Drip Irrigation Information

"Drip Irrigation for the Mojave Desert" written/edited by Joseph Fortier and Kimberly Vilt, produced by the Conservation District of Southern Nevada with funding provided by the Bureau of Reclamation is now available on the web thanks to the Southern Nevada Arborist Group. The guide was developed for homeowners to have a better understanding on drip irrigation and the importance of doing it right the first time.
Please check it out: "Drip Irrigation for the Mojave Desert"

Check an article published in Irrigation Business & Technology, November 1998:
Supplying Moisture for Our Majestic Giants

Following are several articles Joseph H. Fortier and/or Jerry Belt, Jr.  have written for Southwest Trees and Turf and charts for minimum number of emitters per plant and estimated plant water needs for the Las Vegas, NV area.

Successful Drip Irrigation Design

Drip irrigation (Drip) is the slow application of water directly to the root zone of a plant. In the last 20 years, drip irrigation has been gaining popularity in commercial and residential landscapes. Technological advancements in drip system components have flooded the market with different types of products. This vast array of products, and the different ways to design and install them can create confusion, and lead to less than favorable experiences when experimenting with drip.

A basic understanding of the structure of plants root systems is critical.  The major feeder roots of a tree are usually in the top eighteen to twenty-four inches of the soil. Of course there are exceptions, and many trees will root deeper than two feet, but these trees are usually seeking available subsurface water, and irrigating to this depth is typically a waste.  Shrubs feeder roots typically will be found between twelve to eighteen inches deep.  All plants “breathe” oxygen from the soil through their roots. The highest oxygen content in the soil is in the first twenty-four inches.  Since this is the area containing the most oxygen, the most active, beneficial plants roots are found at this depth. Water, nutrients, and oxygen are absorbed most readily at this depth. The next factors to consider are closely related. How far beyond the drip line will the roots spread horizontally, and where within this vast network of roots will the most water be absorbed?   Generally speaking, tree roots will extend somewhere between one and one-half, to four times the diameter of the tree's canopy, and usually, two-thirds of the water is absorbed beyond the drip line.   



Thoroughly understanding these concepts is critical to design an irrigation system to meet the water needs of the plant properly as it matures. Compounding the problem is the typical situations found in most urban plantings, narrow beds, parking lot planters, and streetscapes. Most plant roots will not grow in the compacted, under watered, oxygen-poor soils too often found in these situations.  In these circumstances the irrigation designer needs to work as closely as possible with the planners and designers to make sure the watering needs of the plants are properly addressed.  In an ideal situation, the irrigation designer will have input on the planting details for all installations. A sound irrigation design, and proper planting will help ensure the plant is receiving sufficient irrigation to develop a healthy root system. A well-developed  root system will produce a healthier plant with fewer disease and insect problems. The roots also provide a strong anchor to prevent the tree from being blown over in a strong wind.  In many urban settings this is difficult to achieve, but there are still creative ways to accomplish this objective.

While the concept of drip irrigation is simple, proper design, installation, and management of  a drip system is critical to ensure success. The most common mistakes that lead to plant failure are not providing sufficient drip emitters to meet the needs of the plant as it matures. Confused, well let's look at how to design a drip system to meet the needs for the plant as it matures.

One of the first considerations when designing a drip irrigation system, or any irrigation system, is to identify the purpose of the irrigation system. Is it being designed for establishment and/or to provide supplemental irrigation during drought, or is it being designed to supply regular water to the plant as it matures? In the southwest we should design irrigation systems to provide sufficient water to the plant during initial establishment, and to provide water in a manner to encourage development of a healthy, vigorous root system as the plant reaches mature size.

The next challenge is to estimate how much water the plant will need. Unfortunately, not enough information exists on plant water needs for most of the country. The California Cooperative Extension worked with many plant and landscape experts to try to develop a method to estimate plant water needs for California.  Their research is a good starting place for much of the Southwest. Other sources include local water utilities, Cooperative Extension offices, and professional organizations such as The Irrigation Association's Drip Irrigation in the Landscape. Use the following formula to estimate water use if you know the Landscape Coefficient, and Reference ETo.

    Microsoft Equation 3.0         
    
After determining the water needs for the plant, an understanding of how the plant roots develop will help with emitter placement. For the most part, place the emitters to cover under the mature canopy to a few feet beyond the mature canopy. Properly installed drip irrigation will give the plant a deep soaking, and wet a sufficient area under and around the plant to ensure good development of the root system. Refer to the drawing below to see where to place the drip emitters around a tree.  The emitter placement in the drawing can be accomplished with any type of drip irrigation system. The critical factor is to place the emitters just beyond the area where the tree branches will extend at full maturity. This will provide water to a large area and encourage the plant to develop a vigorous, healthy root system. Some people suggest installing fewer emitters initially, and adding emitters as the tree grows. This approach works well if one remembers to install the emitters at a later date. Honestly how many people will do this? To keep things simple, install all of the emitters at the time of planting. Notice that the emitter closest to the tree trunk is removed after one year. This emitter is needed initially to make sure the newly planted root ball receives enough water. Once the roots have grown past the root ball remove the emitter to prevent diseases caused by too much moisture around the base of the tree.



A more serious problem sometimes exists in a retrofit situation where an existing spray irrigation system is being replaced with a low-volume irrigation system. Careful consideration must be given to the area that received the most irrigation from the old system.  This area will have the most active roots, and ignoring this fact has caused the slow demise of numerous mature trees when turf has been removed from the area under and around the tree, if emitters or bubblers were only placed near the trunk.  Visible stress, and even death has occurred within one year.

How important are these steps? Have you ever seen a large tree toppled over by a windstorm? Upon closer inspection did you notice a very small root mass and wonder how the tree ever stood in the first place? If the tree was in a turf area, it was most likely caused by shallow watering from the lawn sprinklers only. Properly installed drip irrigation can help prevent this. Trees need water to be applied deeper than grass. If the tree was in a xeriscape, the emitters were probably installed from the base of the tree to the edge of the box, and the roots never developed much further. As the canopy extended beyond the root zone area, the tree became unstable, and the wind just pushed it over. Roots can also be confined if the tree is planted incorrectly.

Success with drip irrigation can be easily achieved with a small amount of planning. Considering the cost of replacing a large shrub or tree, it is more than worth the effort. These same concepts should be used for all trees and shrubs.



One Way to Develop a Watering Schedule for Point Source Drip Irrigation

Through our involvement in many areas of the green industry, one issue of water management has not been address very often. How do you schedule point source drip irrigation? We would like to try and provide you information on how to develop a drip irrigation schedule for point source drip irrigation, like any calculated irrigation schedule it is just a starting point. True water management is monitoring the plants health and soil moisture.

In scheduling any type of irrigation there is several pieces of information we need to have in order to develop a schedule. The most logical order we have found to work the best is:

•   what is the plant water needs
•   how much water is being applied through the irrigation system
•   how long to irrigate
•   what kind of soil is the plant surviving in
•   how often to irrigate

Keep in mind the irrigation system and plant water needs will tell you how long to irrigate, the soil will tell you how often to irrigate.

How much water will the plant need/use?
This is one of the hardest questions every asked in scheduling. There has been research for different types of turfgrass but when it comes to trees, shrubs, and groundcovers very little research has been done. Many people are using the landscape coefficient method to estimate plant water use. There are different versions of how to calculate plant water use, the formula that we use is:
      
    Microsoft Equation 3.0

Landscape coefficient is based on type of plants, the density of the landscape, and the micro climate affecting the landscape. The formula for landscape coefficient is:

    Microsoft Equation 3.0

Species Factor (Ks)
Species factor: different plants species can vary considerably in their rates of evapotranspiration. Some species transpire large amounts of water, while other use relativity little.
                                                                    
Vegetation
High
Average
Low
Trees
0.9
0.5
0.2
Shrubs
0.7
0.5
0.2
Ground Cover
0.9
0.5
0.2
Mixed
0.9
0.5
0.2

    

Density Factor (Kd)
Density factor: newly planted and sparsely planted landscape often has less leaf surface area than mature, dense landscape and typically would use less water. Even though individual plants in a sparsely planted landscape may lose more water for a given leaf area, the total water loss from a dense planting will likely be higher due to the greater total leaf surface area for the site.
                                                                                            
Vegetation
High
Average
Low
Trees
1.3
1.0
0.5
Shrubs
1.1
1.0
0.5
Ground Cover
1.1
1.0
0.5
Mixed
1.3
1.1
0.6

                                                                

Micro climate Factor (Kmc)
Micro climate factor: environmental condition may vary significantly within a single landscape. Structure and pave areas typical of urban landscape can greatly affect these conditions.
                                                                                            
Vegetation
High
Average
Low
Trees
1.4
1.0
0.5
Shrubs
1.3
1.0
0.5
Ground Cover
1.2
1.0
0.5
Mixed
1.4
1.0
0.5



ETr Rates Based on Climate    
Climate
Definition
((midsummer)
ETr (worst case,
Inches per day)
Climate
Efficiency (%)
Cool Humid <70ºF
>50% Humidity
0.10 - 0.15
100
Cool Dry <70ºF
<50% Humidity
0.15 - 0.20
95
Warm Humid 70ºF - 90ºF
>50% Humidity
0.15 - 0.20
95
Warm Dry 70ºF - 90ºF
<50% Humidity
0.20 - 0.25
95
Hot Humid >90ºF
>50% Humidity
0.20 - 0.30
90
Hot Dry >90ºF
<50% Humidity
0.30 - 0.45
90


    
Estimated Emitter Efficiency
Emitter Type
Efficiency
Line Source Non-PC
80%
Line Source PC
90%
Point Source Non-PC
75%
Point Source PC
85%

    (PC – pressure Compensating)

There are several sources for information on landscape coefficient method, some are: Drip Irrigation in the Landscape, The Irrigation Association (http://www.irrigation.org); Low-Volume Landscape Irrigation Design Manual, Rain Bird (http://www.rainbird.com/drip/technical.htm); or WUCOLS, University of California Cooperative Extension, University of California (information can be found at http://www.owue.water.ca.gov/docs/wucols00.pdf).

An example: an ash tree (average water use) with a 20’ canopy, in July with a weekly ETr of 2.83 inches (hot dry climate), average density and micro climate, using pressure compensating emitters could use the following amount of water:

     Microsoft Equation 3.0  

    Microsoft Equation 3.0

There for an ash tree in May could use 362 gallons of water in one week.

How Much Water is Being Applied Through the Irrigation System
If you looking at an individual plant then you would just count the number of emitters around the plant and the amount of gallonage output or if you are looking at a complete irrigation zone then ether calculate the total number of emitters and output or use a water meter to calculate the total flow.

How Long to Irrigate
To calculate how long to irrigate, take the total gallons needed per week and divide it by the amount of water being applied. The formula is:

    Microsoft Equation 3.0

An example: the ash tree has 24 each 2 gph emitter around the tree, how long would we water in one week?

    Microsoft Equation 3.0
The ash tree would require a watering time of 452 minutes or 7.5 hours in one week. Actual frequency of irrigation will depend on soil type.


Emitter Quantity

The information is based on information from: Drip Irrigation in the Landscape, The Irrigation Association (http://www.irrigation.org) and Low-Volume Landscape Irrigation Design Manual, Rain Bird (http://www.rainbird.com/drip/technical.htm).

The number of emitters per plant should provide a uniform wetted area to cover a minimum of 50% to 65% of the mature plant root zone or 75% to 100% of the mature plant canopy. Install enough emitters to meet the water needs of the mature plant. When low, low/medium, and medium water use plants are installed on the same zone the low/medium water use plants should have 1.75 times more emitters than the low water use plants and the medium water use plants should have 2.5 times more emitters than the low water use plants. Trees and shrubs should always be installed on separate zones. Suggestion, install the number of emitters to meet the water need of a mature plant and the correct placement at the original irrigation system installation as the possible cost to retrofit an irrigation system after two or more years could exceed the original irrigation system cost. Mojave Water Management holds no responsibility for any damages or losses that might occur from any information provided.

Suggested Minimum Number of Emitters per Plant
Sandy Soil
Sandy-Loam Soil
Loam Soil
Canopy Dia.
1 GPH Emt.
2 GPH Emt.
1 GPH Emt.
2 GPH Emt.
1 GPH Emt.
2 GPH Emt.
Shrub
1'
1
1
1
1
1
1
Shrub
2'
1
1
1
1
1
1
Shrub
4'
2
1 - 2
1
1
1
1
Shrub
6'
4 - 5
3
2
2
2
1 - 2
Shrub
8'
6 - 8
4 - 6
3 - 4
2 - 3
2 - 3
2
Shrub
10'
9 - 12
6 - 8
4 - 5
3 - 4
3 - 4
3
Tree
10'
9 - 12
6 - 8
4 - 5
3 - 4
3 - 4
3
Tree
12'
13 - 17
9 - 12
6 - 8
5 - 6
5 - 6
4 - 5
Tree
15'
19 - 26
14 - 18
9 - 12
7 - 9
7 - 9
5 - 7
Tree
18'
28 - 37
20 - 26
12 - 16
10 - 13
10 - 13
7 - 10
Tree
20'
34 - 45
24 - 32
15 - 20
12 - 16
12 - 16
9 - 12
Tree
25'
53 - 71
37 - 50
24 - 31
19 - 25
19 - 25
14 - 18
Tree
30'
76 - 101
54 - 71
34 - 45
27 - 36
27 - 36
19 - 26
Emitter Spacing*
3'
3.5'
4.5'
5'
5'
6'
 *Spacing between emitters for different soil textures.


Emitter Chart to Balance Water Needs

With larger plants, more emitters with higher flow rates are needed so that water is properly delivered to the larger root zone. Smaller plants require fewer emitters with lower flow rates to prevent runoff. Some drip irrigation manufacturers make emitters that emitters with lower water in excess of 2 GPH. Using emitters with higher flow than suggested on the chart below is acceptable, but remember to cover three-quarters of the plants mature canopy with emitters to develop a sound root system.

Use the emitter selection chart on the facing page as a general guide to find the flow rate and number of emitters needed for each plant based on water needs of the mature plant. Mature plant sizes can be found on the plant identification tags that come with the plant when you purchase it, or in garden books like Sunset's Western Garden Book. To use the chart, locate the information located directly beneath your plant's diameter.

For example, a mesquite tree can have a 25’ diameter and has low water needs. Based on the chart, you would need to install 22 each 2 GPH emitters to provide enough water to meet the mesquite tree's needs at maturity . It is suggested to install all the emitters when planting the tree as roots can grow three times faster (if moisture is in the soil) than the canopy of the tree. With a nice healthy root system, you will have a healthier tree that should withstand windy days and avoid being blown over.

Note: Higher water use plants may require the installation of additional emitters to ensure enough water is applied without the need to increase watering times. Mojave Water Management holds no responsibility for any damages or losses that might occur from any information provided.

Emitter Selection for Shrubs
Plant Diameter in Feet
2
4
6
8
Plant Water Use
Low
# of Emitters
1
1
3
5
Emitter Rate (GPH)
0.5
1
1
1
Low-Moderate
# of Emitters
1
2
5
4
Emitter Rate (GPH)
0.5
1
1
2
Moderate
# of Emitters
1
3
4
6
Emitter Rate (GPH)
1
1
1
2


Emitter Selection for Trees
Plant Diameter in Feet
10
15
20
25
Plant Water Use
Low
# of Emitters
7
8
14
22
Emitter Rate (GPH)
1
2
2
2
Low-Moderate
# of Emitters
6
14
25
40
Emitter Rate (GPH)
2
2
2
2
Moderate
# of Emitters
9
20
36
56
Emitter Rate (GPH)
2
2
2
2


Inline Drip Tubing Lengths (0.6 GPH emitter) for Trees*
Plant Diameter in Feet
10
15
20
25
Plant Water Use
Low
Feet of Tubing
7
8
14
22
Emitter Spacing
24"
24"
24"
24"
Low-Moderate
Feet of Tubing
6
14
25
40
Emitter Spacing
18"
18"
18"
18"
Moderate
Feet of Tubing
9
20
36
56
Emitter Spacing)
12"
12"
12"
12"

* Inline drip tubing at 0.5 GPH emitters use 20% more tubing, inline drip tubing at
0.9 GPH use 33% less tubing and inline drip tubing at 1.0 GPH use 40% less tubing.



Estimated Plant Water Needs - Las Vegas, NV

The following chart are estimated plant water needs and estimated weekly plant water needs based on information from: Drip Irrigation in the Landscape, The Irrigation Association (http://www.irrigation.org). These numbers are estimates only, the actual water needs will depend on soils, mulch covers, weather conditions, type of maintenance (both landscape and irrigation), irrigation system performance, and irrigation scheduling. Mojave Water Management holds no responsibility for any damages or losses that might occur from the use of these estimates.

Plant Type and Water Need
Canopy Dia.
Maximum
GPW *
Estimated Water Needs Per Week
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Low Shrub
1
0.4
0.1
0.1
0.2
0.2
0.3
0.3
0.3
0.3
0.2
0.2
0.1
0.1
Low Shrub
2
1.6
0.3
0.4
0.6
0.9
1.2
1.3
1.3
1.2
0.9
0.6
0.3
0.2
Low Shrub
4
6.4
1.1
1.6
2.5
3.6
4.6
5.4
5.2
4.7
3.6
2.4
1.4
1.0
Low Shrub
6
15
2.4
3.5
5.7
8.2
10.4
12.0
11.7
10.5
8.2
5.4
3.1
2.2
Low Shrub
8
26
4
6
10
15
18
21
21
19
15
10
6
4
Low Shrub
10
40
7
10
16
23
29
33
33
29
23
15
9
6
Med Shrub
1
1.0
0.2
0.2
0.4
0.6
0.7
0.8
0.8
0.7
0.6
0.4
0.2
0.2
Med Shrub
2
4.0
0.7
1.0
1.6
2.3
2.9
3.3
3.3
2.9
2.3
1.5
0.9
0.6
Med Shrub
4
16
2.7
3.9
6.3
9.1
11.5
13.4
13.0
11.7
9.1
6.0
3.5
2.5
Med Shrub
6
36
6
9
14
20
26
30
29
26
21
14
8
6
Med Shrub
8
64
11
16
25
36
46
54
52
47
36
24
14
10
Med Shrub
10
101
17
24
40
57
72