Weather patterns come, and weather patterns go, but that would be the largest contributor to the amount of fresh water available. In the water cycle, water is evaporated into the atmosphere, condensed, and then falls back to the Earth in the form of precipitation. Sometimes that precipitation part gets interrupted by unfavorable weather conditions. The Ogalla Aquifer is one of the largest naturally occuring aquifers in the world, used primarily to irrigate crops in the middle of the United States. If it does not get refilled, the water table in the aquifer dips lower. I can recall several years ago, we experienced a drought-like condition across the deep south. Fresh water had to be conserved; people were banned from watering their lawns and washing their cars. The ground was so dry, we saw very deep cracks in our back yard. So an interruption in the timely delivery of the precipitation part of the water cycle can make a huge impact on the amount of fresh water available.