fact that tree roots can grow up to 3 feet a year, after one year the diameter of the root system could be 6 feet. A single emitter with a 5' diameter wetting pattern could restrict root development as early as the first year after planting. For this reason, it is especially important to allow for the placement of additional emitters early in a plant's life.

The size of the root system at plant maturity must also be considered. The following chart shows the number of one gallon per hour emitters recommended based on canopy width at maturity and wetting at least half of the root zone. This chart assumes two feet between emitters on a sandy soil and five feet between emitters on a clay soil. Use the soil probe technique to decide how wide and deep the water has moved from a single emitter after a certain amount of watering time. Space emitters so wetting patterns meet or overlap slightly based on the results of this test (figure 11).

The large number of emitters recommended, especially on sandy soils, is impractical. However, plants in a landscape share root zones and can share emitters. In an example using a 24 inch soil wetting pattern, a 15 foot canopy tree would require 63 emitters. A planting of five large shrubs, each with a 4 foot canopy would require 5 emitter per plant or a total of 25 for the group. If the tree and shrubs shared the same root zone area the total number of emitters needed would not be 88 (63 + 25), but instead, as few as 38 (63 - 25) emitters.

Drip emitters can be placed under a surface mulch or underground with distribution tubing sticking aboveground. Drip systems also do not lead to surface compaction due to low flow rates.

Quantity of Water and Frequency of Irrigation

Every irrigation should wet the soil to the depth of the rooting zone. The rooting depth of turf, ground covers, vegetables and flowers is typically 12 inches. The rooting depth of shrubs is 18 inches. And the rooting depth of trees is 24 to 36 inches. Use the soil probe test to determine how long it takes to wet the soil to the potential rooting depth. Water this same duration every time. Frequent, shallow irrigations encourage a shallow root system and an unstable plant. Also most plants will use water that is available, although it may not be needed, so frequent irrigations can waste water. The soil should be allowed to dry between irrigations.

Many factors determine how much water a plant needs and how often you should irrigate. We have already discussed soil and plant types, size, and density.Along with soil, plant type, plant size, and planting density is the factor of weather. Plants use more water during the hot summer than in the winter. They will also use more water during an exceptionally hot summer than in an average summer. Many established trees and shrubs- in the hot, low desert areas of the state require between 0.6 and 0.8 inches of water per week in June during an average summer. The following table shows how many gallons this would be for different canopy sizes.

Established trees and large shrubs should not require watering more frequently than once every week or two. Newly planted trees and shrubs, regardless of their drought tolerance, will need daily watering after planting, tapering off gradually until establishment. The following chart pertains to low desert areas. In higher elevations weekly water requirements may be less. Use the following chart as an aid to scheduling water times.

Suppose you have a deep, clay-type soil and it takes six hours to wet the soil to a depth of two feet. You also have a low water shrub with a ten foot wide canopy and five one-gallon-per-hour emitters. Six hours times five emitters equals fifty gallons per irrigation. The chart suggests that a ten-foot wide shrub would require about thirty gallons every week or sixty gallons every two weeks in June. Water the shrub once and watch for the signs of drought: leaves curl, wilt, or turn yellow and drop off. Most plants recover easily from a slight wilt. If you see any of these signs within two weeks, water but keep trying to stretch the durations between waterings. If you do not see signs of stress within two weeks you might want to water or wait until they appear.

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