Environmental Engineering Reference
In-Depth Information
character such as area and alti-tude. The spatial density of gauges, in particular, is low -
e.g. a minimum of one per 100 km
2
in the United Kingdom
CATCHMENTS AND WATERSHED MODELS
systems
The hydrological
catchment
defines 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 are catchment variables (Figure 1). Hydrological studies focus ultimately
on river channels, although they cover less than 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
lag time
(delay)
in water transmission. This moderates the episodic nature of precipitation and sustains
stream flow during dryer 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:
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.
