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  Turf Scheduling Basics
Using weather information to adjust your watering will save water, reduce water cost, reduce the need for fertilizer, and improve the heath of your plants. Weather based scheduling works by calculating how much water your plants need, then calculating how long to run your watering system. Since the need for water varies throughout the year you should regularly adjust your watering times.
We will concentrate on lawn watering systems since they account for a large part of your water consumption and they are the easiest to manage.

First, you must determine how your watering system performs. The application rate is the amount of water (in inches) applied per minute. By determining the application rate you can determine how long to run your sprinklers to apply the proper amount of water. We also need to know how uniform your watering system is. Uniformity is a measure of how evenly your system applies water. If your system were perfect then it would be 100% uniform. As uniformity decreases we must apply more water to insure that your plants remain healthy.

Second, you need to learn how to manage your watering system for best use of the water.

Finally, you can calculate how long to run your sprinklers.

Determine How Much Water Is Being Applied
All sprinkler systems distribute water unevenly. This results in the need to apply additional water to insure that all of the plants receive enough water
Two measurements are used to evaluate how  your sprinkler system performs:  Application Rate and Distribution Uniformity (DU).  The application rate is the average rate (usually expressed in inches per hour in the U.S.) that water is applied by an irrigation system.  You can use the application rate to calculate how long to run your sprinklers to apply a specific amount of water.  The DU measures the application of water in the driest areas as compared to the average.   This is important because we want to water so that all of the grass is healthy.   A system with a DU of 100% would be perfect.  A system of 60% means that the application rate in the least watered 1/4 of your lawn is only 60% of the average.   We  use DU for scheduling to adjust the average application rate to get a modified application rate used for scheduling the length of irrigation,

Well designed home sprinkler systems have typical application rates and uniformity's shown in the table below.  You can use these figures for scheduling, however it is recommended  that you measure these values on your system.
Type of System Typical Application Rate Distribution Uniformity (DU)
Popup Head (standard flow) 1.7 Inches/Hour 60%
Impact Sprinklers (standard flow) 0.5 Inches/Hour 70%
Wateright can help you evaluate the performance of your watering systems and use that information to create a watering schedule.