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Calculating Water Requirements: part 1

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 article we will use the scientific method to demonstrate how to calculate water requirement for individual plants.

How to calculate water requirements for an individual plant with a single outlet drip emitter:
Calculation of water requirement is based on 3 steps and uses 3 formulas.
Step 1: Finding water requirement per day per plant.
Step 2: How many drippers to use
Step 3: Determining the watering time of your system

Step 1: The first part of designing the system is to calculate the amount of water the plant will need. To do so we have below a series of tables that will provide the information needed.
Table A1: Ks =Landscape coefficient
Table A2: ETr =Reference evapotranspiration rate (inches per day)
Table A2: Climate efficiency
Table A3: Estimated emitter uniformity

Formula # 1 will be used to determine the number of gallons of water needed per day per plant. Please be aware that this formula needs to be applied separately to each different sized plant in the design.

 

Gallons per day per plant
 
  =  
 
.623 x plants area x Ks x ETr
climate efficiency x estimated emitter uniformity
(in decimal form)


If you have all the information concerning the individual plant's site, the following pieces of the puzzle will fall into place.

For example: If there is a tree in a cool, humid climate with a 10' diameter canopy,

The Formula to Calculate Canopy Square Feet is: 3.14159 x Radius x Radius = Plant Area in Square Feet

So, 10' of canopy is equivalent to an area of 78.5 square feet. The canopy area is also the same as the plant root zone area.

Using the formula above, the gallons per day per plant can then be calculated as follows:

 

Formula example: 12(GPD)   =   .623 x 78.5' x .90 x .24" Daily ET Provided
1.00 x .90


By using the first formula with the information in the tables below, we calculate that the tree needs 12 gallons of water per day.

Different plant species can vary considerably in their rates of evapotranspiration. Some plants transpire large amounts of water, while others use relatively little. Values for plant factor or crop coefficients are sometime available through local sources such as Agricultural Extension Services and local newspapers. Check in your area. When available, this data tends to be more accurate than the tables below and can save time. Because there is such a wide range of water needs among different plants, the water use in Table A is divided into three user groups: high, average and low.

 

Table A1: Ks, a landscape coefficient

Vegetation High Average Low
Trees 0.90 0.50 0.20
Shrub 0.70 0.50 0.20
Ground Cover 0.90 0.50 0.20
Mixed 0.90 0.50 0.20
Turf Grass 0.80 0.75 0.60

Table A2: ETr and climate efficiency

Climate Definition (mid-summer) ETr (worst case, inches per day) Climate Efficiency (%)
Cool Humid <70 degree F > 50 % H .10-.15" 100
Cool Dry <70 degree F > 50 % H .15-.20" 95
Warm Humid <70-90 degree F > 50 % H .15-.20" 95
Warm Dry <70-90 degree F > 50 % H .20"-.25" 90
Hot Humid <90 degree F > 50 % H .20-.30" 90
Hot Dry <90 degree F > 50 % H .30-.45" 85
  H = Humidity    
ET varies as a function of the mix of plants, density of plantings, and the effects of microclimate.

Table A3: Estimated emitter uniformity

Emitter Type Poor Fair Good
In line dripper 65% 80% 90%
PC in line dripper 85% 90% 95%
Button dripper 60% 75% 90%
PC button dripper 80% 85% 90%
For more information on emitter uniformity ratings check-out Center for Irrigation Technology

Calculating Water Requirements Step 2
Step 2: Determine the number of drippers needed to apply 12 gallons per day to the 10' tree.

How many drippers should you use?
Deciding on the number of drippers to use is part science, part math, and partly a judgment call. Remember that you must wet at least 60% of the plant's root zone. When unsure of the number of drippers needed, always use more. You must find the proper balance based on the formula and actual site conditions. You also need to be aware of the number of drippers to use. If you have one 4-GPH dripper watering a plant, and it plugs and stops working, what will happen to your plant? On the other hand, if you have four 1-GPH drippers, the plant has a better chance to survive and to grow.

Formula # 2 Determine the number of drippers to be used per plant:

 

Square foot of plant root zone area   =   Number of drippers per plant
Square foot of dripper wetted area


Example: Our plant root zone area is 78.5 sq.ft. If we have sandy soil we can see from table 4 that you can expect 7-13 sq. ft of wetted area from a 1 GPH dripper. Let's stay on the conservative side and use 10 sq. ft. of wetted area.

 

78.5 sq ft.   =   7.8 drippers
10 sq. ft. with 1 gph dripper


The number of drippers per formula 2 for the 10 ft tree is 7.8 1-GPH drippers. Let's round it to 8 drippers.

Table A4: Wetting Pattern of Drip Emitter in Different Soils

Soil type Dripper flow rates (GPH) Wetted area(ft) Wetted area (sq. ft)
Sandy 0.5 1-3 1-7
Sandy 1.0 or 2.0 3-4 7-13
Loam 0.5 2-4 2-13
Loam 1.0 or 2.0 3-5 7-20
Clay 0.5 2-3 3-7
Clay 1.0 or 2.0 3-5 7-16
Clay 4.0 4-6 13-28

Calculating Water Requirements Step 3
Step 3: Determine the watering time of your system every day:

Formula # 3 Determining system run timer per day:

 

Plant water requirement(GPD)   =   Run time per day
Flow rates x Number of drippers per plant


Example: If the tree needs 12 gallons per day ( plant water requirement from formula # 1), divided by the number of drippers ( from formula # 2) x the drippers? flow rate, we have a run time of 1.5 hours every day, or 3 hours every two days

 

12 Gallons per day   =   1.5 hours
1 gph x 8 drippers


conclusion: Turn the water on for 1.5 hours every day using 8 drippers at a flow rate of 1 gallon per hour.