Last week Wisconsin’s Governor Walker pronounced drought emergencies for 40-plus counties in the land of cheese. I’m not sure what that means other than the obvious fact that it is dry and it’s been hotter than bejeezus for quite some time, with the peak being the week of Independence Day. A “blocking” high pressure system was parked in the middle of the country. High pressure systems result in sinking air, the opposite of low pressure systems where surface air rises to cold altitudes causing clouds and rain. Sinking air results in no clouds and heat of compression. Heat of compression may seem ludicrous until you consider that just a half inch Hg (mercury) increase in atmospheric pressure (about 1.6%) would increase the temperature by about 9 degrees F, all else equal.
But back to the drought emergency – this I read/heard allows farmers to take water from streams and lakes for irrigation, which like the President traveling to a natural disaster site, sounds like a good idea but is pretty much worthless. It is worthless because who has equipment lying about to suck water out of a stream/pond/lake and water their crops and pastures?
But let’s consider irrigation from city water or privately owned wells. Wasting resources, the basis of many rants, drives me nuts and I frequently see incredible waste in many irrigation operations from lawns to crops. I know little about irrigation “technology” but I am a mechanical engineer with some knowledge of fluid mechanics, heat and mass transfer. I view some systems and curse the waste.
There are generally two inefficiencies with certain irrigation systems. One costs nothing to implement and saves gobs of water and electricity and the other may cost something and saves gobs of water and electricity.
As shown in the first photo, the center pivot irrigation system is common for many types of crops. The grotesque waste with this thing pictured is threefold. First, it is a high pressure system, which obviously wastes pumping energy. Secondly, since it is a high pressure system, it virtually atomizes the water. Hint: efficient humidifiers generate tiny water droplets to evaporate in air with no heat input other than from the air itself. That’s what this thing is – a giant humidifier. It’s supposed to be watering the crop and instead it’s spraying water in tiny droplets into the air and watering the atmosphere. Lastly, it runs during the middle of the hot, sunny day. With the tiny droplets in the hot sun, half the water evaporates before it lands on the crop below and another large percentage evaporates before it drops from the crop foliage to the soil, where it is needed.
I see the same thing happen on smaller scales in city gardens and lawns. Sprinklers and spray heads pumping out a mist during middle of the day, adding moisture to the air and not so much the soil as intended.
One saving grace is golfers don’t like to get wet while playing so courses are irrigated at night and so are many corporate campuses and municipal parks and boulevards. This atomizer sprinkler head (second photo) is common and like for crops, if used during any part of the hot sunny day, is going to result in half the water evaporating, first in the air, then before it drips from the grass to the soil.
Think of it this way: fog isn’t water vapor. Water vapor is invisible at close range. At long range it causes haze but haze never results in driving risk due to poor vision. Fog is actually tiny water droplets aloft. Fog doesn’t work very well for adding moisture to soil, does it? The head pictured works like fog to get even distribution. The spray pattern is fixed so it relies on whatever wind current there is to spread it round. Good grief.
Without geeking into engineering babble, the driving force for evaporation is much greater in the heat of the day when the relative humidity is low. In only the freakiest situations, if ever, is relative humidity lower when the sun is hidden by clouds or the earth (night).
Summarizing, to save water and energy when irrigating:
- Irrigate at night or early morning when relative humidity is highest.
- Use low pressure sprinkler heads producing large water droplets that have low surface to volume ratio to minimize evaporation. This also reduces pumping energy for large pumping systems dedicated to irrigation only.
- The end.
A great option for homes is the tractor sprinkler shown below. It may look like a cute toy, but it is actually self-propelled machine that follows the garden hose as shown. This helps avoid overwatering if you forget to move the sprinkler while watching SpongeBob reruns. It also squirts relatively large water droplets for low evaporation waste.
 State and federal executives traveling to natural disasters is stupid, but hey, they “run the country (or state)” and so they will make it better – not. It’s an expectation from the ignorant masses.