I've only grown one clump of bamboo for around 5 years, so take my reply with a grain of salt and give more experienced responders more weight.
I do water my
Fargesia rufa irregularly during periods of no-rain, and I've given it some chemical fertilizer to partly make up for my not-very-organic, nasty clay soil. So my clump only gets a LITTLE yellow.
Mine always came back fine after I gave them some water and fertilizer, but they never got real yellow.
Since old culms don't grow in size, and at most add some leaves their second year, why cut the old culms? Their existing leaves will provide the plant with energy to store for next year's culms. If the plant can't support those leaves "with a profit", it will probably drop them by itself (turning them into the mulch it craves).
If you're worried about water loss from existing leaves, can't you give the barrel a few gallons now and then? .
A barrel might not give them much root space: I've read that bamboos never send roots very deep. It would be best for it to have rich organic soil near the surface, organic fertilizers and an inch or two of organic mulch. (Mulch will also help keep the soil surface less hot and save water).
If the yellow is caused by lack of Nitrogen as much as heat and drought, almost any soluble fertilizer might give you an "instant cure". (Always include plenty of water when adding any chemical fertilizer or concentrated organic fertilizer.)
Mulching with something like alfalfa pellets would give you mulch PLUS some N.
P.S. Are the roots getting hotter than they really like, in a barrel? I suppose most bamboos have good heat tolerance even in their roots, but you might check on that or give the barrel some shade without blocking air flow.
What species is it? Not all bamboos can take full sun. A cool-shade-and-moisture lover might be demanding a couple of things.
Here's a good place to do bamboo research:
http://www.bamboo.org/BambooSo...
http://www.bamboo.org/BambooSo...
And you probably don't care, but my little clump laughs at snow and ice:
