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You are here: Home > Sprinkler School™ > Lawn Care > Lawn Care Maintenance > How Much To Water

How Much To Water

Sprinkler systems are broken up into individual zones or sections that water specific areas. This provides the flexibility to enter in different watering times based upon the needs of the plant materials and the area being covered.

When entering a watering schedule or "program" into a controller, what’s the best way to determine the correct run time for each zone on your system?

Each system is different. Each landscape is different. Each soil is different. Plus there’s varying plant types and seasonal changes. All of these factors must go into an educated method of calculating "run times" for individual stations within a system.

You need to determine each zone’s precipitation rate (how quickly it applies water) and compare that to the area’s typical weekly irrigation requirements for residential lawns.

The more complex evapotranspiration method takes into account many potential variables and provides a more precise total run time requirement. This method requires a good understanding of the variables and how to apply the results.


Watering Schedules

There is a simple way to arrive at a watering schedule that will work for your lawn and landscape. The first thing you must know is how much water do you want to apply to a given area.

Does the plant material need frequent, light watering (such as newly seeded lawns or newly installed annual color beds), or do you need heavy, soaking applications to deep water existing shrubs and grasses? Your irrigation contractor may not be as qualified as a professional nurseryman, landscape contractor or agricultural extension.

Note that the total daily watering requirement does not have to be applied in one episode. Watering can be distributed over a given day in short bursts. Areas that are prone to runoff because of slope or hard soil conditions can be watered for shorter periods, sometimes spread over early morning watering and late evening applications. What matters is the total inches of water applied over the area during the watering day.

Another concern is "how much water can your soil absorb and hold during the watering event?" An inch of soil over rock can only hold so much water, then it begins to run off. Deeper soil will allow deep rooting of grasses and shrubs, therefore allowing longer periods between watering. The City of Austin requests that landscapes be watered every five days, some lawns cannot go that far between watering without suffering great deals of stress, and in some cases death. This problem is the best argument for choosing Buffalo grass or Bermuda for lawns. Both grasses can accept a greater deal of stress due to drought and, while yellowing out during such periods, they are most likely to respond by greening up again once watering is increased by normal rainfall or relaxed restrictions by local authorities.

Short frequent watering can lead to shallow roots and less stress tolerance. Those horror stories can be best told by your extension agency or landscape contractor.

Typical Watering Requirements For Turf

Texas A&M University has stated that any grass, St. Augustine, Bermuda, Zoysia, or even Buffalo grass will need about an inch and a half of water a week to look green and fresh during the summer months, which is the ideal in residential neighborhoods.

How long do you have to run a particular zone to give the area that inch and a half of water? The answer to that question comes by calculating the rate at which the sprinklers apply water, better known as the precipitation rate.

There are several methods used by professional engineers and irrigation designers to calculate precipitation rates, two of these are explained in the Precipitation Rates section


Zone run times

Determining Zone Run Times Using Evapotranspiration Data

The Handbook of Technical Irrigation Information produced by Hunter Industries provides formulas and other information that can aid irrigators in arriving at efficient watering schedules. The following information is derived from this book.

"The sprinkler run time formula calculates the number of minutes required to apply enough water to replace the water lost by evapotranspiration for a specific crop irrigated with a system at a particular precipitation rate and efficiency."

Followed by the formula below:

T= 60 x Eto x Kc
_____________
P
r x Ea

Where:

T = Sprinkler run time in minutes

Eto = reference evapotransporation rate, in inches

Kc = crop coefficient, percent

Pr = precipitation rate of the area, in inches per hour

Ea = application efficiency of the system, percent

60 = Constant for conversion of area, flow, inches per hour and inches per day into common units.

Example:

Determine the total monthly sprinkler run time for an athletic field with an total monthly ETo of 2.0 inches and a crop coefficient for the warm season turf of 0.70. The sprinkler precipitation rate is .50 in./hr. with an application efficiency of 75%.

T = 60 x 2.0 x 0.70
.50 x .75

T = 224 min. of total monthly watering time

Thus if the system is programmed to water on 10 days during a month, the zone watering time would be 22.4 minutes (1/10th) of the total monthly requirement.

Evapotranspiration Rates

Eto or the evapotranspiration rate is a value given to help determine the rate at which plants lose water through evaporation. Humidity, temperature and other factors are taken in account to determine the Eto rate at a given time. 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.

According to the National Climatic Center in Asheville, North Carolina, the total monthly Eto rates for the Austin area are as follows:

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0.05 1.0 3.0 4.5 6.0 7.0 8.0 7.5 5.0 4.0 2.0 1.0

Crop Coefficient

Kc or crop coefficient is a value assigned various plants that lose water at different rates. This helps adjust for soft leafy plants, like ferns, that would lose water faster than hard waxy leafed plants, such as groundcovers. Warm season grasses such a Bermuda lose water differently than cool season grasses such as rye. See also Texas A & M University's Potential Evapotranspiration data from a number of special weather stations located in central and south Texas.

Application Efficiency

Ea or application efficiency can only really be measured by a thorough sprinkler system audit. This will measure the uniformity of coverage by using a system of "catch cans" placed throughout each zone, running the system, reading the results, and crunching the numbers. A rating of 0.75 is considered a pretty good number and indicates an efficient system.

Do not overlook soil consistency and active root zone depth. All of these are considered when calculating a scientific schedule for watering a landscape.

 
   
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