Geoscience Reference
In-Depth Information
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
capillary suction, as high matric forces draw molecules
from wetter to drier areas. Soil moisture storage is meas-
ured 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 fluctu-
ating 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.
Groundwater
stores are more stable and contribute to
delayed
or stream base flow. Global groundwater stores
amount to 23·4 M km
3
or 0·17 per cent of global water
balance, 55 per cent of which is saline. The remaining 10·5
M km
3
account for 97 per cent of non-frozen terrestrial
fresh water. This maintains 30 per cent of global river flow
and is a major source of human fresh water supplies.
Bedrock generally acts hydrogeologically as an
aquifer
(underground reservoir rock) or
aquiclude
(barrier to
significant absorption/transmission). Low-permeability
aquitards
retard flow between aquifers, whereas
aquifuges
absorb but cannot transmit water. Geological structure is
often as important as individual lithology in determining
catchment groundwater character (
Figure 14.3
).
Water
tables in soils and rock delimit the saturated and overlying
aeration zones and move in response to discharge-
recharge fluxes. Aquifers
confined
by impermeable strata
generate high water pressures and force a
piezometric
surface
in wells above the general water table of
unconfined
aquifers.
Phreatic water
below the water table moves more
vigorously than
vadose water
above it. Rock discontinuity
networks generate high permeability and can be enlarged
by corrosive flow, to such an extent that drainage may
unimpeded overland flow on former lake and fan sediments at
Zabriskie Point, Death Valley in south-east California.
Photo: Ken Addison
macropores
(millimetres) and
pipes
(centimetres or
metres) greatly increase downward
percolation
and lateral
throughflow
, emphasizing that soils are rarely 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 hydrat-
able 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
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
