Time was, the best way to  keep your clients happy was to keep the plant material "green"  regardless of the amount of irrigation water applied. Not only was this  a waste of resources, but overwatering also increased the likelihood of  pest infestation, fungi, runoff and erosion, and even liability.

Hit-and-miss irrigation scheduling is a thing of the  past. Today, irrigation managers are accountable not just for the  condition of the landscape, but for the amount of irrigation water  used and the amount of energy consumed to apply it. When irrigating  with treated effluent, accurate irrigation schedules become more important still.

Now there are quantifiable methods of applying irrigation  water efficiently that can be easily calculated. Evapotranspiration  data, or ET, is the measurement of plant water use and indicates the  two ways in which water moves from the landscape to the atmosphere  -- evaporation, the movement of water from the wet soil to the air,  and transpiration, the movement of water from the plant to the air.

Evapotranspiration data is available in real-time,  measured by an on-site or nearby weather station, or through historical  records for your region, usually available through your local extension  service or state-supported weather-station network (in California,  CIMIS, Arizona, AZMET, Washington State, PAWS and so forth).

Using ET data with plant material needs (crop coefficients),  plant maturity, soil type and slope, as well as irrigation system precipitation  rates and distribution uniformity will enable you to calculate accurate  irrigation schedules. All of these factors must be considered to establish  optimal application rates and run times within your specific watering  window.

Plant Water Requirement

The plant water requirement is the amount of water that  must be applied through the irrigation system to sustain the plant material  over a given amount of time. Irrigating at this level minimizes pesticide  and fertilizer use while helping to cultivate healthy, resilient plant  material.

As an example, warm-season turfgrass in California's  Central Valley, in the month of July requires 4.8 inches of water (ETo)  to remain healthy. Rainfall for June will supply .8 inches of that requirement.  The remaining 4 inches is the `net plant water requirement.' Plant water  requirements for your area are available locally.

Irrigation Water Requirements

Proficient irrigation practices rely on equipment  selection, system design and installation efficiency. A distribution  uniformity (DU) test, or catchment test, will determine exactly how  evenly water for each sprinkler zone is actually being applied.

Let's say for now, that your irrigation system in Central  California has a DU of 65 percent. Remember your net plant water requirement  in the Central Valley is 4 inches for July. To find your actual irrigation  system water requirement, divide 4 inches by 65 percent (.65) and you  get 6.15 inches. In order to deliver a minimum of 4 inches of water  to all of your turf in July, you are going to have to actually apply  6.15 inches to ensure that the least efficient coverage areas get their  4-inch minimum.

Scheduling Requirements

In order to formulate an accurate scheduling  regimen, you need to know the site's soil type, the root depth of the  plant material being irrigated and how much water can be stored in the  soil where the roots live. A soil probe can provide most of the answers.

In probing the soil, you find that the soil  type is a loamy-sand, the rootzone depth is 6 inches and the soil's  water-holding capacity is .08 inches of water per 1 inch of soil. Therefore,  the total water-holding capacity of your soil at the roots is .08 inches  per inch of soil x 6 inches root depth, or .48 inches available water  holding capacity (AWHC). As a rule, 50 percent soil water depletion  should trigger a new irrigation. Fifty percent of .48 inches AWHC is  .24 inches.

Determine how many days the system must operate,  by dividing the irrigation water requirement (6.15 inches) by .24 inches  (the soil water depletion). You get 26 days total during the month of  July. The total run time for the period (July) is calculated as follows:

(Irrigation water requirement) / (precipitation  rate of the sprinklers per hour) x 60 = minutes of irrigation runtime  or, 6.15 / 2.0 = 3.1 x 60 = 186 minutes.

Finally, calculate the total run time per day,  which is the total run time per period (186 minutes) / the number of  irrigation days (26), which equals 7 minutes. Thus, by irrigating a  total of 26 days in the month of July, for 7 minutes each irrigation  day, you will meet the turfgrass plant water requirement in the Central  Valley.

Boosting Uniformity

In the course of performing a catchment test,  check the sprinkler head-tilt (all heads should be perpendicular to  the surface) and nozzling (nozzles should be free of wear and sizes  should match one another). These two simple observations might locate  uniformity problems that can be easily remedied and increase your DU  significantly.

Consider an improved DU of 75 percent for the  same site. Using the same formula mentioned previously, now your system  water requirement is 5.33 inches to ensure that the entire site gets  its minimum 4 ET inches. With your same .48 AWHC and 50 percent soil  moisture depletion (.24 AWHC), you calculate the how many days the system  must operate (5.33 divided by .24, or 22 days).

The total runtime for the month of July with  your new DU is 5.33 divided by 2.0 (precip rate) = 2.7 x 60 = 162 minutes.  Your runtime per day equals 162 minutes / 22, or 7 minutes. Now, irrigating  22 days in the month of July, for 7 minutes each irrigation day, will  satisfy the turfgrass plant water requirement.

You've just shaved off .82 inches of irrigation  water and 28 minutes of irrigation time for one month. Over a 12-month  period, that's almost 10 inches less irrigation water through the system  and 336 minutes less runtime on your system, which saves water, energy,  manpower and irrigation equipment.


Common Causes of Poor Irrigation  Uniformity and Possible Solutions

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