Computing su bsy st ems

2.3.1

The annual volume of primary information received from a single gauging station is in the neighbourhood of 150,000 characters. Other observations such as water quality might produce between 300 and 600,000 characters. The summarizing of these data in yearbooks is a time-consuming job. Moreover, several types of statistical information required for the analysis of hydrologic systems, i.e. flood frequency determinations, involve the use of complex algorithms. The use of complex algorithms has forced the hydrologist to depend on electronic computers for the solution of these problems and for the development of digital models of hydrologic systems.

Processing and analysis of data

2.3.1.1

A continuous record of flow at a gauging station is computed from records of stage and the discharge rating for the station. The type of stage recorder used at the station determines whether the computations will be done manually or by an electronic computer.

In either case the engineer must study the data and prepare what is termed a station analysis before computations are performed.

A station analysis, which documents the results of the study of the data, is prepared for each station at the end of each water year. The study includes the following items:

1. A review of field surveys of gauge datum and a determination of the datum corrections, 2. A review of discharge-measurement notes.

3. An analysis of the discharge rating and the determination of the rating (or shifts) applicable during each period of the year.

4. The preparation of tables which express the discharge rating.

Computation and preparation of records

if any, to be applied to gauge readings during the year.

Description of different systems characteristics

2.3.1.2 M a n u a l computation

If stage is recorded at the station on a strip-chart recorder all computations are performed manually in the following order:

1. Determination and application of gauge-height and time corrections to the gauge- height chart.

2. Computation of the mean gauge height for each day, or for shorter periods if the range in discharge during the day is large. Subdivision is necessary because of curvature in the discharge rating.

3. Computation of discharge for each period from mean values of stage and the discharge rating, including any shift corrections.

4. Computation of peak values of gauge height and discharge.

5. Listing of the values of mean daily gauge heights and discharge and monetary peaks.

6. Computation of mean flow for each month and the year in cubic feet per second (or 7. Review and comparison of the record of discharge with that of nearby streams.

in m3/sec), and inches (or millimetres).

2.3.1.3 Airtonintic coinpiitation

If stage is recorded on a digital tape at the station, the computations outlined above are performed by an electronic computer. The input to the computer is the digital record of stage, with a list of any datum corrections, and the discharge rating with a list of any necessary shift corrections. For stations at which the stage-fall-discharge type of rating is applicable, the digital-tape records of stage from both the primary and the auxiliary gauges are furnished to the computer. In addition to the stage-discharge relation, supple- mentary information such as the stage-fall relation and the fall-ratio versus discharge-ratio relation are supplied.

The computer offers the possibility of different forms of output adapted to the needs of the users. T w o commonly used forms are mentioned here. The first includes a listing of the maximum, the minimum, and the mean gauge height for each day, bihourly gauge heights for each day, and the mean discharge for each day. The second form includes a listing of mean daily discharges and the the monthly and yearly summaries in the same format as is used for publication. Daily discharges and yearly summaries can also be stored on magnetic tape. Corrections are made on the tape where necessary after the computed records are reviewed by engineering personnel in the district offices. More advanced forms of output can be developed.

2.3.1.4 Analysis of water-quality records

Water-quality records generally consist of field and laboratory analyses of water samples.

Generally, these analyses are tabulated and published in yearbooks in a manner similar to that used for presentation of streamflow data. Automation of water-quality records, either laboratory analyses or from an automatic monitor, is accomplished using routines similar to those for streamflow records.

2.3.1.5 Meteorological and other records

The tabulation and analysis of observation well data or meteorological data is a highly specialized science. The reader is referred to the Unesco manual, Ground-water Studies

(this Series, No. 7), for the analysis of geohydrologic data.

For meteorological data, presentation, and analyses the publications of the World Meteorological Organization provide the best international information (cf. for example, W M O Technical Note No. 115); useful information is also given in national publications.

2.3.2 Processing centres

The term'processing centre' means any office within a water data system where raw field data or notes are processed into engineering data. The simplest example would be an engineering office where records of gauge heights are converted to values of daily flow for subsequent use in planning and design of water-resource facilities. A ' processing centre' can also be a rather complex system involving the use of large-scale electronic computers to provide information on the current status of the national water resource.

Each national administration uses the facilities of local field centres and to a degree all have some sort of national centre where water data can be made available to agencies or other entities that require these data to meet their responsibilities. A n outline of some processing centre requirements for a developing country is given in Chapter 3. The systems described in this section relate to the most advanced processing centres and equipment utilized in several developed countries. This information is presented here to guide water- resource authorities in developing systems which will provide compatibility on a world- wide basis. The scarcity of trained manpower in every country necessitates the speedy adoption of the most advanced techniques so that major economic developments can take place as rapidly as possible.

2.3.2.1 Computer hardware

Computer hardware systems are composed of input systems, a central processor, and output devices. The size and complexity of these systems depend on the job to be done.

They can be as simple as a card punch, card reader, and electric typewriter costing $3,000 to a third-generation computer involving the investment of several million dollars.

Computers suitable for the processing of hydrologic data should have the following general requirements.

Input system. Input devices must be able to accept information in one or more of the various forms, such as : punched cards, optical character recognition, perforated tapes, and magnetic tapes.

Centralprocessor. The central processor must have a sufficient operating memory capacity, exceeding 62,000 bytes, capacities exceeding 252,000 bytes are required for some problems.

It should have a compiler for at least one high-level programming language, i.e., Fortran IV, PL/1, Algol 60, Cobol, or their equivalent so that assembly programming can be avoided. The external storage capacity should be sufficient to handle all the data currently available. It should have direct-access capability and be subject to expansion as required by the needs of the operating system.

Output devices. The output devices should be capable of alphanumeric printing of suffi- cient speed to produce tabular data for immediate use and for publication in the yearbook.

Line printers already in use have the capacity to handle several hundred lines of 128 to 160 characters per minute. The system should be able to reproduce punched cards from magnetic tape, and to copy magnetic tape from magnetic tape for use on other computer systems.

Other output devices include automatic plotting systems that are capable of reproducing hydrography from discharge data stored on magnetic tape or cards.

2.3.3

National organizations responsible for operating water resources data and information systems have constantly sought to improve the efficiency and effectiveness of their services.

As modern societies become more dependent on continuous supplies of good water, the Algorithms for computer processing of water data

Description of different systems chavacieristics

need for water data increases through all levels of administration. A n increase in the amount of information generated in a water resources data network invariably leads to an increase in technical personnel to carry out the collection and processing of data. In both developed and developing countries there has been a shortage of trained hydrologists and technicians to perform these activities.

In the developed countries there has been an increasing reliance on automation of both the data collection and processing activities associated with water-resource networks. This has been done to keep the cost of doing business to a level compatible with the value of the information obtained. The following sections of this chapter describe some of the auto- mated computing systems used in some developed countries for processing water data.

2.3.3.1

Gauge heights and discharge measurements are collected either on punched tape (sixteen- channel) or on analogue charts. Data collected on punched tape are converted to the computer-compatible magnetic tape on an off-line magnetic tape translator. Data on analogue charts are either digitized on magnetic tape by a line-follower or sent through an optical character scanner for conversion to magnetic tape (Fig. 2.3). Punch cards are used to enter all supplemental data including station name, programme options, rating tables (Q = K(H)) datum corrections, and shift adjustments.

The computer output consists of one entry per day as shown in Fig. 2.4. A summary of data for each day consists of the daily mean gauge height and equivalent gauge height.

The mean daily discharge is stored on a magnetic disc pack for updating at a later time and is used to print tables for the annual yearbook. For permanent record-keeping the data are stored on seven-channel tape in binary-decimal code. Each record consists of the following information:

Example oj’primary computation of streamflow data

1. Station number;

2. Year, month, day;

3. Daily discharge in hundreds of cubic feet per second or cubic metres per second.

2.3.3.2

The input to automated processing of water-quality data is by punched card. The punched cards are prepared from the analytical notes and inputted to the computer directly. The programmes generally list each constituent in a preselected order, and computer tons-per- day of dissolved solids, sodium-adsorption ratios, percentage sodium, noncarbonate hardness, alkalinity as calcium carbonate, etc. (see Fig. 2.5).

Retrieval of the data from the historical file is done in a form that can be used directly for the preparation of an annual yearbook, as specified by the user. A n example of a retrieval from the historical file is shown in Figure 2.6.

E.uanpIes of processing of water-quality data

2.3.3.3

Most major national administrations responsible for water resources information have hundreds of computer programmes on file for data processing. These programmes have been written for specific computer systems, but they can be adapted to almost any com- puter system. For instance the United States Geological Survey prepares an annual pub- lication entitled Computer Programs for Processing Water Data. This manual contains abstracts and samples of output from computer programmes used to process water data.

As new computer programmes become available they are added to the manual. At the present time over 250 programmes have been developed for data processing in the United States.

Availability of algorithms for data processing