Page 62 - Fundamentals of Geomorphology

THE GEOMORPHIC SYSTEM 45

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a
b
500 Metamorphic and plutonic rocks 50 Cold regions
Volcanic rocks 200mg/l Temperate regions
Sedimentary rocks
Hot regions
Mixed rocks
100 /year) 10
/year) 50mg/l 2
2 50 (t/km 5
(t/km 25mg/l silica
transport dissolved

Chemical 10 5 Sedimentary rocks of Transport 0.5 1 15mg/l 6mg/l
Metamorphic and plutonic rocks 3mg/l
Volcanic rocks

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 (1990) set about the task of solving this problem by con-
Papua New Guinea. sidering three questions: (1) whether indeed relief and
All the general and detailed summaries of global and climate are major determinants of soil loss; (2) if so,
regional sediment yield (e.g. Fournier 1960; Jansson whether relief or climate is the more important deter-
1988; Milliman and Meade 1983; Summerfield and minant at the global scale; and (3) whether other factors
Hulton 1994) split into two camps of opinion concern- known to influence soil loss at a local scale have a signifi-
ing the chief determinants of erosion at large scales. canteffectattheglobalscale.Phillips’sresultsshowedthat
Camp one sees relief as the prime factor influencing slope gradient (the relief factor) is the main determinant
denudation rates, with climate playing a secondary role. of soil loss, explaining about 70 per cent of the maximum
Camp two casts climate in the leading role and relegates expected variation within global erosion rates. Climate,
relief to a supporting part. Everybody seems to agree measured as rainfall erosivity, was less important but
that either relief or climate, as measured by surrogates of with relief (slope gradient) and a runoff factor accounted
rainfall erosivity, is the major control of erosion rates on for 99 per cent of the maximum expected variation.
a global scale. The problem is deciding on the relative The importance of a runoff factor, represented by a
contribution made by each factor. Jonathan D. Phillips variable describing retention of precipitation (which is