Surface water includes water found in streams, rivers, lakes, marshland, snow, ocean water, or any other water found on Earth’s surface. Groundwater is located in the subsurface in reservoirs (aquifers).
Approximately 70% of Earth’s surface is covered by surface water. Of this estimated 1.4 billion cubic kilometers of water, almost 98% is saltwater and between 2 and 3% is freshwater. Surface water can be replenished by precipitation and can pass to the atmosphere via evaporation, and so is a crucial part of the planet’s hydrologic cycle.
The increased temperature and drier climate that are developing in some regions of the world under the influence of global warming—the increasing warming of the atmosphere that is being driven in part by the production of compounds associated with human activity that restrict the dissipation of heat in the atmosphere—is causing some surface water sources to decrease or dry up altogether. In underdeveloped countries, this is a concern to human welfare and survival, where freshwater is scarce.
Surface water is also more susceptible to microbial contamination than is groundwater, and so is an important cause of waterborne illnesses such as cholera.
Historical Background and Scientific Foundations
The surface water that is replenished by precipitation and which can release water vapor to the atmosphere is a small proportion of the total surface freshwater on Earth. Of the estimated 35 million cubic kilometers of fresh surface water, almost 25 million cubic kilometers are unavailable, as they are present as glacial ice, permafrost, or permanently frozen snow.
Evaporation of surface water adds an estimated 580,000 cubic kilometers of water vapor to the atmosphere every year. Without this infusion of water vapor, the planetary water cycle, and so life on Earth, would cease.
Because of its location, surface water is susceptible to contamination by a variety of pollutants and microorganisms. Typically, the disease-causing microbes normally live in the intestinal tract of warm-blooded animals including humans. They enter the surface water in feces. Examples of disease-causing bacteria are those in the genera of Salmonella,Shigella, Vibrio, and Escherichia.
Escherichia coli0157:H7 has become prominent. Contamination of drinking water with 0157:H7 can be devastating. An infamous example was the contamination of the municipal water supply of Walkerton, Ontario, Canada, in 2000. Several thousand people became ill, and seven people died due to ingestion of the contaminated water that had flowed into an uncapped well from an adjacent cattle field.
Protozoa can also contaminate surface water. The main two examples are Giardia and Cryptosporidium, which are naturally present in the intestinal tract of beavers and deer. Waterborne illness due to these protozoans is increasing in North America, as human settlement encroaches on previously natural areas.
Impacts and Issues
The substantial proportion of fresh surface water that has been inaccessible is becoming more available as polar regions of Earth increase in temperature due to global warming. Large regions of glacial ice in the Antarctic have broken away into the surrounding sea. Eventually the ice will melt, increasing the volume of liquid water present in the global ocean.
WORDS TO KNOW
EROSION: The wearing away of the soil or rock over time.
RECHARGE: Replenishment of an aquifer by the movement of water from the surface into the underground reservoir.
RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.
WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.
Measurements of tide levels have been done throughout the twentieth century and satellite measurements have been ongoing since the 1970s. These measurements have established that the global sea level is rising. According to the 2007 report of the Intergovernmental Panel on Climate Change (IPCC), the average global sea level rose by 1.8 millimeters per year from 1961 to the early 1990s, and has been rising by over 3 millimeters per year since 1993.
Rising ocean waters are already affecting coastal regions. For example, according to the Geological Survey of Canada, rising sea levels are contributing to the increased damage of beach coastline in the maritime province of Prince Edward Island caused by coastal storms, which is threatening the province’s nearly $3 million annual tourism industry. If the rising surface waters continue, almost half the coastline could be submerged by 2100.
Florida is another area being affected. The advancing sea level has already affected surface water bodies including coastal wetlands. The low elevation of the state puts even the interior of Florida, including the Everglades, at risk, and so could threaten many species of birds and wildlife.
The fact that many of Florida’s shorelines are gently sloping will increase the height of the sea-level rise, which could exceed 2 ft (0.6 m) by 2100. The state’s drinking water supplies could be challenged.
In contrast, climate change is causing a decline in surface waters in regions of the globe that are becoming warmer and drier. For the 600 million people who already face water scarcities, the further loss of surface water could be life threatening. The World Health Organization estimates that by the year 2025, almost 3 billion people could be affected by inadequate water supplies.
Grover, Velma I. Water: Global Common and Global Problems. Enfield, NH: Science Publishers, 2006.
O’Neill, Karen. Rivers by Design: State Power and the Origins of U.S. Flood Control. Raleigh, NC: Duke University Press, 2006.
World Health Organization. “Water Resource Quality.” http://www.who.int/water_sanitation_health/resources/resquality/en/index.html (accessed April 20, 2008).