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

I.2-24
GUIDE TO HYDROLOGICAL PRACTICES
Optimize operations
Operational procedures account for a significant
portion of the cost of data collection. This includes
the types of instrument, frequency of station visits
and structure of field trips. The minimum-cost
operational procedures should be adopted.
Budget
Based on the identified network and operational
procedures, the cost of the operation of the network
can be established. If this is within the budget, the
next step can be followed. If not, either additional
funding must be obtained or the objectives and/or
priorities need to be examined to determine where
costs may be reduced. The process adopted should
allow the designer to express the impact of insufficient
funding in terms of not meeting objectives or
reduced information and net impacts.
Implementation
The redesigned network needs to be implemented
in a planned manner. This will include both shortand
long-term planning horizons.
Review
Since a number of the above components are
variable in time, a review can be required at the
instigation of any particular component – for
example, changes in users or uses, or changes in the
budget. To be ready to meet such changes, a
continuous review process is essential.
2.4.2 Density of stations for a network
The concept of network density is intended to serve
as a general guideline if specific guidance is lacking.
As such, the design densities must be adjusted to
reflect actual socio-economic and physio-climatic
conditions. Computer-based mathematical analysis
techniques should also be applied, where data are
available, to optimize the network density required
to satisfy specific needs.
As stated in 2.4.1.3.1, the minimum network is one
that will avoid serious deficiencies in developing
and managing water resources on a scale commensurate
with the overall level of economic
development and environmental needs of the
country. It should be developed as rapidly as possible,
incorporating existing stations, as appropriate.
In other words, such a network will provide the
framework for expansion to meet the information
needs of specific water uses.
In the following sections, minimum densities of
various types of hydrological stations are
recommended for different climatic and
geographic zones. These recommendations are
based on the 1991 review of Members’ responses
regarding the WMO basic network assessment
project (WMO/TD-No. 671) and are presented in
Table I.2.6. However, these recommended network
densities are being revisited through a study
undertaken by the Commission for Hydrology and
the revised recommended densities will be placed
on the Website as part of the electronic version of
the Guide.
It is impossible to define a sufficient number of
zones to represent the complete variety of hydrological
conditions. The simplest and most precise
criterion for the classification of zones would be on
the basis of the areal and seasonal variation of rainfall.
Each country could present a good map of
annual precipitation and a minimum network
could be developed from this. However, this would
Table I.2.6. Recommended minimum densities of stations (area in km 2 per station)
Physiographic
unit
Precipitation Evaporation Streamflow Sediments Water quality
Non-recording Recording
Coastal 900 9000 50 000 2 750 18 300 55 000
Mountains 250 2500 50 000 1 000 6 700 20 000
Interior plains 575 5750 5 000 1 875 12 500 37 500
Hilly/undulating 575 5750 50 000 1 875 12 500 47 500
Small islands 25 250 50 000 300 2 000 6 000
Urban areas – 10–20 – – – –
Polar/arid 10000 100000 100 000 20 000 200 000 200 000