Geoscience Reference
In-Depth Information
Catchments range in scale from single
first-order
streams (see below), less than 1 km
2
in area, to major trunk
rivers, such as the Thames (9,950 km
2
) or Mississippi
(3,270,000 km
2
). River flow is measured from single
precipitation event responses, in hours or days, to the
annual
water balance year
. The extent to which actual river
discharge flow differs from the 'instantaneous' rate and the
precipitation pattern is a function of catchment controls
on lag time and routing towards or away from the channel
system. The hydrological system ought to be one of the
most easily understood open systems of energy and
material transfers in the physical environment. In essence,
water passes from available store to store, in sequence via
a set transfer route as the capacity of each is reached. In
practice, the land surface and its cover of vegetation or
buildings disguises much of the system. We can measure
quantities and rates at a limited number of visible points
but depend mostly on estimates or extrapolations.
Hydrometeorological transfers
The (
P - E
) portion of the water balance deals with
primary
hydrometeorological
transfers between atmos-
phere and catchment (precipitation inputs and evapo-
transpiration outputs).
Precipitation
is the total atmos-
pheric input of water or water-equivalent mass (snow,
ice) and varies according to volume, type, intensity,
frequency and annual regime (see
Chapter 5).
Specific
parameters are measured at
points
in the landscape by rain
gauges.
Area
estimates are provided by radar assessment
of rainfall intensity, or gauge-and-radar combinations
with ground truth from gauges refining radar estimates.
Data expressed as millimetres of water depth or rates in
mm hr
-1
are directly applicable only to the point or area
concerned. However, total catchment data are required for
most purposes. Area estimates are therefore made using
statistical weightings, which reflect catchment character
such as area and altitude. The spatial density of gauges, in
particular, is low - e.g. a minimum of one per 100 km
2
in
Catchments and watershed models
SYSTEMS
The hydrological catchmentdefines the geographical surface area and geological subsurface structure which delivers
water to each trunk river. This three-dimensional landsystem is bounded by a
watershed
. The hydrological system
defines the structure of component stores, transfer mechanisms and processes whose individual character and spatial
location provide catchment variables (
Figure 14.4
).
Hydrological studies focus ultimately on river channels, although
they cover 1 per cent of catchment area, but most water flow commences underground and the role of the
hydrological system is emphasized by a hypothetical case. If all precipitation fell directly into channels, stream flow
exiting the catchment would be very rapid, although not instantaneous - a function of catchment shape, stream
connectivity, average slope angles and channel friction, discussed below. Catchment storage, including the channel
itself, creates a
lag time
(delay) in water transmission. This moderates the episodic nature of precipitation and sustains
stream flow during drier spells. It also buys time for other water-using systems, such as the biosphere and humans,
which divert some water away from the channels and reduce overall river flow. This is summarized in a basic water
balance equation:
Q = (P - E) + (S + T)
where Q = stream flow, P = precipitation, E = evapotranspiration, S = net change in storage and T = net
underground (influent - effluent) transfers. Where storage increases and effluence exceeds influence, S and T
are negative (and vice versa) and Q correspondingly falls (or rises).
Management of catchment water balances for human use creates the need for watershed models. Empirical models
link hydrometeorological inputs with catchment properties to predict important hydrological parameters such as the
mean annual flood and annual average discharges. Conceptual or analogue models simulate catchment store and
transfer networks, to calculate their individual and collective water balances, and are also widely used now to predict
the hydrological impacts of climate change.
