When the water returns to earth it can fall in the wrong place at the wrong time. Floods or rain instead of snowpack can be the result. The underground aquifer may not recharge itself rapidly enough from a mortal's point of view. We all impact the hydrologic cycle in a variety of ways. Agriculture is draining natural underground reservoirs faster than they can be replenished.
Industry uses water for production.
And, of course, pollution often occurs faster than natural microbial filters and plant life can clean out the pollutants. The result is a shortage of usable, drinkable water. Probably the best way to avoid a shortage is through conservation. Recent national legislation has made it easier for all of us to conserve our precious natural resource. The National Plumbing Products Efficiency Act, which is implemented through local building codes, has been a step in the right direction.