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.

• Fresh water in lakes and rivers accounts for less than 1% 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 runoff.
• 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.
• The movement of groundwater through highly soluble limestone rock produces karst landscapes .
• Runoff includes both overland flow and flow in stream and river channels.
• Streams and rivers are organized into drainage networks in which the channels collect overland flow from slopes and seepage from groundwater, and transfer this water downstream to larger channels.
• 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 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.
• Human activities can cause pollution of both surface and underground water.

KEY TERMS

hydrology         water table        drainage system    
infiltration      stream             drainage basin     
runoff            channel            flood              
overland flow     discharge          floodplain         
groundwater       hydrograph         lake               
karst                                                   

STUDY QUESTIONS

1. What is infiltration and how does it affect the amount of precipitation which will become overland flow?
2. Does all of the precipitation absorbed by the soil become groundwater? Why?
3. 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.
4. What are some of the distinctive features of a karst landscape and how are they formed?
5. How can the pumping of groundwater from wells induce land subsidence? Give an example.
6. 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.