Water prices should not be subsidized as it is today, and as it has been for the past few decades, and everyone should pay the same amount for water. Water is currently wasted substantially in the US due to the government's subsidization of it, and market prices would lead to better allocation.
The term "farmer" rarely applies anymore to an individual who owns and tends to a plot of land. Corporations and conglomerates run most farms. Monsanto has cornered the market on most things to grow anyhow, so even if you are independent, you are still corporately controlled. Giving a break to "farmers" is really just subsidising water for the agriculture industry.
No, I do not believe that farmers should be charges the same rates as residential customers. Because of what they are using the water for, farmers should be charge a discounted price for their water. It should also be based on the amount specifically; if a farmer goes above a certain amount the price per gallon should be discounted slightly.
I believe it would be better to charge farmers additional fees for the water they use over the normal rate residential customers use. Since most farms are now owned by corporations I think this would be a good policy to balance out their high usage rates and help guarantee that they don't waste a valuable resource. It may also encourage farmers to use rain water more or to develop new stragies that are more friendly to the environment.
Water rates should be cheaper for farmers than they are for residential customers. Farmers tend to consume much more water than residential customers because they use water to grow crops which in turn is sold to city dwellers for food. Residential customers should pay a premium for water because they should be discouraged from wasting water.