Water sources may be variable and unreliable. Water reliability may vary by season, by year, and by location. In some areas, the rains fall mainly during the monsoon seasons, leaving dry conditions at other times of the year. Large scale climate variability such as the influence of El Niño and La Niña may mean that one year is wet while the next is dry.
The quantity of water in rivers and lakes can also be unreliable. Some rivers only flow during part of the year, leaving a dry riverbed and no local source of water. Rivers and lakes can also dry up from overuse.
At the household level, the reliability of the distribution system that provides water to the people is critical to maintaining quantity. If pipes are broken or only intermittent service is available, the quantity of drinking water suffers.
Often the unreliability of surface water can be off set by the use of groundwater. However, if ground water sources are depleted too rapidly, or are not being successfully recharged by either natural or man-made processes, the quantity of drinking water suffers.