Geography 110H
Fall 1997
(Not yet edited for Fall 1998!)

Assignment 6
Due: Tuesday, Nov. 4

This week we will look at a regional comparison of the relative amounts of runoff carried by different rivers. The total volume of water that flows down a river in a given year can be measured in cubic feet, cubic meters, acre-feet, or any other unit of volume. This water is derived ultimately from precipitation, and from our basic introduction to the hydrologic cycle we know that only a fraction of the water that falls as precipitation will actually appear in the stream as runoff. If we divide the total volume of streamflow by the total drainage area of the watershed, and if we convert units correctly, we can express the total amount of streamflow as an equivalent depth of runoff per unit area of the watershed.  We can also express the number of inches or centimeters of runoff as a percentage of the total number of inches or centimeters that fall as precipitation over the course of the year. Your job is to select several major rivers from different climatic regions; to find both the total annual depth of precipitation and the total annual equivalent depth of runoff; and to calculate runoff as a percentage of precipitation. Compare your results from different watersheds and discuss how they are related to regional patterns of climate.

In order to do this you can make use of the link entitled "Water balance in major river basins of the world." This link will take you to a list that includes many of the world's major rivers. For each river the author provides a link to a graph that is the hydrologic equivalent of a climograph: it shows the seasonal patterns of precipitation and potential evapotranspiration. It also shows the seasonal patterns of moisture storage (presumably in the soil; it's not obvious from the labels on the plot) and of runoff, as well as a variable labelled "vapor convergence" (we don't need to be concerned with that one for this assignment). At the bottom of each graph you will also find tabulated annual totals for rainfall, runoff, and evapotranspiration, expressed both in mm of water and in total kilograms of water.

Pick four rivers from the list and use whatever sources you can to find out where they are located. (If you can't do it easily on the web, then look it up in the index of a world atlas). Make sure they come from different climatic regions by checking on the map of global climates. Then examine the plot and describe the seasonal patterns of rainfall, evapotranspiration, and runoff. Make a table comparing the average annual precipitation, evapotranspiration, and runoff, and also calcualte runoff as a percentage of annual precipitation. Try to find climographs, either in the textbook or at one of the following sites (or anywhere else you can find them) that are representative of the same climate type as each of your rivers.

Clickable map of world climate regions
Climographs of selected U.S. cities

Once you have all of this information, write a discussion  that explains the comparison of your runoff patterns by placing it in the context of your understanding of the factors affecting regional climates. In other words, try to explain the total runoff and runoff as a percentage of rainfall, as well as the major characteristics of the seasonal pattern of runoff, with reference to the major factors that control climate in each of the four areas you have chosen. It will be helpful if you let everyone else know, by means of a quick posting on the bulletin board, which rivers you have chosen. There are about 70 rivers on the list, so it will be more interesting if we try to avoid duplicating our efforts and choose the widest possible range of rivers.

Hopefully this assignment will help you to tie together much of what we have been talking about for the last couple of weeks, and it should also be helpful in preparing for the next exam. If you have any questions or comments, send me email or post your comments on the bulletin board.