Now that you have a watering system you have the opportunity and responsibility to “use your water wisely.” The question is: What does that mean?
Every garden and every gardener is different. Some people appreciate guidelines and formulas for determining the proper amount of water to apply. Some people prefer personal observation as the best means to know when and how much to water. We feel that both are important. Personal observation combined with an understanding of how your system is designed to deliver water as well as an understanding of how plants use/lose water in our climate is the best basis for making informed decisions about how to use your water wisely?
First I will give you a basic rule of thumb, and if you are interested, read-on for a more detailed explanation.

Rule of Thumb:

Rotors 15-30 minutes every 1-5 days
Spray heads 5-15 minutes every 1-5 days
Low-Pressure 10-60 minutes every 1-5 days.

In general, it is a good idea to water deeply (without run-off) with less frequency. This encourages the roots to chase the water. Deeper roots promote tolerance to drought and pests.

A. Design and Distribution

1. Design
Your system is designed to achieve the most efficient result for your garden. The best distribution method is selected to fit your landscape based on plant type, microclimate and available flow. We strive to keep the system as simple as possible, and balance the efficiency of distribution with the efficiency of maintenance. A simple system is less expensive to maintain.

2. Efficiency
Your watering system is designed with efficiency in mind. We define efficiency as making the best use of the resources that are invested in your landscape. An efficient watering system will allow you to have a beautiful garden for the lowest possible cost. Cost is measured by both time and money. These are design costs, watering costs, installation costs, plant costs, maintenance costs, etc. A well designed system assists in the minimizing of all these costs. Each of these factors has a symbiotic relationship with the others. I.e. if your landscape is designed efficiently it will be easier to water, if your landscape is watered efficiently it will be easier to maintain. Sometimes greater efficiency is achieved by moving or removing plants, sometimes it is achieved by raising your mowing height or frequency, but all of these issues affect the others and all contribute to overall efficiency.

3. Distribution types:
There are three basic types of output mechanisms for watering your landscape. All three have fundamental differences, and should not (except in rare instances) be mixed within the same watering zone.
a. Rotors: Rotors are sprinkler heads that turn as they water. They are mechanically driven and come with an assortment of nozzles which affect the distance and volume of water delivered. There three types of rotors: impact, single stream, and multiple streams. Rotors are typically used to water larger turf areas and sometimes shrub beds. If rotors are used in a shrub bed they should be multiple streams because multiple stream rotors are much gentler to delicate shrubs. Because rotors are designed to distribute water over longer distances they do not work well in tight spaces.
b. Spray Heads: Spray heads have a fixed spray pattern as determined by the nozzle that is installed. Unlike rotors, they do not turn. They are typically installed in smaller turf areas and shrub beds. Spray heads deliver more water relative to the amount of space they cover and are therefore set to run for fewer minutes than rotors. It requires more zones to water a landscape with spray heads than it would with rotors.
c. Low-pressure: There are several types of “low-pressure” systems. Low pressure systems are commonly referred to as drip irrigation. Low-pressure includes drip, micro-spray, and soaker hoses. Low-pressure systems have filters and pressure reducers (25-35 psi) installed at the valve.

i. Drip systems usually have ½” supply tubing which delivers water to ¼” distribution tubing and a series of emitters at the end of the line which in theory deliver a calculated amount of water to each plant based on its individual need.

ii. Micro-sprays are supplied by the same ½” and ¼” tubing, but at the end of the line are small spray heads that deliver water above ground in a fixed spray pattern similar to a spray head. Effective range is 2-6.’

iii. Soaker systems are typically installed to water sub-surface, and are ideal for Roses. The best soaker hose is a product called Techline developed by Netafim in Israel and is a ½” tube with pressure compensated and flow-regulated emitters every 12-18” along the tubing. You can install very long runs of this product and it will deliver the same amount of water at the beginning of the run as it will at the end. It does not work that well on slopes or very sandy soils. In general low-pressure systems are extremely efficient in the short run and extremely inefficient in the long run. That is, they are efficient in the way they deliver water, but they are high maintenance in the long run, and therefore may result in a greater long term expense to operate. I.e. the efficiency is outweighed by the cost of upkeep. We most often install low-pressure systems in containers and/or rose beds. It is important to note that the low-pressure types can be mixed as long as the output is measured to be appropriate relative to the plant type and micro-climate.

B. Scheduling
Appropriate water scheduling is determined by a variety of technical and common sense factors. A brief description of some of the technical factors is included but should not be relied upon to the exclusion of personal observation and common sense.

1. Uniformity of Distribution
Efficiency of a sprinkler system looked at in isolation is best measured by “uniformity of distribution,” or (DU). DU is a statistical measurement of how evenly the water is applied. The highest DU that is achievable is approximately 85% for rotors and 75% for spray heads. Low pressure systems are not measured by DU, but in theory are the most efficient.

Example:

DU Plant need (inches per week) Amount to apply each week

30% 1” 3.33”
50% 1” 2.00”
75% 1” 1.50”

If you had a DU of 50% you would have to apply twice the target amount of water.

Distribution uniformity is system specific and is determined through a water audit. You should expect the DU of your system to be at or near 75%.

2. Precipitation Rate (PR)
This is a measurement of how much water is distributed, usually expressed in inches per hour per square foot of irrigated area. There is a straight forward formula for measuring PR.

PR = 96.25 x Total GPM
Total Square feet of Irrigated area

i.e. A space 15’ wide by 45’ long covered by spray heads with 14 gpm output.

PR per square foot = 96.25 x 14
675 = 1.99” per hour

3. Evapo-Transpiration Rate
This is the sum of water lost from the soil surface (evaporation) and water used by plants (transpiration). This is expressed as a daily, weekly or monthly number and is essentially the amount of water that needs to be replaced either by rain or irrigation. ET rates are affected by temperature, wind and solar radiation, and are calculated for each region by university extension agencies. The peak ET rate for Portland is .18 inches per day. This means in the later part of the growing season you need to apply approximately .18 x 7 = 1.26” per week.

Example: To replace 1.26” per week you would need to run the zone in the example above 38 minutes per week:

1.26 inches per week
1.99 inches per foot per hour = .63 per hour per week or 38 minutes

These 38 minutes per week would be most effectively applied in two or three applications, or approximately 11 minutes every other day or approximately 16 minutes every third day.

4. Micro-Climate
ET’s are calculated for broad regions. ET’s are further affected by site specific micro-climates. Factors such as soil type (infiltration rate--how quickly the soil absorbs the water), exposure (sun vs. shade), wind, slope etc., all affect the water requirement for your landscape. This is where personal observation and understanding typically supercedes textbook formulas.

5. Manual or Semi-automatic watering
Possibly the most efficient way to water is to press the manual start button on your controller whenever you think you need to water. Sometimes you may water the whole system; sometimes you may water just one or two zones. Especially in the spring and fall when we are getting help from the rain, it makes sense to leave the controller off and only water from time to time when things look dry. Remember to turn the controller off after you water.