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Article 40: Water Returns - Evaporation, by Jeff Richardson

Posted by Jeff Richardson on

The water returns play a huge role in evaporation on your pond.  While water is in motion, depending on the return, you are greatly increasing water temps, surface area, aeration and surface movement which all contribute to water evaporation.

What is Evaporation?: Solid water (snow or ice), or liquid water can change to gaseous water (vapor) and float away in the surrounding atmosphere over your pond.  Condensation is the opposite of evaporation and liquid or solid water is returned from the atmosphere back into your pond.  Believe it or not, both processes are happening all the time at different rates but for the most part this entire process is invisible as it is happening on a molecular level (except during obvious precipitation events such as rain, snow, dew, etc.).  Evaporation can happen even when it is freezing outside (albeit at a very slow rate).  This rate of which this happens depends on your local weather, water temps, total surface area of your water feature, water movement, and aeration. 

Pollutant Levels: As discussed in previous articles, your water feature will dirty over time, not just by external sources, but from within.  The more volume your water system has, the less chemical changes will have an effect on the overall.  Example: a drop of dye in a cup of water compared to a drop of dye in an ocean – you can see how the greater the volume of your water system, the greater ratio of water compared to nitrates, fish and plant waste, environmental heavy metals, minerals, and chemical swings.  But make no mistake; pollutants do not “evaporate” with the water.   Therefore, although the pollutant levels do not change per se, the amount in a gallon of water rise into higher concentrations as the water evaporates from the pond – which can become toxic.

Base Factors of Evaporation: Firstly comes the temperature of the air and water (and the differences between the two).  Secondary is the air pressure which changes with elevation and of course with the ever-changing weather.  And thirdly is the dew point (the tipping point where condensation begins).  Humidity, precipitation and temperatures will affect this.  Your “outdoor” water feature will be subject to all these natural phenomena with little control causing evaporation.  

System Factors of Evaporation:  Sadly, many of the best practices you design into your pond system to make your water happy and healthy also adds to evaporation.  You may want to reduce evaporation, but not at the detriment to the overall wellbeing to the flora and fauna of your pond.  First is surface area – the more surface, the more evaporation.  The surface is the place where the evaporation happens.  Of course, as discussed about climatic zones, weather, and location, the amount of sun on the surface will conspire to increase evaporation, temps and in turn, raise pollutants.  Next, the less volume of water in your system, the more susceptible the water is to swings in pollutants and temps.  Returns that you are designing are another huge cause of evaporation.  The pump discharge and moving the water causes friction which changes water temps.  Mixing air into the water causes huge amounts of increased surface area.  Every bubble is a sphere of surface area.  The return is usually a shallow volume of water with extensive surface area (fountains, falls, stream beds, etc.) – you are exposing nearly all the water to the air.  Splashing will also take a small amount of water out of the pond and lay it flat on a stone – the wet spot is easily evaporated as it is all surface area and almost no volume.  Aerators create lots of surface area, change water temps, and move surface water.  In extreme climes (like a sunny desert with low humidity), you may have to consider shading the pond, cooling the water, pick a return that gives you enough aeration without excessive evaporation.

Calculating Evaporation: You can go online and find water surface evaporation calculators.  Unless we are using an indoor aquarium inside with controlled air temps, water temps, air humidity, etc., you would likely have to take a multitude of measurements every day outdoors – and even then it will change constantly. Rather, try using the following estimates.  Mark the full surface of the main pool or reservoir so you know exactly where the water surface should be. Turn off any auto filling valves.  Do not manually fill.  Choose a week when no large rain events will likely occur.  Calculate the following:

  • Multiply the total discharge at the top of your return by .01 (this is GPD lost while the pumps run - ~1% baseline)
  • High humidity, wet or rainy conditions, still water or air, shady pond, cool temps (-½% to baseline)
  • Low humidity, dry, full sun, windy, high temps, long returns, large surface area with shallow pools (+½% to baseline)
  • Multiply the total surface area in feet by .62 (include return areas – this is the top inch in gallons of your pond)

After a week, measure how far the water has fallen from your full water mark.  You will be able to see if it is what is expected (½%, 1% or 1½% water lost per day).   Adjust your percentage, refill the pond and do a second measure. 

Replacing Evaporated Water:  All pond systems will lose water to evaporations – period.   As you can see from above, how much and how fast will have MANY factors that will likely change often.  Your returns will be probably the largest source of “controllable” evaporation.  In earlier articles when planning your potable water system to add fresh water, I suggested a water meter, water timer, auto and manual fill valves, an overflow to counter this.  Your bacteria does not like large swings and if you have fish, even less so.  Gradual is the key word here.  Using an auto fill valve will constantly keep the pond topped off (but use a timer so it doesn’t mask leaks).  If your evaporation is less than 5%, you want to change the difference weekly (without fish).  If you have goldfish or other hearty breeds and not overstocked, consider changing out 10% per week.  If you have Koi or a large fish load, go as high as 15% to 20% per week.  A water meter, staff head gauge and timer will assist you with this and help you determine water costs.

Previous Article: Water Returns – Capacity

Next Article: Water Returns – Beds

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