Although most of Earth's water is in its oceans, the water humans consume and use for irrigation and in our households is freshwater. Freshwater in the form of surface water is found in lakes, wetlands, rivers, streams, brooks, etc. As groundwater, it is located in aquifers, wells, and springs. Most of us depend on surface water to fulfill our needs, but groundwater in renewable and non-renewable aquifers directly supplies drinking water to about 1.5 billion people. Freshwater systems provide goods and services valued at trillions of dollars per year, but their ability to maintain high quality and quantity is strained. Increased demands from industrial production and agriculture are the primary causes. Agriculture alone uses roughly 70% of all the water humans withdraw and is one of the heaviest polluters. Forests and grasslands filter and purify water, but in many of these ecosystems a significant amount of their original vegetative cover, essential for water filtration, has already been lost.

How extensive are global freshwater systems and how have they been modified?

Surface waters — as rivers, lakes, and wetlands — occupy 0.08% of Earth's surface and account for only 0.01% of the world's freshwater; more than 90% is locked in polar ice. To maximize the potential of the water at our disposal, we've modified rivers and lakes by altering waterways, draining wetlands, and constructing dams and irrigation channels. In fact, the number of large dams increased sevenfold since 1950. We've also built canals and pipelines to connect water basins to make it possible to transfer water between basins. These changes enabled us to increase farm yields and hydropower and control flooding. But they've also changed the natural hydrological cycle in most of the world's rivers. In many cases rivers have become disconnected from their wetlands and floodplains, key components of freshwater ecosystems. Wetlands provide flood control, carbon storage, water purification and goods such as fish, shellfish, timber, and fiber. It is estimated that in the last 100 years, half the world's wetlands were converted to agroecosystems or urban areas.

What are some of the most important goods and services freshwater systems provide?

What about water availability and quality?

Many experts, governments, and international organizations predict that water availability will be one of the major challenges facing humanity in the 21st century. Humans withdraw about 4,000 cubic kilometers (km³) of water per year from global freshwater sources. Scientists estimate that the amount of runoff (the renewable supply of water that flows through rivers after evaporation and infiltration) totals between 39,500 km³ and 42,700 km³ per year. But only about 9,000 km³ is readily available for us to use; another 3,500 km³ is stored in reservoirs. Scientists view as problematic those regions where water availability is less than 1,700 km³ per person per year. Currently 2.3 billion live in such regions. It is estimated that by 2025 that number will increase to 3.5 billion, 48% of the world's projected population. Between 1950 and 2000, water availability per person per year went from 16,800 m³ to 6,800 m³.

Agricultural Waste Water

Many rivers, lakes, and aquifers are so polluted, they're unfit for drinking, further reducing the amount of available water for humans and other species. Worldwide the quality of freshwater systems has worsened almost everywhere large urban and industrial areas exist and intensive agriculture is practiced. In the developing world, water-borne diseases from fecal contamination of surface waters continue to be a major cause of sickness and death. In the United States and Europe, surface water quality actually improved in the past 20 years with respect to some pollutants. But nutrient loading from agriculture continues to be a problem in many parts of the world. In China and India, the use of fertilizers is increasing the nitrate pollution in surface and groundwater.

We don't know just yet how extensively chemical contamination has impaired the biological functions of freshwater systems. But we do know that algal blooms and oxygen depletion have become widespread in freshwater ecosystems worldwide, an indication that freshwater systems are seriously affected by pollution all over the world.

There are, however, courses of action to take, alone or in combination. We can resolve to stop degrading our freshwater systems, or we can risk having to pay billions of dollars to either build artificial water treatment plants or restore natural wetlands. The people of South Africa and Florida's Everglades are already working toward these ends.

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