Geology Reference
In-Depth Information
()
a
()
b
500
50
Metamorphic and plutonic rocks
Volcanic rocks
Sedimentary rocks
Mixed rocks
Cold regions
Temperate regions
Hot regions
100
10
50
5
10
1
5
0.5
1
0.1
1
5
10
50
0.5
1
5
10
50
2
2
Runoff (l/s/km )
Runoff (l/s/km )
Figure 2.5
Dissolved loads in relation to runoff. (a) Chemical transport of all major ions plus dissolved silica versus
runoff (specific discharge) for various major drainage basins underlain by sedimentary, volcanic, and metamorphic and
plutonic rocks. (b) Evolution of the specific transport of dissolved silica for cold, temperate, and hot regions.
Source:
Adapted from Meybeck (1987)
has been measured in an area draining karst terrain in
Papua New Guinea.
All the general and detailed summaries of global and
regional sediment yield (e.g. Fournier 1960; Jansson
1988; Milliman and Meade 1983; Summerfield and
Hulton 1994) split into two camps of opinion concern-
ing the chief determinants of erosion at large scales.
Camp one sees relief as the prime factor influencing
denudation rates, with climate playing a secondary role.
Camp two casts climate in the leading role and relegates
relief to a supporting part. Everybody seems to agree
that either relief or climate, as measured by surrogates of
rainfall erosivity, is the major control of erosion rates on
a global scale. The problem is deciding on the relative
contribution made by each factor. Jonathan D. Phillips
(1990) set about the task of solving this problem by con-
sidering three questions: (1) whether indeed relief and
climate are major determinants of soil loss; (2) if so,
whether relief or climate is the more important deter-
minant at the global scale; and (3) whether other factors
known to influence soil loss at a local scale have a signifi-
cant effect at the global scale. Phillips's results showed that
slope gradient (the relief factor) is the main determinant
of soil loss, explaining about 70 per cent of the maximum
expected variation within global erosion rates. Climate,
measured as rainfall erosivity, was less important but
with relief (slope gradient) and a runoff factor accounted
for 99 per cent of the maximum expected variation.
The importance of a runoff factor, represented by a
variable describing retention of precipitation (which is
