What is Surface Water?
Water is continually moving around, through, and above the Earth. It moves as water vapor, liquid water, and ice. It is constantly changing its form. Water on Earth is known by different terms, depending on where it is and where it came from.  

* Meteoric water - is water in circulation
* Connate water - "fossil" water, often saline.
* Juvenile water - water that comes from the interior of the earth.
* Surface water - water in rivers, lakes, oceans and so on.
* Subsurface water - Groundwater, connate water, soil, capillary water
* Groundwater - exists in the zone of saturation, and may be fresh or saline.

The movement of water is referred to as the global water cycle (hydrologic cycle). Precipitation, evaporation/transpiration, and runoff (surface runoff and subsurface infiltration) are the primary phases in the hydrologic cycle. The global water budget is based on the recycling (movement, storage, and transfer) of the Earth’s water supply.

The direct process by which water changes from a liquid state to a vapor state is called evaporation. In transpiration, water passes from liquid to vapor through plant metabolism. Plants are classified as hydrophytes, phreatophytes, mesophytes, or xerophytes. Hydrophytes take their nutrients directly from the water. Mesophytes are plants that grow under well-balanced moisture supplies. Xerophytes are plants that are adapted to dry conditions. Phreatophytes are long rooted plants that absorb water from the water table or directly above it. Golden tamarisk and mesquite are phreatophytes.

How Much Water is There In and On the Earth?
The volume of the Earth’s water supply is about 326 million cubic miles. Each cubic mile is greater than 1 trillion gallons. Although water is abundant on a global scale, more than 99% is unavailable for our use. A mere 0.3% is usable by humans, with an even smaller amount accessible! The oceans, ice caps, and glaciers contain most of the Earth’s water supplies. Ocean water is too saline to be economically useful, while glaciers and icecaps are "inconveniently located." Click here to see a chart with these data.

Surface water supplies, primarily river runoff, are about 300 cubic miles. That means we have about 1/10,000th of 1% to use! Conservation is important!

Surface runoff plays an important role in the recycling process. Not only does it replenish lakes, streams, and groundwater; it also creates the landscape by eroding topography and transporting the material elsewhere.

A stream typically transports three types of sediment- dissolved load, suspended load, and bed load. Chemical weathering of rocks produces ions in solution (examples- Ca2+, Mg+, and HCO3+). Hence, a dissolved load. High concentrations of Ca2+ and Mg+ are also known by another name - hard water. Some of you may be very familiar with hard water! Take a look at some water chemistry.

Suspended sediment makes water look cloudy or opaque. The greater the suspended load, the muddier the water. Bed load (silt- to boulder-sized, but mostly sand and gravel) settles on the bottom of the channel. Bed load sediment moves by bouncing or rolling along the bottom. The distance that bedload travels depends on the velocity of the water.

Factors Affecting Surface Runoff
Several factors can affect surface runoff. The extent of runoff is a function (ƒ) of geology, slope, climate, precipitation, saturation, soil type, vegetation, and time. Geology includes rock and soil types and characteristics, as well as degree of weathering. Porous material (sand, gravel, and soluble rock) absorbs water far more readily than does fine-grained, dense clay or unfractured rock. Well-drained material (porous) has a lower runoff potential therefore has a lower drainage density. Poorly-drained material (non-porous) has a higher runoff potential, resulting in greater drainage density. Drainage density is a measure of the length of channel per unit area.  Many channels per unit area means that more water is moving off of the surface, rather than soaking into the soil.

Drainage basins or watersheds have different shapes and sizes. Large drainage basins are usually divided into smaller ones. Size and shape have a direct effect on surface runoff. Refer to Module 3 to see information about drainage basins.

Which Type of Drainage Basin Has the Greatest Effect on Surface Runoff?
Long, narrow drainage basins generally display the most dramatic effects of surface runoff.  They have straight stream channels and short tributaries. Storm waters reach the main channels far more rapidly in long narrow basins than in other types of basins. Flash floods are common in long, narrow drainage basins, resulting in greater erosion potential.

Topography (relief) and slope (gradient) are additional factors affecting water velocity, infiltration rate, and overland flow rate. Water velocity, infiltration rate and overland flow rate affect surface and subsurface runoff rates.

Climate is also important. Precipitation (type, duration, and intensity) is the key climatic factor. Infrequent torrential downpours easily erode sediment-laden topography, while soft drizzly rain infiltrates the soil.

Vegetation aids in slope stability. Removal of vegetation by fire, clear-cutting (logging), or animal grazing often results in soil erosion. The eroded material is washed into streams, adding to the sediment load.

Runoff Paths
There are three runoff paths that water follows to reach a stream channel- throughflow, overland flow, and groundwater flow.

Throughflow is a shallow subsurface flow that occurs above the groundwater table. A major requirement for throughflow is a good infiltration capacity. Throughflow commonly occurs in humid climates containing thick soil layers and good vegetation cover. In such locations, saturated soil conditions result in surface runoff (overland flow).

Overland flow occurs when precipitation exceeds infiltration rates. Overland flow is common in semi-arid regions, sparsely vegetated and/or disturbed areas, and locations containing dense, clay-rich layers.

Surface Water /Groundwater Interaction
Surface streams have an effect on the groundwater table. Influent streams recharge groundwater supplies. Influent streams, located above the groundwater table, flow in direct response to precipitation. Water percolates down through the vadose zone to the water table, forming a recharge mound.

Effluent streams are discharge zones for groundwater. Effluent streams are generally perennial (flow year round). Groundwater seeps into stream channels, maintaining water flow during dry seasons.

The Big Lost River in Idaho is a good example of an intermittent, ephemeral influent stream. Natural flow of the Big Lost River terminates in the Big Lost River Sinks, located on the INEEL. But, local irrigation now diverts the Big Lost River from its natural terminus

Groundwater supplies 30% of the water present in our streams. Recall that effluent streams act as discharge zones for groundwater during dry seasons. This phenomenon is known as base flow. Groundwater overdraft reduces the base flow, which results in the reduction of water supplied to our streams.

Equally important is water quality. Salinity, a by-product of water flowing over salt beds, salt springs, and irrigation and evaporation, increases with distance downstream.
Images by DAI production staff, 2001
For more information found on the web, visit our hydrology web link page.