Fluvial systems: catchments and rivers - Fundamentals of the Physical Environment - page 317

LONGITUDE

180° 120° 60° 0°

60° 120°

80° 80°

8

8

60° 60°

6 8

6

6

4

4

4

40° 40°

20° 20°

0° 0°

20° 20°

40° 40°

60° 60°

60° 120° 180° 120° 60° 0°

Ice sheets, glaciers
and permanent snow

Stable snowpack
(duration in months)

Unstable snowpack
in most years

No snowpack

5

Figure 14.5 Global terrestrial snow and ice cover. Global warming will cause rapid reductions in snowpack.
Source: After Mackay and Gray (1981)

and is inversely proportional to precipitation intensity and
duration. Water also reaches the ground by direct
throughfall , or indirectly by stem flow and leaf drip . Ta b l e
14.2 shows some typical values.
Soil, and to a lesser extent bare rock, surfaces receive
water directly or from the vegetation cover. Shared
properties of porosity (void space) and permeability
(ability to transmit water) provide both storage capacity
and transmission routes. They vary substantially over
short distances in soils and at soil horizon boundaries, and
owing to other pedological processes (see Chapter 18).
Intense rain reduces infiltration rates in bare soil by
raindrop compaction and can cause surface crusting upon
drying. Cultivation provides aeration through seedbed
preparation (ploughing and harrowing) but compaction
by vehicle wheels or trampling, which drastically reduces
infiltration on cultivated sandy soils and induces gully
erosion ( Plate 14.2 ). Soil fissures (millimetre scale),

15

10

5

0

80 60 40 20 0 20 40 60 80

N Latitude S

+ + +

Figure 14.6 Global water balance. Run-off is P (red col-
umns)-E (blue columns); note the subtropical areas of large
water deficit (hot deserts) and very small polar water surplus
(cold deserts).

_ _