Guide to Hydrological Practices, 6th edition, Volume I - Hydrology.nl

I.2-22
GUIDE TO HYDROLOGICAL PRACTICES
relationship, usually by means of correlations, with
one or more of the base stations. A new secondary
station can then be established with the equipment
and funds that had been in use at the discontinued
site. Records can be reconstructed at the discontinued
site by means of the base-station records and
the inter-station relationship. At times, it may be
necessary to re-establish secondary stations if it is
believed that the conditions either at the secondary
site or at its related base station(s) have changed.
The perpetual nature of the principal stations in the
basic network provides a basis for monitoring longterm
trends in hydrological conditions in the
region. This is particularly important in the light of
potential changes in the hydrological cycle that
could be caused by land-use changes or by increases
in stratospheric greenhouse gases.
2.4.1.4 Integrated network design
The hydrological cycle is a continuum, and its interconnections
permit the partial transfer of
information obtained in one part of the cycle to
another. The efficiency of such transfers is proportional
to the degree of hydrological understanding
that is captured in the models that are used to route
the water (and the information) between the parts
of the cycle. For example, precipitation records on
or near a gauged drainage basin permit the reconstruction
of streamflow records during periods
when the stream-gauge malfunctions if a valid
precipitation-runoff model has been calibrated
during times when all gauges were functioning
properly. A groundwater observation well may
perform a similar role for malfunctions of the
stream gauge if the well is monitoring the water
table of an aquifer that is directly connected to the
stream.
To date, little has been done to include these interactions
in network designs in an explicit manner.
Ideally, the complementarity between the raingauges
and the stream gauges that are operated in a
flood-forecasting network could be used in designing
a network for water resources assessment, for
example. If the economic trade-offs between the
two networks could be defined, they could be optimized
together and peak efficiencies in information
generation could be attained for both. In spite of
this technological shortcoming, networks should
be designed iteratively, and the outcomes of an
existing design should become starting points for
subsequent designs. By extension of the above
example, this can be illustrated. The flood-forecasting
network will probably have stream gauges and
precipitation gauges at rather specific locations to
meet its information needs. As the water resources
assessment will generally have less specific requirements
for its information sources, it will be likely
that many of the gauges of the flood-forecasting
network can be incorporated into the assessment
network and used as initial given conditions for its
design. This iterative approach is particularly useful
when designing generalized networks, like the basic
network on the basis of networks, with more restrictive
information demands. Networks with more
restrictive demands include benchmark stations,
representative basins and networks for operational
purposes.
2.4.1.4.1 Stations for operational purposes
Stations may be established for such specific
purposes as reservoir operation, irrigation, navigation,
water-quality monitoring or flood forecasting.
Benchmark or reference stations would also belong
to this category. The length of operation of special
stations is related to the purpose for which they
were installed.
In some cases, the specific purpose to be served
may require observations on only one particular
aspect of an element, or be confined to one season
of the year. For example, a hydrometric station
may consist of a crest gauge for recording only the
maximum flood peak or a storage gauge for measuring
the total precipitation during a season.
Although such stations may perform a valuable
function, they do not provide the data required
for general hydrological analyses. Consequently,
such stations may or may not be included in a
basic hydrological network.
2.4.1.4.2 Benchmark stations
Each country and each natural region of large countries
should contain one benchmark station to
provide a continuing series of consistent observations
on hydrological and related climatological
variables. Hydrological benchmark stations should
be established in areas that are relatively uninfluenced
by past or future anthropogenic changes.
Since long records are the essence of a benchmark
station, consideration should be given to existing
stations if they meet the other requirements. The
Reference Hydrometric Basin Network of Canada is
one such example (Harvey and others, 1999).
Climatological benchmark stations are known as
reference stations.
2.4.1.4.3 Representative basins
A representative basin is desirable in each natural
region – especially in those regions where great