Causes and Trends in Water Surplus and Deficit

Surplus and deficit are a balance

A place's water situation depends on both supply and demand. Supply is the usable water available; demand is the amount people need or want to use. A water surplus occurs when supply is greater than demand. A water deficit occurs when demand is greater than supply.

This means that low rainfall does not, by itself, define a deficit. A dry area with very few people may have enough water to meet its limited demand. Equally, a wetter area can experience shortages if its population, farms and industries require more water than can be supplied.

Why water supply differs

Precipitation adds water to rivers, lakes and underground stores. Where precipitation is low throughout the year, little water is available to replace what people use. High temperatures can also increase evaporation, returning liquid water to the atmosphere and leaving less available for use. Together, low precipitation and high evaporation help explain shortages in regions such as North Africa and the Middle East.

High precipitation and lower evaporation favour a surplus because more water remains available. Canada is an example of a country with substantial freshwater supplies. However, physical conditions are only part of the explanation: water must also be stored, managed and delivered to people. Poor water management can restrict usable supply, while effective management can make available water go further.

Why demand can create a deficit

Population size matters because every additional person needs water for drinking, washing and sanitation. Farms also need water to produce food, while industries use it in their processes. A growing population, expanding agriculture and industrial growth can therefore push demand above supply.

India illustrates the importance of considering demand as well as supply: population, agriculture and industry place substantial demands on its water resources. In contrast, areas with relatively low demand are more likely to have a surplus if supply is plentiful. Parts of Canada and Russia have water surpluses through a combination of available water and comparatively low demand.

The causes often reinforce one another. An area with little rainfall may also have a growing population and expanding irrigation. Its supply is limited at the same time as its demand is rising, making a deficit more likely.

Why demand has risen over the past 50 years

Over the past 50 years, human activity has increased water demand in many places. Population growth has increased direct household use and the amount of food that must be produced. Agricultural expansion consequently increases demand for water, including water used to irrigate crops.

Industrial growth adds further demand as more factories and production processes use water. Meanwhile, rising living standards can change how much water each person uses. Households may acquire washing machines and dishwashers, take more showers and baths, and use water for washing cars or watering gardens.

Leisure activities can add to this growth: swimming pools, hot tubs and the watering of golf courses all require water. Changing diets matter too. Greater demand for meat increases pressure on water resources because meat production uses more water, including water needed to grow animal feed.

These changes are not identical everywhere. Population, industry, wealth and lifestyles have changed at different rates, so the increase in demand varies between places.

How human intervention has changed supply

People have changed water supply as well as demand. Dams and reservoirs allow water to be stored and made available for use rather than simply flowing away. This can improve the reliability of supply for the places they serve.

However, a river connects different places. Holding back water in a reservoir or abstracting it for use upstream can leave less water available downstream. An intervention that improves supply for one area may therefore increase the risk of a deficit elsewhere. Greater abstraction can meet growing demand locally while putting pressure on the remaining water supply.

Human-caused climate change also affects supply by changing precipitation patterns. Some areas receive less precipitation; others receive rainfall at unusual times of year. Timing matters because water may not arrive when demand is greatest, increasing the importance of storage and management.

The overall trend is therefore not simply that water supply has increased or decreased everywhere. Human intervention has made more water accessible in some places, but has also reduced or made supply less reliable in others. Whether a place develops a surplus or deficit depends on how these supply changes compare with changes in demand.