Physical Geography

Consider the hydrological cycle again.

This exercise is concerned with the portion in which precipitation infiltrates into the soil or runs off as surface runoff into streams and rivers.  The total land surface that sheds water to any particular river channel is its catchment (also known as its watershed or at a larger scale as its drainage basin.)
For surface runoff to occur, the following must have already happened, if they are going to happen at all:

• vegetation intercepts rainfall.
• infiltration wets the soil (water can remain here for a long time).
• if the soil cannot infiltrate all the rainfall, surface depressions fill up (puddles.)
• if the rainfall continues, the depressions overflow, creating overland flow.

This graph shows the normal relationship between interception, depression storage, infiltration, and surface runoff over time, during an extended rainfall in a forested area.

1.  After 40 minutes of rainfall, what percentage of the precipitation is being used for:
(a) infiltration.
(b) surface runoff.

2.  After 2 hours and 20 minutes?
(a)
(b)

3. At what time do you think 'flooding' may become a concern and give a reason why? Plot on the graph, where you think flooding will become a concern.

 Important factors dictating the amount of runoff are the properties of the surface material and the duration and intensity of the rainfall.
4.  This graph shows how the infiltration rate changes as the storm continues.

(a)  Describe and explain what happens to the infiltration of water into the soil as a rainstorm progresses.
(b)  What is the link between infiltration and surface flow?
(c)  What do you think will happen to the amount of surface flow as the rain storm progresses?
(d)  Explain how runoff would be affected if the material were (i) clay, (ii) impermeable rock, (iii) forested land.

5.  A hydrograph is a graph of both the rainfall (as a histogram), and river discharge (as a line) versus time.
 

(a)  Which letter on the diagram corresponds with:

• flood peak
• lag time
• storm flow
• base flow.

The effect of land use, particularly urbanization but also artificial drainage systems under agricultural land, is significant.
6.  This graph shows the relationship between total rainfall and surface runoff, in urbanized and natural settings.

(a)  Describe the relationship shown by the graph.
(b)  Suggest 3 characteristics of urban areas that cause this relationship.  (Think back to the information at the beginning of this exercise.)

7.  This graph is a model hydrograph for a natural drainage basin, an urban drainage basin, and a drained agricultural system.

(a)  Which line do you think represents which land use?  Explain.
(b)  How might the following measures improve a river's flood response?
       (i) Keeping as many trees as possible.
       (ii)  Detain rainfall in hollows on the surface, or in check dams in the river.
       (iii) Man-made canal systems connected to the river (Toronto has these).