August 08, 2015

How to Reduce Water Waste from Sprinklers: The Science of Spray

Everyone knows that water availability is tight this summer in our region due to the lack of snowpack and the extraordinarily warm temperatures this summer.  And water costs are very high during the summer in Seattle and some other cities.

So it is maddening to see folks waste large amounts of water by using their sprinklers mid-day when temperatures are highest and evaporation is largest.  But there is more, and I will talk about it a bit in this blog.
How much of the water is evaporating here?

When you water your lawn (or plants of any kind), there are a lot of ways for the water to be waylaid before getting to where you really need it:  the roots of your plants.  Let me list the big ones:

  • Water can evaporate in the air.
  • Water can be blown away by the wind
  • Water can run off on the surface
  • Water can evaporate on the surface before soaking in.

Typical home sprinklers lose 25-50% of the water due to these factors.  Not good.  But you can significantly reduce the losses with a little knowledge of some basic facts and Northwest weather.

The first issue is evaporation of the water droplets.

Evaporation of water droplets in the air increases when relative humidity drops and temperature rises. Below is a figure showing a measure of potential loss from evaporation, called vapor pressure deficit, as function of temperature and humidity.  The larger the deficit, the more evaporation.  As you can, at say 50% relative humidity there is nearly four times more evaporation potential at 90F than 60F.  

But the relative CAN vary over the 24-h day.  Here in the Northwest, our relative humidity (RH) is much lower during the day than at night.  At 90F with 30% RH there is nearly 10 times the amount of potential for evaporation loss than at night with 70% RH and 60F.  This has big implications for us, making the daytime particularly bad times to use sprinklers.

To illustrate,  look at the temperatures and relative humidities at Sea-Tac airport for the last 4 weeks.  This includes both warm and normal periods.   The temperature (first panel) show a large diurnal (daily) temperature change:  roughly 90 to 60F during warm periods and 75 to 55F  during cooler ones.

During all periods, there are large variations of relative humidity during the 24-day, from roughly 60-80% at night and 20-40% during the day (second panel above). Warm and dry during the day, cool and moist at night.

Clearly to reduce evaporation you want to water when the air is cool and the relative humidities are high.  The best time around here would be between roughly 3 AM and 6 AM, but doing it as early in the morning is still MUCH better than mid-day, when loss due to evaporation can be 5-10 TIMES AS GREAT.

Strangely, evaporation is LESS of a problem in torrid Florida because the relative humidities are so high all the time (at 100% RH there is NO evaporative loss).  That lack of evaporation makes one feel sticky and hot.  Better here!

There is another reason NOT to water during the middle of the day:  even if the water reaches the surface without evaporation, it can evaporate on the surface before sinking in.   Obviously, surface evaporation is much greater when the air is warm and dry.  Furthermore, if the sun is out and reaches the wet surface, that will greatly enhance evaporation by further warming the soil.

There is another factor that effects water loss and that is wind.   As noted above, wind can cause drift, whereby the water droplets move away from their intended target.  This can be a huge effect in strong winds.    In most places, and certainly here in the Northwest, the winds are stronger in the day.

Why you ask?  Several reasons.  During the day, the sun heats the surface causing the air to convect, which brings down stronger winds from aloft.   Also during the day, the differences in heating between land and water (and between mountains and low levels) cause diurnal winds, like sea breezes and upslope winds.   For example, here in central Puget Sound we have the late afternoon and early evening SOUND BREEZE.   In Ellensburg, where they irrigate like mad on the hay fields, westerly winds get much stronger during the afternoon.  I could give you a dozen more examples.

So it makes sense to water during the night or early morning when winds are light, so that the water droplets are not blown away.

The bottom line of all this is clear:  NEVER water between roughly 9 AM and 9 PM unless you want to help us run out of water or enrich local water suppliers.  Watering during the wrong times can cost you hundreds of dollars over a summer.

And there is more.  The size of the water drops in your sprinkler makes a big difference.  Small droplets (as in the picture above) have more surface area for the amount of water they possess, so it is good to have BIG DROPLETS.   So if your sprinkler is producing a mist (small droplets) rather than big droplets, you might find something better.   And high water pressure tends to produce small, wasteful drops.  A pressure limiter can help.

Finally, there are other issues.   It is far better to water less frequently but with greater amounts of water, an approach that fosters deeper penetration of the water into the soil.  And often the best times to water is AFTER IT RAINS.  No, I am not crazy.  Very dry soil can become hydrophobic, meaning it does not readily absorb water.   Thus, a light watering might stay on the surface without getting to the roots.   After an extended light rain, the soil can start moistening, opening the pathways of water into the soil. And watering after it rains, promotes deeper penetration of water into the soil.

Of course, the best thing is to avoid sprinkler technology completely and use drip irrigation, but that is not always practical, particularly for lawns.  And we need a movement for making dormant lawns the accepted norm in our region.   Perhaps we can push them as Golden Lawns, with anyone with a green lawn considered to be non-PC.

A "Golden Lawn"

A picture taken by John Switten on July 19 west of Boeing Field shows most folks in the community were letting their grass go dormant.   The golf course is quite a contrast.

And someone should figure out why the weeds stay green throughout the summer with no water.   Can't we figure this out and put the correct genes into into the grass DNA?


  1. I have to say, I am in favor of survival of trees, wild animals, fish, and birds, and I am less in awe of mostly-barren manicured grass. I don't give a whit about green lawns and actually think LESS of people and businesses that have them (yes, I'm looking at you, Microsoft, watering with waist-high/waste-high sprinklers at 5:30 pm on sunny days, getting your staff wet as they leave work but not doing as much as it should for your precious, evil grass!)

    Our "lawn" (required for neighborhood compliance) is brown and has been brown since April or May, very early this year and I don't care. The moss is still alive as it greened right up after the recent rain shower, so all is well. ;-)

    I do have some perennials that I am trying to establish; I water them deeply in the evening once or twice a week, either turning the hose on directly at their roots for ~10 minutes (the dirt is deliberately bowl-shaped around their stems) or carrying 10-20 gallons of water to them in buckets. The first minute or gallon I pour on quickly and then I wait five minutes before giving it the rest. This has worked really well, keeping these plants alive and growing, with their more-established neighbors benefiting a bit as well but the lawn not getting too much overspill and keeping the water bill minimal. Normal summers, I water plants like this once a week, but this has not been a normal summer!

    I have actually watered some of our trees for fifteen minutes a week this year, again with the hosepipe directly against the trunk. In normal summers, i have not needed to do this but the dogwood especially looks really parched this year and I want it to survive.

    We lecture each other to stop wasting water because "the salmon need the water" and try hard to keep it reasonable when it's tempting to use tons of water (really long showers at hotels with endless hot water).

    Ultimately, though, our water-reduction efforts are mostly about saving our household money, because even a 50% reduction in our use is not very many gallons. True water savings comes from industrial-scale users like golf courses, turf farms, and agricultural - farms and dairies - who reduce their usage or use water in ways that minimizes waste.

  2. I hand water my vegetable garden out of watering cans with obviously huge droplets. I put the water right where it is needed. My back is killing me this year. But it minimizes the weeding. Weeds don't sprout where this is no water. Imagine that. Especially this year in West Seattle...nine-tenths of an inch of rain since May the first. Yes...I added it up...

  3. The gene that grass has is to go dormant over the summer. Most weeds don't have that, they would die, therefore they have to stay green.

    (If all your grass had long taproots I don't think you'd actually be happy with it.)

  4. JewelyaZ aptly notes that we "lecture" each other about water usage ~ and that quoted word is the unfortunate reality. No one wants to be lectured. No one wants to be patronized or spoken to from the Soapbox of Those Who are Doing Oh-So-Much Better When it Comes to Saving the Planet. Those who are the worst offenders are not going to respond positively to that sort of approach. Suggest, yes. Encourage, definitely. But lecture? Eh, don't waste the breath & finger-wagging.

  5. Cliff - you have been one of the few (in fact one of the only) in the scientific community calling for calm and reason throughout this water year. And now we get this? SPU (as you specifically noted in an earlier post) stated clearly in May that they could be forced to raise rates if people stopped using water normally in the summer (and that does include lawns for some in our community). Those rate increases could have come as base increases which would affect virtually everyone, not just heavy water users (in fact heavy water users would likely be the most unaffected). This is from SPU, the engineers and professionals who run our water resource. You were one of the few who pointed that out while others were running around crying wolf and scaring people.

    Btw, SPU has one of the most progressive rate structures in the country to prevent water waste. You have pointed that out as well.

    This post just doesn't sound like the same guy. Why?

  6. JewelyaZ- don't water your trees directly against the trunk, most of the absorbing roots are at least several feet out from the trunk near the dripline. Also it's not good to wet the trunk for prolonged periods. Glad to hear you're watering your trees though!

  7. Colleen, I'm talking about in my family, and yes, I will lecture my kids to remind them of what they already know, not only because they have seen the low reservoirs and the fish struggling (other years), but because they know the rules of this house. I can't change other people's behavior but I do try to get them to think about the impacts of their choices. My electric car is probably the biggest conversation-starter in that direction. :-)

  8. WTF does "PC" have to do with any of this. Does the 'P' mean Political, Polite, Pansy, or Puget?

  9. @sunsnow12 - I don't see your point. This is factual information about minimizing waste of water in keeping plants alive. The water utility rate structure or costs didn't factor in Cliff's piece. All this information except maybe the meteorological details is well known to gardeners who are concerned about conservation of limited natural resources like water.

  10. Green lawns and plants help stop fires. There is fire danger to letting it all go dormant and dry out.

    The sleepy hollow fire couldnt spread in one direction because of a green orchard.

    There is a balance, and right now it's pc to be anti-green grass. I am going to water my lawn and make a fire resistant space around my home because it's the smart thing to do in my area.

    If a fire rips through your neighborhood, dont blame the fire dept. Their priority is life, not your house.

  11. Kathead,
    Fire doesn't spread over a shortly cropped lawn, even if it is dormant. So I don't think that is an issue. We are talking about urban lawns...cliff

  12. My city gets water from Spada Lake which right now is at
    1406.6 feet. Normal lake level is 1,420 feet to 1,450 feet\
    and an article in the Everett Herald had this quote
    "Between Spada and (Lake) Chaplain we've got about a seven-
    month supply in the reservoir. That's without any more measurable

    I think we are likely to get quite a lot of rain in the next
    seven months--especially in months #4 through 7 from now.

  13. Ruth – I agree, it does include factual information, absolutely. It is a hallmark of this blog and one which makes it highly valuable in this day and age. What I was referring to was this: “And we need a movement for making dormant lawns the accepted norm in our region.” That is not factual information, it is opinion. And it is Cliff’s blog, so he is entitled to it. But it indirectly contradicts many of the very facts he has stated previously regarding our water supply this year. I also, personally, feel that that particular movement – to shun people who enjoy a green lawn between gardens, or a place for children to play in their yard – is divisive in our community. Do we really judge everyone based on their yard? The size of their garbage can? The cars in their garage? Just exactly when does anyone become a friend and not an enemy, only when they are exactly like us?

    I also take issue with the lead statement: “Everyone knows that water availability is tight this summer in our region due to the lack of snowpack and the extraordinarily warm temperatures this summer.” Extraordinarily warm temperatures, yes. But as Cliff has pointed out multiple times, the snowpack situation this year has had virtually zero impact on our water supply. Our reservoirs were full because precipitation was normal and our water managers responded. I could elaborate on that further but Cliff has articulated it better. So why lead this article with that, when it is clearly not true, and reinforces misinformation that “everyone knows” but is in fact wrong?

  14. I never water my lawn and my lawn is green, but it is 100% weeds.
    Weeds look alright, when you keep them trimmed with the lawnmower.

  15. This is all a very foreign idea to me - grass going dormant in the summer, turning green in the fall, and turning brown in the spring. On the East Coast, it's the opposite. Grass goes dormant in the winter, and is lush and green in the summer. It seems a little counterintuitive to have grass go brown in the spring; normally we associate spring with renewal and rebirth, not dormancy. On the East Coast, we go from a completely barren landscape (brown grass and leafless trees) to a gradual "greening" of everything: grass, flowers, trees, etc. So, in Seattle, in the springtime, the trees turn green, but the grass turns brown? Is that the case? That'll be really interesting to see.

  16. To be blunt, I do not believe a single thing that city official tell me. Not only that, but they hate anyone who owns a single family home. I'm not going to make any changes whatsoever.

  17. Soaker hoses are an easy way to water plants, flower beds, and shrubs without generating spray. I snaked a couple around the beds in my yard and leave them in place hooking them up with an extension hose when I want to water.

    I don't generally bother watering the grass. Summer makes a good time to weed since the green dandelions stand out so boldly against the dormant lawn.

  18. More people need to understand how it is that they can reduce water waster from their sprinklers. This is even something that I have been wanting to know about myself. Just hope that I can so I can use my sprinklers as efficiently as possible. Like you said though, there is a science behind it all and you need to make sure that you go about things the right way.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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