Calculating Drip Irrigation Water Requirements

Title: Calculating Water Requirements
Author: Drip Depot Staff

There are two basic methods in determining how much water to apply.

The first method "Calculating water requirements for an individual plant", calculates how many gallons per day to apply to an individual plant. This method is typically used to figure water requirements for individual trees and spaced plantings, and requires the use of the three formulas below
The second method "Calculating water requirements for dense plantings", is used to determine how many inches per day to apply, and is typically used for dense plantings, such as ground cover and flower beds.

Method #1 - Calculating water requirements for an individual plant

First formula to determine gallons per day per plant

Gallons per day per plant =	.623 x Plant Area x Ks x ETr
	Climate efficiency x Estimated emitter efficiency (in decimal form)

Ks=Landscape coefficient. (Table A3)
ETr=Reference evapotranspiration rate(inches per day). (Table A1)
Climate efficiency(Table A1)
Estimated emitter uniformity(Table A1)

If you have all the information concerning the individual plant site, the following pieces to this puzzle fall into place. For example: we have a tree with a 10' diameter of canopy (equivalent to an area of 78.5 sq.ft ) in a cool, humid climate (the canopy area is the plant root zone area)

The formula for sq.ft: 3.14159 x radius x radius = plant area in sq.ft

We find out our tree needs 12 gallons of water per day. Now we need to determine the number of drippers needed to apply 12 GPD to the 10' tree.

Table A1 - 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 A2 - 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%

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 and can save time. Because there is such a wide range of water needs among different plants, the water use is divided into three user groups: High, average and low ( see table A3, below)

Table A3 - Landscape coefficient (Ks)
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

Chart C: Distance you can run your drippers on 1/4" or 1/2" drip line

Maximum length run of 1/4" micro tube and 1/2" drip hose with button and flag drippers
Flow Deviation 7%		Dripper spacing in feet on 1/4" micro tube					Dripper spacing in feet on 1/2' poly hose
Products	Flow rates	1'	2'	3'	4'	5'	1'	2'	3'	4'	5'
Button dripper	1 gph	17	26	36	24	45	140	230	310	350	400
Button dripper	.5 gph	26	40	50	64	70	195	320	430	525	610
Button dripper	2 gph	10	16	21	24	30	80	145	175	230	270
Flag dripper	1 gph	17	26	36	40	45	125	220	300	330	370
Adjustable dripper	1 to10 gph	Depend on flow rates

Maximum length run of 1/4" micro tube or 1/2" drip hose with PC dripper
Flow Deviation 7%		Dripper spacing in feet on 1/4" micro tube					Dripper spacing in feet on 1/2" poly tube
Products	Flow rates	1	2	3	4	5	1	2	3	4	5
PC dripper	1 gph	35	50	72	88	105	320	530	670	820	970
PC dripper	2 gph	25	36	45	56	65	190	310	420	510	610
PC dripper	4 gph	14	22	30	36	40	120	200	250	320	450