Why did farmers need to develop a system to control their water supply?

Farmers needed to control their water supply because they can either have too much water or too little. Too much and their crops drown, too little and they dry up. Another reason is because they needed to provide dams to control flooding when rivers overflowed when things thawed in the spring, or during heavy rain fall, and to be able to have water during a drought. Speaking of a drought, they can use up too much water and cause a drought in big farming areas. Hopes this answers your question!