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.
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?
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