are there any rules of thumb for evaporation of water?
I have a water feature that the water levels goes down, about 1/2 inch a day in full sun.
consists of a lower pond (~100sq ft), falls (~3 ft) into upper pond (30sq ft) small falls into 50′ stream (1,5 ft wide) into lower pond via small falls.
have check for leaks and suspect that between splash from large falls and evaporation is where the water is going, but not sure.
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bobl Volo, non valeo
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Replies
There are far too many variables to determine anything close to your own situation's evaporative rate, but you're right is assuming that yours is mainly due to splash and agitation of the water. As we know, surface exposure to air increases the evaporative process, hence, splashes and agitated water, which creates a huge number of water droplets, both large and small, of which each droplet's surface is exposed to the evaporative process.
The site below might help:
http://www.grow.arizona.edu/water/temperature/evaporation.shtml
Sonny is right. Sprinklers, falls, and splashing can increase the amt lost to evaporation by as much as ten times. That is why flood irrigation is more efficient in dry climates vs sprinklers, and why lawn waering is more efficient at night than in the full heat of day.
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I have dealt with precisely this problem. I called up the weather bureau and they said 1/4" per day is the typical loss from evaporation [in San Diego]. The trick is to call the library and they will give you the true number for the weather bureau so you can talk to a real person.
Our lake was considerably losing more than 1/4". Efforts were made to check for leaks in the pipes, ling of the lake, etc. New coating was applied [Sika-Dur or Coutouliet]. Eventually a well was installed to supply the lake.
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The vapor pressure of water doubles for every 21F increase in temp. So at 102F, air can hold 4 times more water than at 60F. But the driving force is the difference between what the air CAN hold and what it DOES hold, i.e. the relative humidity.
Wind vastly increase the evaporation rate. In still air, the layer of air closest to the water gets saturated and for more evaporation, water vapor must difuse through that stagnant layer. Wind constantly replaces that layer of air with new, dryer air. Moving streams, falling cascades and droplets create their own wind.
Sounds like you have enough water movement to account for the losses you are seeing. Can you turn it off for a day and see if the losses drop to the 1/4" per day range?
This is shooting from the hip so be forewarned.
I remember reading a thread about mosquito habitat control and some suggested a baby oil skim the surface. Would this cut down the evaporatoin? Might filters and pumps get clogged up?
Jon Blakemore
A low volatility oil would 1) eliminate mosquito breeding and 2) greatly reduce evaporation.
But with moving water there are complications. More oil would dissolve in the water. Some would move in the water as suspended micro-droplets. And, if the oil ever got to the pump inlet, the impellor would emulsify it (looks like chocolate mouse').
Adding oil to the surface largely eliminates evaporation but it also largely eliminates oxygenation of the water. Without O2, the animals and some plants die. And the dead material decays anaerobically (stinks to on high) rather than aerobically.
Fire water in a storage drum? Yes, a bit of mineral or vegatable oil will eliminate the mosquito issue. Moving water in a decorative pond? I wouldn't add oil.David Thomas Overlooking Cook Inlet in Kenai, Alaska
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Jon Blakemore
bobl,
Judging from my experience with my small pond (25 foot diameter), 1/2" per day is very realistic for evaporation.
My pond has no moving water It seems that the fuller the pond is, the faster it goes down. Presumably because there is a larger surface area. On the rare occasions it has gotten extremely low (so now it is almost just the center deep hole) the evaporation rate drops off dramatically (because the deeper ground keeps the water cooler??)
Rich Beckman
Another day, another tool.