Recipes for smoking brisket typically recommend a temperature of 225 F. My weber smokey mountain has a water pan to help regulate the temperature. But what if I am able to maintain that temperature without a waterpan? Would it cook the same way or is the humidity necessary when smoking brisket?
The water pan does affect humidity in the cooking chamber and therefore, it does change the how quickly moisture evaporates out of the meat. The question really becomes - "Will the meat be moist enough without the water pan?" Which, to a degree, is personal preference. Some choose to not use a water pan and baste instead, some use both and some neither.
I would recommend trial and error. Brisket is relatively pricey so I would recommend using pork shoulders. Smoke one with the pan and one without and see if you think the moisture content is satisfactory. The briskets won't cook exactly the same but the relative moisture level in the shoulders should give you a decent estimate.
I personally always use a water pan, but I don't usually fear meat from a smoker being "too moist", and it does wonders for temperature stabilization. I also have a larger smoking chamber than a WSM so it's possible that too moist is a larger concern for you.
One last note, while cooking too hot for too long will inevitably dry out your meat, cutting a brisket or shoulder without letting it set for at least an hour (ideally 2) after cooking will also dry it out.
According to Big Green Eggic, the main effect of the water pan is help keep the heat even and low in the cooker by:
This makes it easier to hold even, low temperatures (like 225 F) for long, low and slow cooking.
They claim the humidity change is not really terribly important, as water evaporates from the surface of the foods relatively quickly in any case.
The article Physicist Cracks BBQ Mystery in the Huffington Post blog (which is very interesting in its own right) mentions that many successful competition barbequers are choosing to barbecue at slightly higher temperatures, which would make the entire point of the water pan moot.