I think that many folks may have been misled by smoker manufacturers as to the purpose of the water pan, and it's contents, and it has become a standard practice to use water in vertical smokers as a result...many smoker brands don't have anyone actually cook with them in real-world scenarios, they just market their design and have someone fabricate and distribute the product to retailers. Until recently, I was one of those who didn't fully understand the concept, using a dry pan only occasionally, for actual meat drying (jerky), when high humidity and cold winter temps were an issue, or just when playing with variables during an experimental smoke.
Water in the pan is mostly intended for control of high-temp spikes, and is the most effective for this purpose in vertical charcoal or propane smokers, as well as non-thermostatically controlled electric heated smokers. As water temperature rises, it produces more water vapor...water vapor cools it's surroundings. Higher heat input = higher water vapor production...this is how water in the pan helps to reduce those high-temp spikes, just by balancing the chamber temp vs heat input...this works to a point, in terms of controlling a desired chamber temp, but becomes very inefficient at higher chamber temps, over 225* (water boils @ 212* @ sea-level). Less water in the pan will allow for higher chamber temps, hence, when the water pan is running dry, you experience high-temp spike with charcoal, propane or non-thermostatically controlled electric smokers. The pan itself, while it can be used for water if you so choose, lends it's most effective purpose as a heat baffle to direct the heat away from the meat for a more controlled indirect cooking method, and as a drippings catch so we don't have flare-ups from contact with the heat source.
As water evaporates, it cools it's surrounding environment through evaporation. Liquid water does add some thermal mass at lower temperatures for control of low-temp swings, but a clean, solid/dry mass such as sand, bricks or pea-gravel will serve this purpose quite well. The use of water in the pan is the most effective media for control of high-temp spikes, so, if you have high-temp spike issues with your smoker, water is your best choice for this.
One objective that many of us may find confusing, or we may not understand the basics of the concept, is that when meat is cooking, water from the meat needs to be able to evaporate...this is a normal part of the cooking process (water loss/shrinkage). In a high humidity cooking environment, such as when using water in a pan, and especially when the pan is in close proximity to the heat source causing higher water evaporation rates, internal water evaporation rates from the meat may be reduced, thus causing longer cooking time at a given temperature and ventilation rate of the cooking chamber. Here's the trade-off with using water in the pan: better control of high-temp spikes along with longer cooking times, and, more fuel to maintain the desired cooking chamber temperature. Water in the pan reduces temps, thereby increasing the required heat input. The higher the evaporation rate from the water pan, the more heat is needed to maintain chamber temps.
Will a dryer cooking environment cause dried out meats? Only if they are cooked to high finished temperatures, or cooked too slowly. Generally speaking, if a cut of meat, poultry cuts or whole birds, or fish is dry, it was over-cooked. Are there times when we may want to hold onto some higher humidity in order to finish a particular cut of meat? Yes. If cooking a less tender lean cut to higher finished temps, such as a center-cut brisket (lean-trimmed flat-cut), as just one example, and in these cases, adding water to the pan will help, though foiling the meat towards the end of cooking is likely your best choice. If higher finished temps are in order, a less lean cut should be used. Fat-cap on with a whole packer brisket and shoulder cuts is best for the highest degree of interior moisture retention. But, does the fat cap slow-down water evaporation from the meat? Yes, to some extent, though I think of it as more of a controlled/balanced rate of evaporation, unlike with bare meat. The fat cap lends a self-basting to the meat as it renders down, and also slows the cooking process of the meat somewhat, all without any additional intervention from the chef.
Will my chicken or turkey breast dry out when cooked in a drier environment? No. These should be cooked strictly to temp (165* minimum internal temp for USDA inspected), and if temps don't run away before you remove them from the cooker, they will be juicy. I like to cook to ~168* (with carry-over) for breast meat and ~172* for dark meat...always very moist, even when skinless and lean-trimmed.
To help you to get a better understanding of how water evaporation works, consider evaporative water coolers (swamp coolers) for home comfort cooling: I live in a semi-arid climate where 20-40% R/H (relative humidity) is very common. We use evaporative water coolers instead of central air conditioning for comfort cooling. Although this works quite efficiently and requires far less energy than A/C, it will not work effectively in a higher R/H climate. Why? To promote water evaporation from a given media, you need a reduced humidity environment in order for the exchange of water vapor to occur.
If you live in a humid climate, while outside on a hot day, your clothing may be soaked in sweat and you can't cool-off. Why? Your surrounding environment is already saturated with water vapor, thereby greatly reducing the potential for evaporation of sweat...the sweat is heated up and wants to go some where, but it can't make the transition from liquid to vapor to cool you off because the air is already saturated, so, your sweat just hangs around with nowhere to go and makes you feel sticky and miserable.
So, how does all of this relate to cooking chamber humidity and meats? If you were a piece of meat in a cooker, wanting to get up to temp, and you were loaded with water and there is no where for the water to go through evaporation because your environment was too humid to pick-up much of this water, well, you will be waiting longer until you will reach the table. In essence, the effects of high water vapor content in the cooking chamber can be a double-edged sword. For certain cuts of meat that you want to cook more slowly for the purpose of tenderizing, such as brisket and most shoulder cuts, a slightly increased humidity in the cooking chamber can be beneficial, yet can hinder getting through the dreaded plateau. Using slightly higher chamber temps can offset cooking time, though achieving the higher temps can be more difficult and use more heating energy with high humidity. Poking a couple small holes through interior muscle membranes can speed up the process when a long stall hits, simply by allowing the water to be released from the meat in an unconventional form (liquid, instead of vapor/steam). EDIT: also note that leaner meats contain higher water densities than do those with more marbling...fat has less water than lean meat.
I have recently been using washed pea-gravel in the water pan of a charcoal gourmet smoker (with a foil catch pan fashioned on top), and noticed far less fuel use overall when compared to water. Temp swings seem to occur less often and are not as extreme with proper fire control methods, and the finished product exterior textures are more firm, while interior texture is as would be expected for the given finished temp of the meat. I credit all these benefits to a more balanced cooking chamber humidity, allowing the water evaporation from the meat to be the only source of humidity, as well as just using a thermal mass instead of water in the pan for better temp control. Humidity plays a far larger role in cooking than we may realize. As with many other variables, the amount of cooking chamber humidity can be used to your advantage when you want to try a new twist with methods, yet with the wrong application, it can have adverse effects on your finished product as well.
One last thing to consider regarding humidity and smoke reaction with meats: if you have multiple cooking grates in a vertical smoker, the meat closest to the water pan (when using water) will get less smoke than that which is farther above the pan. This is not due to the baffling effect of the pan, but instead is due to steam generated by the water in the pan, and is the highest concentration closer to the pan. If your entire smoke chamber is saturated with water vapor, you will get less smoke reaction throughout the smoke chamber. I get more evenly smoked meats without water in my pan. I moved my lowest grate upwards (stock install rests directly on top of the pan), to within 4" of the upper grate in my gourmet, in order to compensate for this last summer, and now I am finding that it would not be necessary if I don't use water. Although I would still need a gap for a catch pan above the now pea-gravel filled water pan.
Food for thought...
Edited by forluvofsmoke - 5/30/12 at 1:14pm