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Water supplies may seem stable in places where water emerges whenever one turns on the tap. In reality, however, water supplies vary significantly because of seasonal changes, weather patterns, and even long-term shifts in climate conditions. These variations are felt most acutely in developing countries which do not have the water supply infrastructure needed to even out these fluctuations.

The water cycle is intimately tied to Earth’s climate, and is subject to the same types of variability we observe in the weather. Precipitation (rain, snow, sleet, hail, frost and even dew) produces much of the world’s drinking water. Total precipitation averages from less than 100 mm/yr in dry climates to more than 3,400 mm/yr in wet locales—a remarkable difference.

Unfortunately, annual averages are unreliable predictors of precipitation in a given year. Actual totals can be subject to seasonal or annual shifts as well as prolonged periods of drought that seriously limit water availability for multiple years.

Like precipitation, freshwater that is frozen in permanent glaciers or snow is a critical drinking water resource in many regions. Though this solid water is ever-present, the amount of runoff that is usable for irrigation and drinking is highly variable. Shifting seasons and temperatures regulate the water that is available from such sources.

Long-term climate change will have multiple cumulative effects that change over time. For example, because the size of glaciers dictates their runoff rate and because some of the world’s glaciers have been rapidly shrinking during the past several decades, there have been short-term increases in runoff rates (and therefore water availability) from these valuable water resources. However, in the long term the disappearance of glaciers represents a loss of multi-year water storage that may threaten the water supply of those populations that depend on meltwater.

River runoff, whether fed by permanent ice or simply by precipitation, is often highly variable—even a single storm can swell rivers to their banks and increase flow several fold. Some seasons typically feature greater runoff due to higher precipitation, lower evapotranspiration, or snowmelt, and these seasonal variations are relatively easy to predict—at least in a qualitative sense.

But river flows also undergo long-term variations, with wet or dry periods that can last many months, years, or decades. Predicting these long-term variations is extremely difficult. The uncertainty leaves many people at risk of water shortages.

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