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The Practical Gardener


How to have a green lawn with less water

Posted on August 30, 2010 by Paige

They sure aren't letting this lawn go dormant!

The key to having a green lawn with less water is to pay attention and do a little digging.

  • Make sure you’re giving your grass only the water it needs to stay green and healthy – no more. You’ll need to know your grass type and its typical water needs, and your soil type (sandy soils dry out more quickly than clay ones).
  • Check the soil before you water – dig down through the root depth of the grass – have things dried out?
  • Only water to a little bit below the root depth of the existing roots – again digging is the best way to find this out.
  • Know how long it takes your sprinkler (irrigation system) to lay down 1″ of water.
  • Realize that water loss (and therefore its replacement needs) varies throughout the season so you can’t just say, “I’ll add 1″ a week from June-September.” Sometimes that 1″ will be too much, others too little. A good tool to use to figure out how to adjust your watering is to look at the evapotranspiration rate in your area.

Evapotranspiration is the amount of water that evaporates from the soil and is transpired by plants – the water loss from the soil in other words. Of course these rates vary from place to place even within a small area. Is the soil mulched? What kinds of plants are growing and how densely? For example,” an acre of corn gives off about 3,000-4,000 gallons (11,400-15,100 liters) of water each day, and a large oak tree can transpire 40,000 gallons (151,000 liters) per year.”*

Nothing beats knowing your own yard but if you’re not willing to dig around in your lawn, using the evapotranspiration rate and the effective rainfall (if you can get that number, the table below has it as about 1/3 of total rainfall) will allow you to estimate pretty well how much you should water.

Average ET, Actual and Effective Rainfall and Irrigation Index for the Seattle area (for the irrigation season)
Month ET Actual Rainfall Eff. Rainfall (33%) Irrigation Index
April 2.38 2.87 0.89 53%
May 3.14 2.60 0.86 70%
June 3.50 2.00 0.66 78%
July 4.46 0.52 0.17 100%
August 3.71 1.11 0.37 83%
September 2.54 1.29 0.43 57%
Totals 19.73 10.24 3.38

(Information in table from the Irrigation Water Management Project and the Saving Water Partnership. See http://www.iwms.org/seattle_area.asp)

Note the huge difference in actual rainfall (what ends up in the rain gauge) and the effective rainfall (what your plants can use) and compare that to the water loss (the ET rate). Cumulatively, from April to September, water loss in alost 20″ and what falls naturally and is available to plants is only 3″ and change. The rest you need to make up. Mulching will help retard evaporation in garden beds but you can’t really mulch the lawn so you need to replace that loss.

If your lawn is irrigated via an automatic timer, you need to change the settings over the course of the watering season or you will water too much (or not enough).

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