Water shortages occur whenever the use of fresh water outstrips the supply from precipitation and the water cycle. While wasted water re-enters the water cycle and is eventually available again for re-use, the time necessary for water to pass into the water table and replenish aquifers means that overuse can temporarily drain these important water sources. Dry weather can also cause shortages due to a lack of replenishing precipitation.
A major source of water overuse is agriculture. Agriculture accounts for 70 percent of the freshwater use in the world, and as much as 60 percent of that water is wasted due to poorly maintained irrigation equipment or poor irrigation techniques. This excess water simply runs off of farmland or is absorbed through the ground back into the water table, but it can take a considerable amount of time for it to return to the aquifer that initially supplied it.
Water shortages can also be economic in nature. Especially in dry regions of the globe, access to water can be extremely important, and communities may feud over rights to a particular reservoir. In some cases, water may be pumped from a source over a significant distance to provide water for citizens in dry areas, leaving those in the source region lacking their natural water supply.