A permanent reflection pool offers many more opportunities for photography - the birds are habituated to it for drinking and bathing, plus it can be planted up or set-up and left to naturalise. At Pikelow we have two reflection ponds at the hides. Each are kept topped up with fresh water. Another option would be to channel rainwater from the hide roof to the ponds and have a butt to store water in as well.
The back of the reflection pool should be near enough to photograph the birds with your main working lens, not forgetting that you also want to get the reflection in the photo. Our pools our approximately 2,400mm x 1,200mm. The best conditions for photographing reflections are when there is no wind and blue sky.
Below are the design drawings for the pools at Pikelow, but the principals are the same as for the portable pool.