EXERCISE #7:  SOIL EROSION AND SEDIMENT YIELD

To complete this assignment, you will need your textbook, your lecture notes, and a calculator.  You will also need to print out a copy of a U.S.G.S. fact sheet on Soil Erosion of the Palouse River Basin of Washington and Idaho.  As you print this document, also print enlarged versions of Figure 2 and Figure 3.  The following questions are based on this fact sheet.

History of the Palouse Erosion Problem

1.  Why would the conversion of land use from hay and pasture to grain production cause an
      increase in soil erosion?

 2.  How does the erosion rate estimate of 14 tons/acre/year for cultivated cropland in the Palouse
      River Basin compare with rates given in Figure 6.19 a. of your textbook (p. 186)?

Effects of Erosion Control Practices

The U.S.G.S. estimates that erosion control practices implemented in the Palouse River Basin from 1979 to 1994 have reduced soil erosion by 10 percent.  Apparent reductions in suspended-sediment yield from the basin through time and the correlation in Figure 2 are used to support this hypothesis.

1.  Why might a reduction in soil erosion not cause suspended-sediment yields to decrease?
      (Hint: consider how the Palouse River might adjust to reduced soil erosion).

2.  Figure 2 shows the correlation between calculated soil erosion rates and measured suspended-
     sediment yields for the Palouse River Basin during the 1962-1971 time period.  Based on this
     plot, do you think it is possible to detect a 10 percent reduction in soil erosion by measuring
     suspended-sediment yields?

3.  Figure 3 suggests that the average 1962-1971 suspended-sediment yield is two times the  
     average 1993-1996 suspended-sediment yield.  Why might this figure be misleading?

4.  If the value from 1963 is ignored, what would the average suspended-sediment yield be for
     the 1962-1971 time period (see Table 1, below)?

5.  Do you think that the data presented in this study  allow us to draw a conclusion about the
     effectiveness of erosion control practices on soil erosion rates in this basin?

TABLE 1:  Suspended Sediment Yields¹ in the Palouse River Basin by Year

¹yields given in units of tons/acre-foot, rounded to the nearest significant digit

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