IMO: You want to recirculate it continuously from a master reservoir that stays cool and has a constant level of nutrients in it. Or....do simple one bucket deep water culture.
To pull this off you would need to:
1. Understand recirculating hydro systems
2. Understand terrestrial cooling systems
3. Be willing to do a lot of digging in your grow space
4. Be able to fully test a system to make sure it doesn't leak into the ground
5. Be willing to maintain a sub terrestrial system (many issues here)
Also, you would need to consider:
1. this is a difficult experiment with a high likelihood of failure
2. Outdoor plants get huge and in hydro all the more, which means you need large buckets
3. Deep water is probably the best choice because Nutrient Film is subject to sudden loss of the crop due to a simple pump failure. Also NFT requires gravity and that really wouldn't work underground
4. Large buckets and reservoirs require a lot of water and nutrients (expensive)
Single Bucket DWC:
Maybe a simpler experiment would be to bury a 15 gallon bucket in the ground and put an air pump next to it and cover the pump to protect it from the weather and run an airline and stone into the bucket and put some kind of net pot over the top and insulate the top just leaving an opening for the plants trunk. In-other-words a single bucket non recirculating deep water culture setup buried in the ground for cooling.
Hope this makes sense. I have done NFT, DWC and RDWC as well as top feed RDWC in my own grow with systems I designed and fabricated so I have a pretty good grasp on the technology.
Dee
RDWC Undercurrent system
https://www.thcfarmer.com/community...formance-under-current-for-300-or-less.94487/
Top Feed RDWC
Top feed drippers
Nutrient Film Technology (growing strawberries):