Are Pan Evaporation Rates applicable to pools?

[JoyfulNoise]

After seeing several posts with users wondering how certain chemicals could build up in their water over time (e.g., salt and CH) and considering my on-going plans to eventually install a rainwater collection system from my flat roof gutters, I got to wondering about the applicability of pan evaporation rate data to pools. If the monthly average pan data for a given area is X inches of water, then what is a good correction factor to apply for that?

I won't bore anyone with the details but simply point you to a good primer on the science of Class A Evaporation Pans from our good friends at Wikipedia. There is also some good, State-by-State data of measured pan data HERE. It is possible to evaluate evaporation rates from large bodies of water theoretically using Penman's Equation (see here and here) but I don't think that is necessary when it comes to pools. For the most part, evaporation pans are typically used in agricultural sciences to measure the evaporation rates near a large crop field. One then applies a correction factor (typically 0.7 to 0.75 for moist soil) to the pan data to determine evaporation rates. There are also commercially available atmometers (see here) for measuring evapotranspiration rates, but they typically cost hundreds of dollars. Some have devised simple, DIY atmometers (see here and here), but again it would be preferable to simply use existing data especially when trying to help people who post and may not realize that sometimes evaporation and refill makes a difference.

Of course, I could simply attach a 3/4" brass water volume meter (here) onto my autofill line and measure evaporation that way. I've tried the measuring stick in the pool method but find it unsatisfactory.

 

[mas985]

They are better than nothing but there are a few factors that change the evaporation rates from the pan rates.

The pan rates are usually shown as annual averages and the ones I have seen include rain fall which is fair. The problem with this is it does not reflect the variability in the evaporation rate that occurs over the year. For me, our pan rate is 60" per year or a little over an 1" per week. That is a pretty good average from when I have done a measurement but the worst case I have seen has been close to 3" per week. So worst case can be over 2x the average.

Second, pools are sometimes heated which of course increases evaporation so that will definitely be inaccurate.

The heat transfer spreadsheet in my signature attempts to calculate the water temperature and evaporation rates given the local weather conditions for a particular series of days. I have calibrated it with my own pool and seems fairly accurate and gives a better indication of evaporation given pool heating and local weather conditions.

 

[JoyfulNoise]

Mark,

Thanks for the response. The second link to the Western Regional Climate Center has pan data in a more granular, monthly form along with an annual total. There's two data points for Tucson and they cover a very good span of time -

TUCSON UNIV OF ARIZONA | 1894-2005 |
TUCSON U OF ARIZ # 1 | 1982-2005 |

But I guess the pan data is, at best, an "average" scenario with lots of possible variance. As for my personal circumstance, I rarely ever heat the pool (costs too much) and, for the last few seasons, have had it completely open to the atmosphere. This winter I plan on closing the spillway & covering the spa, adding on separate spa run times to the schedule from the main pool and running the IntelliFlo on only enough RPM's to move the water and run the suction side cleaner. I think I'll invest in a bubble cover in the Spring and just cut it up into three sections (my pool is freeform and therefore a real pain to cover).

As for water conservation, I think the first step would be to get a water flow meter on the autofill line and play more with covering the pool. I'll have to save for the rooftop water collection drums but the company that did my gutters also does water harvesting. So I'm hoping to add that to the house as a means of offsetting some of my fill water demand and for lowering my constantly rising CH.

Reagrds,
Matt

 

[bdavis466]

Not that I'm keeping tabs on you, but your posts indicate that your CH has been at 750 for quite a while. Have you done something to maintain that? I'm just curious that you may have finally found the calcium eating bacteria you talked about and haven't shared with the rest of us!

My calcium level continues to rise alarmingly fast. In 6 months it has risen 425 ppm. I will more than likely be dumping water this winter, which is very sad since the pool didn't even make it a year on its initial fill. The fill water isn't horrible either at 125.

I too am considering a rain collection type of system since a cover is not feasible

 

[JoyfulNoise]

Not that I'm keeping tabs on you, but your posts indicate that your CH has been at 750 for quite a while. Have you done something to maintain that? I'm just curious that you may have finally found the calcium eating bacteria you talked about and haven't shared with the rest of us!

My calcium level continues to rise alarmingly fast. In 6 months it has risen 425 ppm. I will more than likely be dumping water this winter, which is very sad since the pool didn't even make it a year on its initial fill. The fill water isn't horrible either at 125.

I too am considering a rain collection type of system since a cover is not feasible

STALKER!!!!! (I'm actually flattered )

I wish there were calcium eating bacteria...there is actually a form of algae, it might be a red-algae, that forms a calcified shell. Some have referred to it as "sand algae". Obviously I have no intention of polluting my pool with an intentional algae bloom, but I thought that would be a really cool way to reduce CH...assuming you could kill the algae and properly filter out the remaining carcasses.

My pool water has crept up over the years by at least 250ppm/season or more. Right now, at the season's end, I'm probably closer to 800ppm. It's straight evaporation and refill with city water. Tucson intentionally leaves the CH high (no better than 250ppm hardness) because the water is a mixture of deep well water and CAP water (Central Arizona Project water is derived from wells and the Colorado river). When Tucson water first started delivering CAP water many years ago, people's pipes were corroding and rotting away at an alarming rate. It was hypothesized that the CAP water had a substantially different chemical balance than the local ground water and that was causing excessive galvanic corrosion. So now Tucson water keeps the mineral content high and mixes the CAP water with local well water to ensure that is not repeated (many lawsuits filed and much paid in damages). I'm further away from the CAP sources and closer to the foothills, so my water is mostly local, deep-well water.

So for my current circumstance I have about 4000 sq. ft. of flat roof. About 2000 sq. ft. of it is close enough to the pool, with existing gutters and downspouts, that I could reasonably get some large volume cisterns installed. Using this chart (which is probably a bit optimistic), I could potentially capture almost an entire pool volume's worth of water per year. Even if I only captured half that amount, I'd be happy with that as I could offset a huge portion of my summertime water bills. We get "monsoons" in Tucson starting in mid-July as well as winter rain storms - the down-pours are epic but brief. In fact, last night, we got a huge down-pour, the storm alone added 4" of water to my pool. But water was literally pouring out of one of my gutters at a rate of maybe 50 gallons a minute or more. Right now, all that water just washes out into my front yard and is totally wasted. So I'm sad every time it rains...

 

[chem geek]

See the WaterEvaporation spreadsheet I wrote. It is based on formulas from actual pool evaporation studies and heat loss models. This sheet will account for increased temperature of the pool water if it is heated. Note that the day vs. night air temperature and humidity will both change but unless there is dew the vapor pressure of air won't change day vs. night. The real wildcard is wind since the evaporation rate increases rapidly with wind and this wind measurement is at the water's surface. Fences, trees, and even the water in the pool being below ground level help reduce wind at the surface of the water.

The pan evaporation rate, especially when given monthly, is nice in that it is a net result including temperature, humidity, wind, and rain.

So I don't understand why pools in Arizona don't have white or reflective opaque pool covers. That would eliminate evaporation and having them be white or reflective would not add significant heat to the pool since it would reflect most sunlight. Such covers could be thin if one wanted to allow cooling at night (i.e. average day/night temp for the water).

 

[JoyfulNoise]

See the WaterEvaporation spreadsheet I wrote. It is based on formulas from actual pool evaporation studies and heat loss models. This sheet will account for increased temperature of the pool water if it is heated. Note that the day vs. night air temperature and humidity will both change but unless there is dew the vapor pressure of air won't change day vs. night. The real wildcard is wind since the evaporation rate increases rapidly with wind and this wind measurement is at the water's surface. Fences, trees, and even the water in the pool being below ground level help reduce wind at the surface of the water.

I will definitely play with the spreadsheet.

The pan evaporation rate, especially when given monthly, is nice in that it is a net result including temperature, humidity, wind, and rain.

So I don't understand why pools in Arizona don't have white or reflective opaque pool covers. That would eliminate evaporation and having them be white or reflective would not add significant heat to the pool since it would reflect most sunlight. Such covers could be thin if one wanted to allow cooling at night (i.e. average day/night temp for the water).

Well, it's certainly an option. Online there appears to be suppliers of such covers but locally all the stores that sell covers carry the standard transparent blue polyethylene bubble covers. The best I can find online are aluminized (blue/silver) covers which would stop evaporation but would likely transfer heat to the pool as the aluminized side is the bottom layer. But it's a very good idea and I'll keep searching online for what's available. Maybe there's a good reflective white cover out there. It has to be something that I can cut as my freeform pool shape doesn't lend itself to easy covering.