Surface Water

Surface water supplies are among the most basic needs of humankind, but water resources are unevenly distributed in both location and time. Surface water includes the oceans, but the term is generally used to refer to nonoceanic water that is most abundant in lakes, both fresh and saline. Streams and rivers are sources of freshwater that are constantly replenished.

Overview

Surface water is a crucial commodity for humans, plants, and animals, but it is available in only a limited quantity. Furthermore, it is unevenly distributed geographically and temporally, such that vast areas experience perennial, seasonal, or intermittent shortages, while other regions have excessively abundant water. Replenishment of surface water supplies through the hydrologic cycle assures that water will always be available, though not necessarily at the time and place that it is needed or in the desired quantity.

Surface waters include water in freshwater lakes and streams, and in saline lakes such as the Great Salt Lake, the Caspian Sea, and the Dead Sea. Research by the US Geological Survey has revealed that at any given moment, only about 0.001 percent of all water is fresh surface water held in freshwater lakes, ponds, rivers, and streams; a similar amount is held in saline lakes. The total volume of water present in freshwater lakes and streams is nonetheless impressive, equaling more than 125,000 cubic kilometers.

Humankind is dependent on surface waters for a wide variety of uses besides direct consumption, including irrigation of crops, generation of electricity through hydropower facilities, industrial activities, food processing, transportation, and recreation. Globally, these water uses result in the consumption of around four trillion cubic meters of freshwater each year. In moderate climatic zones, humans may ingest only three to four liters of water per day to survive, and typically less, but water use in even the least industrialized societies is far higher than that rate. The annual per capita water consumption in underdeveloped nations is approximately 350 cubic meters annually. In the United States, the annual consumption of water exceeds 2,300 cubic meters per person per year. Only one-tenth of this water is consumed domestically, with the remainder going to irrigation, industry, and the generation of electricity. As developing nations become industrialized, their demands for usable water are increasing significantly.

Runoff

Surface waters are produced and replenished through precipitation and diminished by evaporation, infiltration to subsurface storage, and return flow to the sea. Precipitation falling on forest or complete grass cover is intercepted by vegetation. Interception is of great importance because it reduces the kinetic energy of raindrops considerably, reducing their erosive potential. This water eventually drops off leaves or runs down the branches of trees, bushes, and blades of grass, where it encounters the ground surface. Studies of steep forested terrain at Coweeta Hydrologic Laboratory in North Carolina revealed that no measurable overland flow was generated in seven years of observation. All runoff reaching the stream channels first infiltrated the soil, later reemerging in drainage ways. It is believed that all storm flow and base flow of streams draining forested landscapes is generated by infiltrated water, which moves in the shallow unsaturated zone as throughflow, displacement flow, saturated flow in fluctuating belts adjacent to streams, or slow groundwater seepage. In arid areas or on lands disturbed by farming, paving, or compaction, infiltration is greatly reduced, and surface waters are largely direct surface runoff or overland flow. Runoff generated by overland flow reaches stream channels much more quickly than that which moves by subsurface routes, and it is less substantially depleted by evapotranspiration (the combined loss of water by direct evaporation and transpiration, which is the process by which plants expel water as vapor in their metabolic functions).

All the land area that contributes runoff to a particular stream constitutes the watershed, or drainage basin, of that particular stream. Land-use changes within the watershed will affect the runoff timing, volume, and water quality of the stream. Activities outside the watershed do not ordinarily influence runoff rates or volumes within the watershed. Thus, the watershed is the basic unit of study in most hydrologic investigations.

The global pattern of seasonal and annual runoff is quite complex; the amount of runoff at any location is determined as much by rates of evapotranspiration as it is by the amount of precipitation that falls. Plants obtain water from soil moisture, water that adheres to particles in the soil in much the same way that a sponge holds water. Runoff is surplus water left over after soil moisture is completely replenished. Thus, the tropical and subtropical regions, which receive abundant rainfall, do not necessarily experience the highest rates of runoff because high temperatures throughout the year cause high rates of evapotranspiration, greatly reducing the water surplus.

The average depth of runoff is approximately 27 centimeters, but there is considerable variation from that average. Only a few areas in the world produce more than 100 centimeters of annual runoff. They include tropical areas such as Central America, the lower Amazon basin, equatorial West Africa, Bangladesh and northeast India, Madagascar, and the East Indies, where exceptionally heavy precipitation overcomes the effects of high evapotranspiration. Outside of the tropics, annual runoff in excess of 100 centimeters occurs primarily in coastal alpine settings, where cool temperatures and forced lifting of moisture-laden air over mountain ranges produce high rates of runoff. Such areas include coastal Alaska and British Columbia, Norway, Chile and Argentina, Tasmania, and New Zealand. Each of these belts of exceptionally heavy runoff is surrounded by areas receiving 50 to 100 centimeters of runoff per year. The largest areas in this range are the Amazon basin, the Congo basin, Southeast Asia, the Appalachians, and Japan.

Areas producing less than 10 centimeters of runoff per year are extensive. The largest such contiguous area covers Africa north of 10 degrees north latitude, the Arabian peninsula, Iran, Afghanistan, Pakistan, and much of interior Asia. The interior of North America west of the hundredth meridian produces little runoff except from the higher mountain ranges, and the Atacama and Patagonian regions of South America also yield less than 10 centimeters of runoff per year. Water is a scarce commodity in all these areas, except where unusually abundant groundwater occurs or where streams such as the Nile River pass through.

The large remaining continental areas generally produce in the range of 25 to 50 centimeters of runoff per year. Such moderate rates of runoff still produce substantial streams when drainage basins are large. Eastern North America, northern Europe and Asia, East Asia, most of India, eastern Australia, and East Africa are included in this category. Surface water shortages in these regions occur primarily in watershed divide areas and during seasonal and intermittent droughts.

Mountains, Streams, and Lakes

In alpine areas, the snowpack generated by winter storms provides the majority of the stream flow. Conversely, areas on the lee side of such mountain ranges are deprived of moisture through the rain-shadow effect. The net result of heavy precipitation in the mountains, accompanied by cool temperatures and reduced evaporation rates, is that mountain ranges are islands of moisture that generate abundant stream flow during the spring melt. The numerous streams generated by snowmelt flow to the lower, drier adjacent areas and provide them with a source of usable freshwater. Almost all of the streams of the western United States are generated in this manner: the Colorado, the Snake and Columbia, the Missouri, the Arkansas, the Sacramento, and the Rio Grande being but a few examples. The mountain regions of Europe, Africa, Asia, and Australia perform a similar function for adjacent lowlands. When a stream from such locations flows through arid lands, it is especially valued and is known as an exotic stream. Prominent examples include the Nile, Indus, and Colorado Rivers.

The five largest streams of the world account for more than one-third of the world’s total stream flow. The Amazon River alone accounts for almost 20 percent of the world’s stream flow, as it drains the largest watershed in the world and gathers waters from the largest tropical rainforest. The Amazon watershed is almost as large as the forty-eight contiguous states of the United States and, 800 kilometers above the mouth of the river, its channel averages 2.5 kilometers wide and 50 meters deep. The discharge of the next largest stream, the Congo River, is only one-quarter as great, while the discharge of the mighty Mississippi River is only one-tenth that of the Amazon. The combined discharge of all the world’s streams is roughly 30 percent of the precipitation that falls on the continents; the remainder is returned directly to the atmosphere by evaporation and by transpiration.

Freshwater lakes cover some 1.5 percent of the land surface and contain the vast majority of liquid freshwater, a total of approximately 125,000 cubic kilometers. Saline lakes contain only slightly less water, storing an average of 105,000 cubic kilometers of water. Although the freshwater lakes are generally of greater utility to humans, the saline lakes provide important resources as well, including fish, salt, and magnesium.

Lakes occur where natural basins have developed, such that ponding of surface waters occurs. Because the normal cycle of fluvial (stream-related) erosion of the landscape does not produce such depressions, extensive areas of the continents are without naturally occurring lakes. The vast majority of lake basins were produced by the erratic deposition and scouring action of glaciers, but almost two-thirds of the water held in lakes is contained in six structural basins: the Caspian Sea, the Aral Sea, and Lakes Baikal, Tanganyika, Nyasa, and Victoria. These structural basins were produced by a combination of the downfaulting of basins and uplift of adjacent terrain. Lake Baikal in Siberia contains almost one-fifth of the world’s liquid freshwater because of its large size and depth exceeding 1,700 meters. The rift valleys of East Africa, formed by the downfaulting of linear basins associated with the rending of the continent by plate tectonic movement, contain a series of large, deep freshwater lakes.

Glaciers passing over North America scoured out preexisting river valleys to form deep depressions, which were further deepened (relative to their outlets) by postglacial tilting and deposition of recessional moraines. The Great Lakes were formed in this way, as were Canada’s Lakes Winnipeg and Winnipegosis, Great Slave Lake, and Great Bear Lake, along with several hundred thousand smaller lakes. Hundreds of thousands of lakes were formed in the recently glaciated terrains of Europe and North America by erratic deposition of loose materials by glaciers. Such uneven deposition eradicated the well-integrated preglacial drainage networks of the areas affected, leaving vast areas of uneven topography with many shallow depressions, which are now filled with water. This type of hydrologic system is referred to as a deranged drainage system.

Preferential solution of certain areas of limestone and other soluble rock types often results in collapse features known as dolines, or sinkholes. When these depressions are filled with impermeable clays, or where the regional water table is high, the depressions tend to hold water. The Lake District of north-central Florida provides numerous examples of such lakes. Other causes of lake basins include landslides, volcanic flows (Lake Tahoe), volcanic eruption and collapse (Crater Lake), scouring by wind, and meteorite impact. More than 100,000 small, elliptically oriented lake basins, most of them now dry, occur on the coastal plain of the United States from New Jersey to Georgia. The origin of the Carolina Bays has been vigorously debated. Human-made lakes are increasingly common on the landscape, built by damming streams to produce reservoirs for water supply, flood control, power generation, transportation, and recreation.

Lake basins are sediment traps because flowing water entering a lake quickly loses velocity and drops its sediment load. As a result, only the largest and deepest lakes last more than a few thousand years, making them among the most ephemeral geographic features on the landscape over geologic time.

Study of Surface Water

People have been concerned with the origins and volumes of stream flow since the beginnings of civilized culture, as many early civilizations began on the banks of alluvial streams. The ancient Egyptians kept detailed records of the dates and heights of the annual floods of the Nile from as early as 3500 BCE., dependent as they were upon its delivery of water and nutrient-rich sediment for agricultural activities. Modern hydrologists depend upon similar measurements for much of their research. At more than 16,000 stream sites in the United States alone, gauging stations measure the height of the stream surface every fifteen minutes. These stage recordings are converted to flow rates on the basis of the previously measured cross-sectional area of the stream channel below each gauge height and the flow velocity. These measurements may be recorded on paper at the gauging station, to be retrieved later, or are sent electronically to the managing agency. The stream-flow data are stored in a computer for later tabulation and retrieval. Lake levels are similarly monitored and converted into water volume contained in the lake. In the United States, most stream and lake gauges are operated by the U.S. Geological Survey (USGS), which issues annual reports for all its gauges, organized and distributed by state (available from state offices of the USGS upon request). In addition, the flow data are stored in a computer system that is capable of providing much more detailed flow statistics than the printed reports. Stream gauges provide valuable warning of approaching floods in some locales. The cities of Bath, England, and Gatlinburg, Tennessee, have been heavily damaged by floods in the past but now have electronic gauges upstream that warn of approaching floods. More commonly, such readings are used to allocate limited water resources to downstream claimants.

Hydrologists frequently plot stream-flow rates against time to produce a graphical representation of discharge that is called a hydrograph. Analysis of the hydrograph can provide a wealth of information concerning stream-flow generation and timing mechanisms. By analyzing the form of the hydrograph of a small storm, it is possible to forecast the runoff characteristics of a large storm, such as the peak rate of flow and the time of peak flow.

To study how stream flow is generated, hydrologists employ various research methods. On small upland catchments, dye tracers are often used to determine paths and rates of movement. Dye tracers are also used in tracking the paths of disappearing streams in regions of soluble rock. To determine how much water is being added to or lost from a stream channel through bank seepage, stream reach surveys, which involve stream gauging along successive longitudinal segments of a stream between tributaries, are undertaken.

In areas where snowmelt is an important component of stream flow, regular snow surveys are carried out. Most commonly, this involves manual coring of the snowpack along predetermined transects to determine both the snow depth and the water content of the snow. Depth gauges can be checked electronically from low-flying planes at some locations, and telemetered weighing devices are also used. In the most remote areas, where manual surveys are not possible, aerial photogrammetry and satellite imagery are of great utility in determining snowpack extent and water content. Such information is critical in formulating a basic understanding of snowmelt hydrology as well as for allocating stream flow to irrigators and determining how much flood storage is needed in downstream reservoirs.

Significance

Water is a critical commodity in even the most primitive society. In warm weather, humans require a continual intake of water to maintain body functioning and, in very hot climates, cannot survive much longer than a day without water. In agricultural and industrial societies, immense quantities of water are needed for the production and processing of food, and the manufacture of various goods, and delivery must be reliable. A single hectare of cotton grown in Arizona requires on the order of fifteen million liters of water per year, while the production of a metric ton of steel requires roughly 250,000 liters. The production of a single Sunday newspaper typically involves the use of more than 1,000 liters of water. As a result of water-intensive agriculture and industry, the annual per capita use of water in the United States exceeds 2,300 cubic meters. To increase global living standards, large quantities of water are needed for various agricultural, industrial, and domestic purposes. In a world with nearly eight billion inhabitants, the acquisition of such volumes of water is increasingly difficult, and many experts believe that water supplies will be one of the primary limitations on future population growth. Water shortages have already constrained industrial and agricultural expansion in many areas and present a growing concern, with more than two billion people regularly experiencing water insecurity and more lacking access to safe drinking water. To monitor and address these challenges, scientists use surface measurements and satellite imagery to track Earth's surface water.

Surface water is unevenly distributed across the globe. Some locations have such a problem with excess water that vast areas are not arable and remain largely uninhabitable. Such an area is western Siberia, where more than two million square kilometers of land is annually flooded for extended periods by the Ob and Yenisei Rivers. Far larger areas are without adequate water supplies, many of them in Africa and Asia, the regions of the world with the greatest population pressures. The largest proportion of liquid surface water is in freshwater lakes, but only ten lakes account for two-thirds of all such water. Thus, a few areas (such as the Great Lakes area) are very well endowed with lake waters, while most parts of the world have very little. Most freshwater lakes are very small, contain small volumes of water, are very easily fouled, and are short-lived. Human-made reservoirs are no exception.

The second most voluminous source of surface water is saline lakes. The Caspian Sea alone contains three-fourths of such water supplies, with lesser bodies such as the Aral Sea, Great Salt Lake, and Dead Sea falling far behind. These bodies of water are not potable, or suitable for drinking, but they greatly influence local weather patterns and provide many important resources.

Although they contain far less water at any given time than do fresh or saline lakes, streams are the most important component of the surface water system because of the tremendous volumes of water that they convey throughout the year. Although not as limited in extent as lake water, stream flow exhibits great discrepancies in abundance across the globe. The equatorial zone is very well endowed with stream flow, as are the midlatitudes in general, but vast areas of the subtropical arid belts have very little surface water. Some such areas are blessed with exotic streams, which flow from areas having higher, cooler, wetter climatic regimes.

Principal Terms

deranged drainage: describing a landscape whose integrated drainage network has been destroyed by irregular glacial deposition, yielding numerous shallow lake basins

drainage basin: the land area that contributes water to a particular stream or river system; the edge of such a basin is a drainage divide

lake basin: an enclosed depression on the land surface in which surface waters collect; basins are created primarily by glacial activity and tectonic movement

saline lake: a lake with elevated levels of dissolved solids, primarily resulting from evaporative concentration of salts; saline lakes lack an outlet to the sea

stream: a body of flowing water that delivers surplus water from the land to the sea; this term covers all such moving water, including creeks and rivers

throughflow: the subsurface movement of surplus water through the soil to a stream

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