Environmental Geology - geol 406/506

SURFACE WATER - PROCESS, SUPPLY & USE

Module 7 - Part 1

By Scott Hughes

• Reading:  Chapters 10, Keller, 2000

• Review of topics covered in this module in pdf format or in powerpoint version

INTRODUCTION

"Water, water everywhere but not a drop to drink" is a line from the "Rime of the Ancient Mariner" (Lyrical Ballads). Why use this particular phrase as an introduction to water basics? Because fresh water is critical to survival. The more we know about water, the better we can manage the resource.

Water on Earth is known by different terms, depending on where it is and where it came from.

GLOBAL 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. 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."

GLOBAL WATER SOURCE AND VOLUME

SOURCE: USGS, 1984, The Hydrologic Cycle- Pamphlet

Surface water supplies, primarily river runoff, are about 300 cubic miles. That means we have about 1/10,000^th 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.

To re-familiarize yourself with erosion processes, refer back to Module 3.

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! ....Study this closely before exam ! ! !

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. These drainage basins 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).

CONSERVING AND MANAGING WATER

Frequently population density and major water supplies do not coincide. Therefore, water must be transported great distances to the consumers. California is a good example. Population density is greatest in the lower 2/3 of California, south of San Francisco, while water supplies are greatest in the upper 1/3. Major water diversion projects were implemented to transport water to densely populated areas.

Los Angeles and Owens Valley (opposite sides of the Sierra Nevada) are fighting over water rights. This has been an ongoing problem since the early 1900’s! The LA-Owens River Aqueduct was constructed, completed in 1913. So much water has been diverted to LA that the Owens Valley has suffered from desertification (transformed into a more desert-like environment). By limiting water diversion, environmental degradation may be reduced.

As our population increases, the need for water conservation and management also increases. An example of river management is the Colorado River. The Colorado River is the most regulated river in the U. S. Numerous dams, reservoirs and canals are "part of" the Colorado River system. The Colorado basin encompasses parts of Wyoming, Colorado, Utah, New Mexico, Arizona, California, and Mexico. All want their fair share of water! Thus, water management is important.

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. A large desalination plant is under construction on the lower Colorado River, upstream from the Imperial Dam. After treatment, water should be of usable quality. Mexico would then be able to use it for agricultural purposes.

WATER USE IN THE U.S.

Sources of water in the United States include surface water, groundwater, and desalination of seawater.

In the next section, Part 2 of Module 7, "Groundwater" will be discussed in detail.

Surface water use includes instream and offstream uses. Offstream use either removes or diverts the water. Consumptive use, a form of offstream use, is water used by industry, irrigation, and households. Eventually, water is returned to the stream or groundwater system. Instream use is not removed or diverted. Examples of instream use include cooling, navigation, salmon runs, and fishing.

Almost half the U.S. population uses groundwater as a primary source for drinking water. Groundwater accounts for ~20% of all water withdrawn for consumption. Unfortunately, in many locations groundwater withdrawal exceeds natural recharge rates. This is known as overdraft. In such areas, the water table is drawn down "permanently"; therefore, groundwater is considered a nonrenewable resource. The Ogallala aquifer underlies Midwestern states, including Texas, Oklahoma, and New Mexico, while California, Arizona and Nevada use the Colorado River as their primary water source. All show serious groundwater overdraft.

For information on the Ogallala aquifer, visit this sites:

http://www.meteor.iastate.edu/gccourse/issues/society/ogallala/ogallala.html

Groundwater supplies 30% of the water present in our streams. 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.

Desalination is the removal of salt from seawater. It is a very expensive process (~ 10 times that paid for traditional water supplies) and is considered a last resort alternative.

WETLANDS

Not too long ago, wetlands had a bad image. Wetlands were just dank, murky swamps. Fortunately, we now realize wetlands have a vital role in our environment. Wetlands are a natural filter system. Wetland plants remove toxins from water and sediment. Freshwater wetlands act as sponges and soak up excess water, reducing flood conditions. Coastal wetlands are buffer zones. They reduce the erosion impact of storms and high waves. Wetlands also provide a habitat for numerous wildlife and plant species.

To learn more about wetlands, you may wish to visit the following sites:

Recommended Reading:

The textbook "Rocks, Rails and Trails" by Paul K. Link is found on the Digital Atlas of Idaho Web Site. I has information about Southern Idaho. Take a look at Rocks, Rails and Trails, Chapter 8

Terms to Know:

This site can help if you need: http://www.groundwater.org/kc/kidsvocab.html

Other websites with information on water:

	This is USGS site with multiple links containing lots of information on water: http://water.usgs.gov/
	http://www.groundwater.org/
	http://observe.arc.nasa.gov/nasa/earth/hydrocycle/hydro1.html

End of Module # 7 - Part 1

Go To Module #7 - Part 2

Did you email your assignment?

Back to Environmental Geology Home

Back to ISU Geosciences Web Courses Home