Please bear with me.

There is some very interesting information in this thread on temperature drop via evaporative cooling in degassing columns:
"Some big trout pics down memory lane"
This correlates with information on the web about small "bong coolers" for computers and diy swimming pool coolers for people in Texas.

I live in SW Washington near Portland and have a small garden pond that is 50 x 35ft by about 6ft deep. It's not a simple shoebox shape and I estimate the volume around 30k gallons. I have about biofilter/bog arrangement that uses 22 tons of pea gravel and use airlift pumps to circulate the water through the bog. There is no power at the pond, but I can plumb water or an airline without issue. I would like to plant a few trout (just for amusement). West of the Cascades we don't have major freezing issues but summer daytime temps can go up to where it would make Trout difficult without a lot of incoming cold water
However, given the relatively small volume of water, some sort of DIY cooling tower pumping a some reasonable fraction of the total volume per day, may be able to keep the temps down sufficiently. To recap the bucket based degassing column in the referenced thread dropped the water temp by 7F at 8gpm. This was achieved by evaporative cooling in ambient air temp of 72F.

I assume that a fountain would offer similar benefits and would not require a structure. I think I would want something in the 20 gpm range...
Does anyone have any relevant information or experience they could share?
Any ideas on how best to do this with solar?

with thanks
Beardy

Beardy, the only thing I can suggest is that you consider brown trout. They are more heat tolerant than brook, cutthroat, or rainbows. As for cooling & aeration, other folks here can speak knowledgably.

I don't think a fountain would get you the same result. My other question is where the study was done, what was the relative humidity at the time of the study? You'd get much more evaporation in 20% RH than 90-100% RH.

and night time temps would get much lower too.

Hi Esshup,

I agree with you; relative humidity would be a critical factor. The thread I referenced was from Ryan B in Southern Ontario CA in July, I would anticipate that our summer time humidity is no more than in Ontario. As a happy coincidence the weather is least humid in summer and most humid (typically @100%!), at most other times.
direct link and data at bottom of page 2: https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=325786&page=2

I accept a fountain might not be as effective although I am not sure why it wouldn't be. I wonder if most fountains in small ponds have too low a flow rate to make a measurable difference and the large fountains in big waters are equally ineffective as the pond is just so massive. A 20gpm fountain is a lot in a garden pond considering a typical shower head might be 2.5gpm....

cheers

The thing about a fountain is that you are starting out with pond temp water, where both Ryan and Cecil are starting out with well water (ground water temp) water. The packed column will put more water in contact with the ambient air than a fountain will, as it is breaking up the water column many, many times as it falls thru the bio media in the buckets. A stream coming from a fountain is still largely a stream when it re-enters the pond.

If you are not using a bottom diffusion aeration system in your pond, then pulling the bottom anoxic water and shoving it thru the packed column will be a great benefit to the pond. How much will the water temp drop? I can't answer that, but going by the info on page 3 of the linked thread it looks like 5°F would be a good WAG.

What is your ground water temp and what is the water temp at the bottom of the pond?

Good points although a lot would depend on the airflow in and around the bio media.
I don't yet have any data on any of the relevant temperatures yet but I agree 5F seems like a good WAG. Dew point is likely quite low here so I am hopeful.

I don't yet have aeration installed but expect to do so soon. The air lift pumps inject quite a lot of air in the water as it gets pumped to the biofilter/bog. I am running two HiBlow 80-HP pumps. One pump for each airlift and then adjusting the airflow down to optimize the pump performance, minimize the excess air burping and splashing and dedicating the remainder for a small aeration source.

However, I have been doing a little more internet digging. There are several pages highlighting the use of spray jets, spray bars and fountains for cooling pools. In summary I am sure it's possible but I get the impression it might some trial and error.