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Calculating Water Requirements: part 2

In the first article we provided a little background about the soil, soil - water relationships, and basic watering schedules for drip and micro sprinkler systems. In the second part of the three part series, we use the scientific method to demonstrate how to calculate water requirement for individual plants. In the third and last article we will demonstrate how to calculate water requirements for dense planting.

How to calculate water requirements for dense planting using a drip line with pre-inserted emitters:
Calculation of water requirement for dense planting is based on 4 steps and uses 2 formulas.
Step 1: Determining the application rate per hour
Step 2: Determining how many laterals are needed
Step 3: Determining system water run time per day
Step 4: Calculating water requirements per zone

Calculating water requirements for dense plantings using drip line with a 1 GPH dripper with 12" spacing
Example:
Area to be watered: 10' X 8'
Soil type: sandy
Drip line flow rate: 1 GPH per each drip emitter.
Drip spacing: 12" apart,
Lateral spacing: 16" apart.

Step 1: Determine the application rate per hour

 

Formula # 1   231.1 x 1 gph dripper   =   1.20" per hour

16" row spacing x 12" dripper spacing


We plug all the numbers into the formula and determine that our application rate is 1.20" per hour. The example, using a drip system, gives a higher precipitation rate than would be obtained for a sprayer or sprinkler system. If you wish to lower the precipitation rate, increase the spacing between drippers and laterals.

Average precipitation rate for a sprayer or a sprinkler system is around .50 inches per hour.

Step 2: How many laterals are needed'

  1. Our ground cover area is 10' x 8' = 80 sq. .ft.
  2. We can see from Table 1 that we can use a drip line with 1 GPH drip emitters spaced 12" apart. From Table 1 we will use 16" spacing between the laterals and start with the first line 4" from the edge of the area being watered (see drawing below).
  3. We start 4" from the edge of the area being watered, leaving the same margin on each side. We subtract this amount from the total area using this calculation: 10' x 12" = 120" - 8" = 112"
  4. Dividing the 112" by 16" spacing allows for 7 drip laterals.

We determine that we need 7 laterals for full coverage of this area

After determining the application rates, we need to determine the run time necessary to apply 1.20" per hour

Step 3: Determining system water run time per day

 

ET Plant (water needed per day in inches)   =   Daily run time

Application rate of drip system per hour


From the first formula, our application rate is 1.20" per hour. Assume a daily ET rate of 0.28" per day (see Table 2).

 

Formula # 2   0.28   =   23 minutes per day

1.20


The last thing to do is to compensate for the system application efficiency by dividing the run time of 23 minutes (see formula 2) by application efficiency. For this example we will use a high efficiency of 90%: 23 divided by 90 =25.5 minutes per day.

System run time based on formula 2 is 25.5 minutes of run time per day, or 51 minutes every other day.

Step 4: Calculating water requirements per zone

  1. Once we calculate our water requirements, we need to calculate water requirements per zone so we can select the size of the valve, pressure regulator, filter and main lateral pipe.
  2. Calculate total feet of drip line. Example for 1 zone: 7 drip laterals, each 8' long = 56' of drip line divided by 1' for drip spacing = 56 drippers with flow rates of 1 GPH = 56 gallons per hour divided by 60 minutes = .93 GALLONS PER MINUTE.

conclusion: System water requirements per zone = .93 gallons per minute

  • Recommended filter mesh for drip line: 155 mesh, available in 3/4", 1", 1 1/2" and 2" sizes
  • Recommended filter flow rates: 3/4' - 10 GPM, 1" - 16 GPM, 1 1/2" - 60 GPM and 2"- 80 GPM
  • Recommended pressure for drip line: 20 to 30 PSI

The total amount of water that should be supplied to a landscape must be sufficient to replace water loss from the soil surface by evaporation, which is affected by humidity and temperature and by the plant during the transpiration processes: this is called evapotranspiration or ET. ET is usually expressed in terms of inches of water per day or inches of water per month. These rates vary with the season and realistically they vary by the hour, but for lawn irrigation purposes, average ET rates are more than enough to schedule watering. The best ET data per week or month is usually obtained from local sources such as the Cooperative Extension Service, local newspapers, or the National Weather Service, or you can do it yourself by measuring the depth of water evaporated from an open pan.

This is the last part of three articles, and we hope we have provided all the information needed to design a complete system.

 

Table A1 Drip line recommended spacing
  Sandy Soil Loamy Soil Clay Soil
Dripper flow rates 1 gph .6 - 1 gph .6 gph
Spacing Drip Line Laterals Drip Line Laterals Drip Line Laterals
Ground cover & shrubs 12" 14" - 18" 12" - 16" 16" - 24" 16" - 20" 18" - 24"
Flower beds 12" 12" - 16" 12" 16" - 18" 18" 16" - 20"
Turf 12" 12" - 16" 12" 16" - 20" 18" 18" - 24"

 

Table A2 Generic estimates of ET (inches per day)
AZ .22 to .37 FL .22 to .27 KS .25 to .27 NM .28 to .35 UT 20 to .32
CA .15 to .27 GA .22 to .25 NE .18 to .25 OR .17 to 22. WA .14 to .21
CO .15 to .20 ID .20 to .25 NV .20 to .25 TX .26 to .32    

 

Head loss in PSI 1/4" micro tubing (red background = not recommended)
GPH 2' 4' 10' 20'
10 0.3 0.6 1.5 3.0
20 1.0 2.0 5.0 10.0
30 2.1 4.2 10.5 21.0
40 3.7 7.4 18.5 37.0
50 5.5 11.0 27.5 55.0

Head loss in PSI 1/2" poly tubing (red background = not recommended)
GPM GPH 10' 20' 50' 100' 200' 300' 400' 500' 600' 700' 800' 900'
2 120 0.2 0.4 0.9 1.8 3.6 5.4 7.2 9.0 10.8 12.6 14.4 16.2
3 180 0.4 0.8 2.0 3.9 7.8 13.7 15.6 19.5 23.4 27.3 31.2 35.1
4 240 0.7 1.3 3.4 6.7 13.4 20.1 26.8 33.5 40.2 46.9 53.6 60.3
5 300 1.0 2.0 5.1 10.1 20.2 30.3 40.4 50.5 60.6 70.7 80.8 90.9
6 360 1.4 2.8 7.1 14.1 28.2 42.3 56.4 70.5 84.6 98.7 112.8 126.9

 


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