Soil Erosion and Conservation - Environmental Modelling: Finding Simplicity in Complexity - page 357

Table 22.1 Percentages of map grid cells in which changes over time in erosivity values, RP, calculated using
precipitation were inconsistent in sign with changes in the values of erosivity, RF, calculated using the
Fournier coefficient.

Inconsistencies in erosivity between RP and RF
For all Data Where also both | RP | and | RF | > 10%

Model scenario 40-yr. interval 80-yr. interval 40-yr. interval 80-yr. interval

(%) (%) (%) (%)

HadCM3 17.2 22.2 1.0 1.5

+

CGCM1 HG A1 17.4 19.4 0.7 7.6

Table 22.2 Percentages of map grid cells in which changes over time in erosivity values calculated over
the 40-year time interval were inconsistent in sign with changes in the values of erosivity calculated over
the 80-year time interval.

Inconsistencies in erosivity between 40- and 80-year time intervals
For all data

| |

Where both the 40 y. R and

80 yr | R | > 10%

Model scenario RP RF RP RF

(%) (%) (%) (%)

HadCM3 22.2 15.2 1.5 1.0

+

CGCM1 HG A1 7.6 23.6 0 5.6

The RF values tended to show a somewhat greater mag-
nitude, in terms of the average of the absolute value of per-
cent erosivity change, than did the RP values (Table 22.3).
The difference between the two models in this regard
was striking. The Canadian model indicated a much
greater level of erosivity changes overall as compared
to the Hadley Centre model (Table 22.3). Both models
suggested erosivity changes which generally increased in
magnitude from the 40-year to the 80-year comparison.

experiencing a greater than normal number of wet days.
The results given by Nearing (2001) discussed above pro-
vide a broad view of expected changes in erosivity based
on the statistical models, but an important question not
addressed is the expected differences in erosivity that
come about relative to rainfall intensity versus a simple
increase in the average number of rain days in a year.
Erosion is not linearly proportional to rainfall intensity
(Wischmeier and Smith, 1978; Nearing et al ., 1990).
Pruski and Nearing (2002) recently performed com-
puter simulations to obtain estimates of potential runoff
and soil-loss changes as a function of precipitation
changes. In particular they studied the different responses
of the erosional system to changes in precipitation as they
occurred with changes in rainfall intensities, including
the amount of rainfall that occurs on a given day of rain,
versus responses to changes in simply the average num-
ber of days of rain. Assessments were made using WEPP
for several combinations of geographic locations, soils,
crops, and slopes. Geographic locations included West

22.3.2 Effectsofprecipitation-intensitychanges
versusnumberofdaysof rainfall

Now we take a look at another study of the effects of pre-
cipitation changes on soil-erosion rates, but this time we
use the WEPP model. As we mentioned above, historical
weather records analysed byKarl et al . (1996) indicate that
since 1910 there has been a steady increase in the area of
the United States affected by extreme precipitation events
as well as an increase in the proportion of the country