How much water will I use to irrigate my property?
by Damian Zawacki,
John Deere Landscapes Technical Training Manager

I hear this question all the time. It’s impossible to predict the weather thus it’s impossible to predict exactly how much water we will use. However, there are some numbers that we should know to help us with a theoretical amount.

Determining how many square feet we are irrigating is the first item we have to do. Measuring the length and width of each turf area and then multiplying them by each other, will give us the square footage. Suppose our front yard is 60’x100’, back yard is 100’x100’ and lastly our two sides combined are 100’x55’. We multiply the three sets of numbers and add each set together. Thus, 6000 sq. feet plus 10,000 sq. feet plus 5500 sq. feet totals 21,500 square feet. Now, we have to determine how much water we will use for 21,500 square feet of turf.

The big question of “how much water will we use” is called the “Evapotranspiration Rate.” Evapotranspiration (ET) Rate is a fancy way of saying “how much water did we lose for the day?” It’s a combination of transpiration through the plant and evaporation from the soil and plant. Obviously, the ET Rate is constantly changing. Temperature, humidity, wind, etc, are all factors affecting our ET Rate. Essentially, we want to replace the same amount of water each week with what we lost to ET (for the week).

This is where the prediction comes in (however, using historical data can help). Researching the historical ET Rate for our area with the help of irrigation or landscape associations is one of the easier ways to find our ET. This historical data is usually numerical data given in a daily, weekly or monthly amount.

We will base our number on a weekly amount and use a nice even number. If we need more or less water, we can multiply it out later. The number we will use is 1” a week. It’s a nice round number and very easy to use. Once again we can adjust it up or down depending on what part of the country we live in, what time of the year, what material are we irrigating, etc. Remember, 1” is not etched in stone and will have to be adjusted weekly or possibly daily.

The last two numbers we must know is 43,560 square-feet & 27,154 gallons. 43,560 are how many square feet are in an acre. In our earlier example we were irrigating 21,500 square feet. 21,500 divided by 43,560 is .493 acres, thus we are irrigating (approximately) a half-acre. The last number is 27,154 is how many gallons of water are in an acre-inch of water. We are irrigating a half-acre (in our example), so lets now do the math.

27,154 gallons for an acre, divided by two is 13,577 gallons for a half-acre. Thus, we will use 13,577 gallons of water per week (if we had an ET Rate of 1” a week and we are irrigating a half acre). If we discover our ET Rate to be 2” a week, then multiply our answer by two. Again, it all depends on the time of year, climate, etc. Actually doing the math is the easy part. Figuring out our ET Rate is the difficult part. Since it’s constantly changing day-by-day, it’s best getting the data from a reputable group or local weather station.

As usual, irrigate early in the morning when the daily ET Rate is at its lowest and only irrigate enough to replenish what we lost due to ET. Any additional irrigating will cause water wastage. Please call John Deere Landscapes if you have any questions.