Environmental Engineering Reference
In-Depth Information
associated species) and concentrating on the solely
hydrological issues - and focusing on explanation rather
than prediction - scientists, policy makers and citizens
are mainly interested in:
are widely used. The term 'watershed' is usually reserved
for the boundary of a catchment but in the US and Canada
it is also used to represent the catchment itself. Catch-
ments are a component of the hydrological cycle capturing
water from the atmosphere through the interception
of rainfall (and other forms of precipitation such as
fog - Zadroga, 1981) by land and plants and transferring
it either back to the atmosphere from land and plants, or
to the sea by river or groundwater flow. The drainage basin
is thus a 'cog' in the 'machine' of the hydrological cycle.
Catchments are very clearly spatially separated through
their watersheds and are, indeed, one of the more
easily categorized aspects of the natural world. Catch-
ments are multi-scale with many small subcatchments
joining to make up the estimated 6152 global catch-
ments with coastal outlets that can be defined using the
30-minute global topological network of rivers produced
by Vorosmarty
et al
. (2000). These catchments are simi-
lar in many ways even though they range enormously
in size. Vorosmarty
et al
. (2000) define the Amazon
as the largest catchment in the world with an area of
5
.
854
the reasons behind a devastating flood event or events;
•
the reasons for a devastating hydrological (as opposed
to climatic) drought;
•
the potential impact of climate change on flooding and
water resources;
•
the potential impacts of land-cover and/or land-use
change or housing development on flooding and water
resources;
•
the seasonal and long-term regime of a river or set of
rivers (for engineering purposes); and
•
the potential downstream impacts of channel engi-
neering, flood alleviation or other river- or near-river
engineering developments.
•
Answering these questions is not solely a matter of
modelling but, at the catchment scale (because con-
trolled experiments are not possible), modelling plays an
increasing important role alongside field monitoring and
(paired) catchment experimentation.
10
6
km
2
and a length of 4327 km followed by the
Nile, Zaire and Mississippi catchments. The Amazon has
33 main tributary catchments and a multitude of tributary
streams. Much more detailed assessments of catchment
topology are now available based on high-resolution dig-
ital elevation models (DEMs) such as the Shuttle Radar
Topography Mission (SRTM),
1
which provided high res-
olution DEMs from 56
◦
Sto60
◦
N and from which the
90-m spatial resolution HydroSHEDS flow networks have
been derived (Lehner
et al
., 2008).
×
11.2 The complexity
11.2.1 What arecatchments?
The catchment of any point in the landscape is that
area which provides water to the point through lat-
eral flow over the surface and underground. Catchments
are usually delineated on the basis of watersheds deter-
mined from surface topography. In this way the complex
and long-term processes that determine landscape form
(geomorphology) also determine the topographic and
thus topological properties of catchments at all scales. A
watershed is a positive topographic feature that defines the
boundary between two catchments such that the water
either side of the watershed flows into different catch-
ments (
often
to different outlets). Topographically defined
watersheds may not always be an accurate representation
of the actual watershed because subsurface lateral flow
within the soil and particularly within the bedrock will not
always follow surface topographic control. Indeed sub-
terranean flows between catchments can be significant,
especially in the saturated zone for some mountainous
basins. Nevertheless, topographically defined watersheds
are appropriate for most surface hydrological studies and
11.2.2 Representingtheflowofwater
in landscapes
Potential flow paths for surface water are usually evaluated
using a neighbourhood operation on a digital elevation
model (DEM) of topography, though we should bear
in mind that flow paths are modified by surface fea-
tures such as field boundaries and agricultural activities,
infrastructure such as roads and dams and water-transfer
schemes, which are not usually resolved by DEMs. Digital
elevation models are common in hydrological modelling
and provide a raster (square cellular, grid-based) rep-
resentation of the topography of an area (O'Callaghan
and Mark, 1984; Tarboton
et al
., 1992). Each cell has
a single value for elevation and the cell-sizes of DEMs
vary according to the area under study and the technique
used to develop them, but those applied in hydrological
1
See www2.jpl.nasa.gov/srtm/.
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