Environmental Engineering Reference
In-Depth Information
uniformly porous. Fissures develop seasonally through desiccation or shallow mass
wasting. Macropores may be single or connected pores, often reflecting soil structure,
and develop into pipes by enlargement during percolation. Pipes also develop through
animal burrows and in the presence of swelling clay minerals.
During rainfall, water percolation advances along a
wetting front
in unsaturated soil,
filling voids and hydratable particles. If sustained infiltration exceeds onward drainage
the soil above this front becomes saturated, with implications for
overland flow
.
Gravitational drainage, taking one to two days, does not leave soil absolutely dry. Water
molecules, adhering to soil particles and each other, develop a
matric force
of 1 × 10
9
Pa
(approximately 10
4
Table 14.2 Interception as percentage of rainfall
Vegetation type
Condition
Intercept
(%)
Crop type
Condition
Intercept
(%)
Tundra
Dwarf
shrubs
45-55
Larch
Plantation
20-25
Pine
Woodland
35-42
Spruce
Plantation
24-32
Spruce
Forest
36-45
Sown
grassland
Full cover
18-23
Deciduous
woodland
Winter phase
12-15
Maize
Growing
15-18
Deciduous
woodland
Leaf phase
25-40
Maize
Full cover
40-50
Tropical hardwood
Forest
12-22
Wheat
Winter
cover
3-8
Grassland
Full cover
22-25
Wheat
Full cover
18-20
times atmospheric pressure) which binds thin water films (< 0·06 mm) around particles.
They are invulnerable to gravity but available to plant roots and evaporation by capillary
suction, as high matric forces draw molecules from wetter to drier areas. Soil moisture
storage is measured by tensiometers and neutron probes. It is at
field capacity
when
capillary
and
hygroscopic
water is at a maximum after gravitational drainage and at
wilting point
when accessible capillary water has been removed. Soil water tables,
measured in dipping wells, mark fluctuating saturation zones. Water is diverted laterally
towards channels as throughflow where onward drainage is inhibited by less permeable
soil horizons or bedrock. Global soil moisture stores of 16,500 km
3
account for only
0·0012 per cent of global water, or 0·07 per cent of non-frozen terrestrial water. Despite
this, its short cycling time (0·04-1 yr) and rapid fluctuation according to weather, land
use and hydrogeological conditions give it a major influence on water flow (see Figure
5.22).
Groundwater
stores are more stable and contribute to
delayed
or stream base flow.
Global groundwater stores amount to 23·4 million km
3
or 0·17 per cent of global water
balance, 55 per cent of which is saline. The remaining 10·5 million km
3
account for 97
