Estimating Turf Water Use, Part 2, How to Arrive at Sprinkler Run Times

By Michael Vogt, CGCS

In the first installment we calculated several variables that are necessary to arrive at how much water the plant may need in a given time. These calculations are needed to arrive at the irrigation systems ability to meet those needs.

It’s vital to know what the Plant Available Water (PAW) that will exist in the root zone, expressed in inches, as well as the Management Allowable Depletion (MAD) expressed as a percent. AD is a factor used to trigger an irrigation event. The formula for arriving at an Allowable Water Depletion is:
       
                          AD = PAW x MAD

AD = Allowable Depletion

PAW = Plant Available Water

MAD = Management Allowable Depletion

Distribution Uniformity

Distribution uniformity (DU) is the measure of how uniformly water is applied and made available over a given turf area. DU is expressed in percentage form and generally represents the major component of irrigation efficiency. DU is also a statistical value measuring uniformity of coverage, ie, a DU of .8 denotes a DU of 80%.

Irrigation efficiency is affected by both site management and equipment at the site. Uniformity is related to the design and mechanical performance of the irrigation system.

Estimated DU for sprinkler types

During an irrigation audit the nozzle size of a rotor sprinkler and operating pressure should be determined.The best way determined nozzle size is by using a drill bit inserted into the nozzle orifice.

Run time mulitplier as a function of DULQ

Flow

Station flow can be determined by the following equation:

GPM = 28.62 x D² x √P

GPM = Flow Rate of One Sprinkler

28.62 = Conversion Factor

D = Nozzle Diameter
P = Pressure at Nozzle

Precipitation Rate (PR)

Precipitation Rate (PR) is the rate at which irrigation water is applied per unit of time, usually in inches per hour. Depending pressure, spacing and type of sprinkler selected, each individual sprinkler may have a different precipitation rate. If the flow rate of a specific irrigation area is known it is possible to estimate the average gross precipitation rate, also referred to as application rate.

PRgross = 96.3 x Q

              HS x RS

PRgross = Gross Precipitation Rate in Inches per Hour

96.3 = Conversion Factor

Q = Flow Rate from one Sprinkler

HS = Spacing Between Sprinkler Heads (ft)

RS = Spacing Between Rows of Sprinkler Heads (ft)

Net Precipitation Rate

Net precipitation rate is measured as the amount of water that actually reaches the turfgrass stand. The net is the gross less the losses that occur between the nozzle and the stand of turf.

PRnet = Vavg x 3.66
            TR x CDA

PRnet = Net Precipitation Rate (inches per hour)

Vavg = Average catch volume (milliliters)

3.66 = Constant that coverts milliliters to inches³

TR = Testing Run Time

CDA = Catch devise throat area (Inches²)

Run Time Multiplier

The Run Time Multiplier (RTM) is used to increase the number of minutes that would be required to apply a given amount of water depending on precipitation rate of the sprinkler and to compensate for a lack of perfect uniformity. The RTM also accounts for lateral water movement in the soil. RTM is based on the equation:

RTM =                 1                

                0.4 + (0.006 x DULQ)

RTM = Run Time Multiplier

DULQ = Lower Quarter Distribution Uniformity (Percentage)

0.4 And 0.006 = Constants used because we catch water before it reaches the surface and can move laterally in the soil.

Base Run Time

To calculate Base Run Time for a sprinkler zone, the plant water requirement (PWR) is divided by the precipitation rate of the zone. Multiply by 60 to convert faction of an hour to minutes.

RTb = PWR x 60

   PR

RTb = Base Run Time in Minutes (Minutes)

PWR = Plant Water Requirements (Inches)

PR = Precipitation Rate (Inches per Hour)

Example: where plant water requirements are 1.20 and precipitation rate is 0.59 then base run time would be 122 minutes per week.

Adjusted Run Time

To compensate for the lack of uniformity in the sprinkler zone, additional time will be required to be added to the irrigation schedule. The Run Time Multiplier (RTM) is used to calculate the total number of minute’s necessary based upon this equation:

RTadj = RTb x RTM

RTadj = Adjusted Run Time

RTb = Base Run Time

RTM = Run Time Multiplier

Example: If base run time is 122 minutes and the run time multiplier is 1.22 then the adjusted run time would be 149 minutes.

Maximum Run Time per Cycle

To arrive at a Maximum Run Time per Cycle to avoid run-off the best procedure is to time a zone to see how long it takes for water to run-off the site.

Maximum Run Time per Cycle = Infiltration Rate (Inches per Hour)  x 60      

                                      Precipitation Rate (Inches per Hour)

Example: Where there is an infiltration rate of 0.75 inches per hour and a precipitation rate of 0.59 inches per hour the maximum run time would be 76 minutes before run off would occur.

Summary

The importance of the actual catchment test to determine the DULQ is critical in the above calculations to arrive at a runtime for weekly irrigation time and cycle requirements. Once the irrigation system is programmed with run times based on these equations you’ll benefit from reduced water usage and be confident that the irrigation system has been programmed with the greatest of accuracy. And when ask the question of how you arrived at your water use quantities you can honestly say that the system was programmed within guidelines set forth by the Irrigation Association to the strictest irrigation accepted standards and best management practices.

References:

1.Aronson, L.J., A. J. Gold, and R.J. Hull. 1987a. Cool Season Turfgrass Responses to Drought
Stress. Crop Science. 27:1261-1266
2.Carrow, Robert N. 1995. Drought Resistance Aspects of Turfgrasses in the Southeast
3.Web resource, http://www.fao.org/docrep/X0490E/x0490e06.htm accessed 8/14/2001
4.Mecham, Fortier, Moore, Berrett, Thompson, McKernan, LaFleur, Norum, 2003, Certified Golf Irrigation Auditor Manual