Chapter 16
The Cycling of Water on the
Continents
OVERVIEW
This chapter examines what is probably the single most important environmental agent
acting at the Earth’s surface: water. The circulation of water within the Earth’s surface
system is described by the hydrologic cycle. This chapter examines two parts of that
cycle: water at the land surface and water underground.

Water on the continents represents on three percent of the hydrosphere’s total water.
Fresh water in lakes and rivers accounts for less than one percent of the Earth’s water.

This fresh water is derived from precipitation over the continents generated in the
operation of the global hydrologic cycle.

The soil layer plays an important role determining if precipitation will be directed to
the atmosphere by evapotranspiration, to groundwater by percolation, or to streams
and rivers as overland flow.

Groundwater refers to water beneath the surface that fully saturates the pore spaces
in bedrock, regolith, or soil.

The water table marks the upper surface of the groundwater zone where the pore
spaces in rock and regolith are completely filled or saturated with water.

Groundwater moves slowly underground to eventually return to the surface by
seepage into streams, rivers, ponds, and lakes.

An aquifer is a layer of rock or sediment that holds abundant free flowing ground
water.

The movement of groundwater through highly soluble limestone rock produces
distinctive karst landscapes.

Human use of groundwater resources have resulted in depletion of groundwater
resources, ground subsidence, and groundwater contamination.

Runoff includes both overland flow and flow in stream and river channels.

Streams and rivers are organized into drainage systems in which the channels collect
overland flow from slopes and seepage from groundwater and transfer this water
downstream to larger channels. Each stream has a drainage basin, which includes
all the land around the stream that supplies water to the stream. The drainage basin is
bound by drainage divides, which mark the ridges between adjacent drainage basins.

Discharge, or the flow rate of water past a location on a channel, increases
downstream through drainage networks as flow from tributary streams is collected.

A hydrograph is a plot showing the variation in the discharge of a stream over time.

The lag time between peak discharge and peak precipitation reflects the time required
for water to move down slopes and through progressively larger stream channels.

In urban areas, the high proportion of impervious surfaces and the storm sewers that
move water to stream channels very rapidly act to reduce the lag time and increase
the peak discharge levels of urban streams.

In humid regions, annual stream hydrographs show discharge peaks from individual
rainfall events superimposed on base flow derived from groundwater.

Floods occur when discharge exceeds the capacity of a river channel and water flows
over the low-lying floodplain adjacent to the channel.

Lakes are important elements of the drainage system because they are sources of
fresh water and are used for recreation and hydroelectric power generation.

Lakes without a surface outlet often become saline as they lose water primarily
through evaporation. Saline lakes are characteristic of arid climates.

Humans living in desert areas have relied on exotic rivers for their water supplies.
Many of these areas have experienced problems from salinization and water logging
as a result of irrigation.

Human activities can cause pollution of both surface and underground water.

Our industrialized society demands huge quantities of fresh surface water. The
increasing environmental impacts of these high levels of demand necessitate very
careful planning for future water developments.
KEY TERMS
hydrology
infiltration
runoff
overland flow
groundwater
karst
water table
stream
channel
discharge
hydrograph
drainage system
drainage basin
flood
floodplain
lake
exotic streams
STUDY QUESTIONS
1.
2.
3.
4.
5.
What is infiltration and how does it affect the amount of precipitation which will
become overland flow?
Does all of the precipitation absorbed by the soil become groundwater? Why, or why
not?
Draw a simple cross-section diagram of a valley. Show the likely configuration of the
water table and the flow paths of the groundwater on your diagram.
What are some of the distinctive features of a karst landscape, and how are they
formed?
How can the pumping of groundwater from wells induce land subsidence? Give an
example.
What is stream discharge? How does it change as the velocity and cross-sectional area
of the flow change?
7. How do cross-sectional area and velocity change through pools and rapids along a
stream? Why?
8. Where does the highest flow velocity in a stream usually occur? Why?
9. What is the relationship between drainage basins, drainage divides and drainage
networks?
10. Sketch a hydrograph to show how stream flow responds to a rainfall event. Indicate
base flow on your diagram.
11. What are floods and under what conditions do they usually occur?
12. Lakes are relatively short-lived features on the geologic time scale. What processes
can cause lakes to disappear?
13. Identify some of the human activities that can cause pollution of surface water and
groundwater.
6.
CHAPTER QUIZ
Multiple Choice Questions
1. The absorption of precipitation by the soil at the land surface is termed:
a) evapotranspiration
b) infiltration
c) saturation
d) percolation
2. Caverns, travertine deposits, and sinkholes form in landscapes dominated by
________ rock.
a) granite
b) marble
c) limestone
d) shale
3. The immediate source of stream base flow is:
a) groundwater seepage
b) stream flow from tributaries
c) overland flow from slopes
d) snow melt
4. If the discharge of a stream remained constant while the channel became much
steeper, one would expect:
a) velocity increase and cross-sectional area increase
b) velocity increase and cross-sectional area decrease
c) velocity decrease and cross-sectional area decrease
d) velocity decrease and cross-sectional area increase
5. Which of the following is likely to result from the urbanization of a drainage basin?
a) reduction in the lag time of runoff
b) decrease in the frequency of flood peaks
c) increase in the base flow to streams
d) decrease in the magnitude of flood peaks
True/False Questions
1.
2.
3.
4.
5.
The elevation of the water surface in shallow wells is approximately the same as the
elevation of the water table. (T/F)
Floods are completely unexpected natural disasters. (T/F)
A lake is considered a landform. (T/F)
The Mississippi flood of 1993 was unusual, because it occurred in the spring. (T/F)
Discharge increases as one moves downstream through a drainage network. (T/F)
Short Answer Questions
What are the most important sources of water for stream flow?
How can the movement of groundwater through limestone produce a system of dry
caverns?
3. Why does the lag time between precipitation and stream flow discharge peaks tend to
increase as drainage basins become larger?
1.
2.
Short Essay Questions (1 - 2 paragraphs)
An intense thunderstorm causes heavy rainfall over a small drainage basin. Describe
the pathways that the water delivered by the storm will follow to the streams and how
this water will affect stream discharge.
2. The irrigation of arid lands for the purpose of enhancing agricultural production can
lead to soil salinization and water-logging. Discuss why these problems result, why
drainage systems might help alleviate the immediate problem, and what additional
problems might arise from the drainage systems.
3. Assume that the development of a large livestock feedlot has been proposed for a site
adjacent to a small lake. In what ways might the proposed development affect the
adjacent lake?
1.
Internet Resources
1. NASA's Observatorium – The Hydrologic Cycle:
<http://observe.ivv.nasa.gov/nasa/Earth/hydrocycle/hydro1.html>
2. The Nature of Water – includes discussion of the hydrologic cycle and groundwater:
<http://www.ec.gc.ca/water/en/nature/e_nature.htm>
3. US Environmental Protection Agency – Watershed Protection :
<http://www.epa.gov/owow/watershed/>
4. National Watershed Network: <http://www.ctic.purdue.edu/KYW/nwn/nwn.html>
5. The Grand River Conservation Authority: <http://www.grandriver.ca/default.html>
6. Hydrologic models –a discussion of the KINEROS model:
<http://www.tucson.ars.ag.gov/kineros/>
7. NEED INTRO: <http://wwwga.usgs.gov/edu/measureflow.html>
8. Water Measurement Manual of the US Department of the Interior:
<http://www.usbr.gov/wrrl/fmt/wmm/index.htm>
9. Groundwater basics: <http://www.groundwater.org/GWBasics/gwbasics.htm>
10. The UNESCO Water Portal: <http://www.unesco.org/water/about.shtml>