Water for the Future: Hydrology in Perspective (Proceedings of the Rome Symposium, April 1987). IAHSPubl. no. 164, 1987.

Advances in technology in hydrology — a view from the United Kingdom

R. W. HERSCHY 2 Queensborough Drive, Reading, Berkshire RG4 7JH, UK

ABSTRACT Considerable advances in hydrological methods and instrumentation have taken place in recent years. Some of these are still in the experimental stage, some in the pre-operational stage but others can now be considered as operational. Some of the more important of these advances are described in the paper relating to the United Kingdom. Of these, a most significant advance has been made in data collection which in many countries is the major cost area in hydrology. New methods of data collection presented in the paper include the optical shaft encoder and the solid state logger with take-away memory. The paper also reviews the new methods of streamflow gauging including ultrasonic and electromag­netic methods. A brief introduction of the application of satellite remote sensing to hydrology is also presen­ted. The paper shows how the introduction of weather radar enables precipitation to be determined on an areal basis as well as forecasting the movement of storms. Finally advances in telemetry and data processing are described.

Progrès importants dans le technologie hydrologique -une évaluation faite dans le Royaume Uni RESUME Des progrès importants sont apparus ces dernières années dans le domaine des méthodes hydrologiques et de l'instrumentation. Certains d'entre eux sont encore à l'état expérimental, d'autres sont en voie de devenir opérationnels, mais d'autres peuvent être considérés comme déjà opérationnels. Certains de ces progrès, parmi les plus importants, sont décrits dans ce rapport. Parmi ceux-ci, l'un des progrès les plus significatifs a été réalise dans la collecte des données qui, dans beaucoup de pays, constitue le domaine le plus onéreux. Les nouvelles méthodes de collectes des données présentées dans ce rapport comprennent l'utilisation de codeurs optiques et de centrales de mesure a mémoires amovibles. Le rapport passe également en revue les nouvelles méthodes de jaugeage telles que les méthodes ultrasoniques et électromagnétiques. Une brève introduction a l'emploi de satellites pour la télédétection est également présentée. Le rapport montre comment l'utilisation d'un radar météorologique permet de déterminer les précipi­tations sur le plan régional et de prévoir le mouvement des perturbations. Enfin sont décrits les progrès dans

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le domaine de la télémétrie et du traitement des données.

INTRODUCTION

Microelectronic technology has added significantly to the methods available for hydrological data collection, processing and analysis. Hydrologists have not been slow to take advantage of new technologies and apply them to their own needs. Many of these applications are described briefly in the following paragraphs. Some of the methods and instrumentation described are in common usage, some are under evaluation and others, still in the research stage, look promising. The definition of hydrological measurement has necessarily been widened to include not only simple sensing technology such as water level floats, but also complex systems such as radar, remote sensing and ultrasonic and electromagnetic flow gauges.

WATER LEVEL MEASUREMENT

A number of recent developments or adaptations have tkaen place that, together, make the task of the hydrologist significantly easier. In the field of water level sensing, there have been notable advances in the interfacing of float-driven sensors with electronic recording systems, and in overcoming some of the inherent disadvantages of transducer-based systems for use in river, borehole and sewer environments.

Shaft encoders

The shaft encoder has come to be considered as a potential alterna­tive to the digital or autographic recorder. As a piece of effective technology, the shaft encoder has been in successful use for many years.

The two basic types of shaft encoder are the absolute and the incremental, each with its own attributes. The absolute encoder is the more mechanically complex of the two, relying on conducting tracks and trailing wipers that rotate with the shaft itself and pick up the binary pattern encoded on the tracks. The simpler, cheaper optical incremental shaft encoder (Fig.l) provides a basis for an effective water level measuring system.

Systems are now available commercially that are capable of discriminating water level to 1 mm over a range of 10 m. They provide a digital display of level, are incorporated in or provide an output to an associated telemetry device, and are powered for a year or more by a few flashlight batteries.

DATA RECORDING

As computers have become more and more user friendly, cheaper, smaller and portable, the limitations of punched paper tape and charts as recording mediums have come to be exposed. At the same time, the "no-moving-parts" recorder offering great flexibility, ease of use and low maintenance costs through the use of microelec-

Advances in technology in hydrology 489

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FIG.1 Two examples of float-driven optical incremental shaft encoders (courtesy of Adam Hilger Ltd).

tronic technology, has come on the scene and a number of solid state loggers are now available. They can be loosely classified as being either "dumb", "smart" or "intelligent".

Another sub-classification distinguishes those that use take-away memory from those that do not. There are some designs that allow a memory module to be detached from the logger itself, so that logged data can be returned to base for processing, such as a roll of paper tape which is extracted from a digital recorder. There are designs that require total logger substitution to facilitate data recovery, but where the logger is small enough, rugged enough and cheap enough (relative to the application) for this to be a practical proposition. Other loggers can talk to a portable retriever (a portable computer) which can extract data from them for transport back to base. More up to date still is a class of device that can communicate directly with base by way of a telecommunications link (Walker, 1986).

Solid state data logging

In the United Kingdom the use of solid state data logging for hydrological purposes dates from about 1979, when simple event recorders were introduced into rainfall measurement. These were capable of recording the time at which the bucket in a tipping-bucket raingauge tipped, to the nearest half minute, on a detachable unit of solid state memory known as a Cachette (a word coined by a particular logger manufacturer, but which has come to be used as a generic term). Cachettes (Fig.2) come in different memory sizes, typically from 4K through 8K up to 16K nibs capacity (a nib being shorthand for a nibble, which equates with half a byte, in other words a four-bit character representation.

The loggers described are easy to install, cheap to maintain and reliable in operation. The take-away memory modules are easy to handle (typically about the size of a cigarette packet) and suitably rugged, so that data loss through Cachette malfunction is exceedingly

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FIG.2 Solid state memory module (Cachette) (courtesy of Adam Hilger Ltd).

rare. Field life between Cachette change can be up to 56 days (at 15 minute logging intervals) in the largest Cachette size. The time taken to read a memory module into a computer is five to six times less than an equivalent paper tape. Figure 3 shows a typical Cachette reader and associated microcomputer.

FIG.3 Cachette reader and associated microcomputer (courtesy of Adam Hilger Ltd).

ULTRASONIC (ACOUSTIC) FLOW MEASUREMENT

The principles and practice of this technique are well understood and described adequately in the literature (Herschy, 1985). However, some recent innovations have added greatly to the versatility and, in particular, the reliability of this method. Single-path acoustic

Advances in technology in hydrology 491

flow gauges have always been of limited use in locations where a wide variation in stage occurs, and it has taken the arrival of cheap and powerful microprocessors to enable multipath gauges of great com­plexity to operate reliably enough to be properly viable, and to fill the performance gap left by the simpler single-path systems.

The principal breakthrough has been the adoption of a systems engineering approach to the design of the newer multipath systems (Fig.4), that considers, in particular, exactly what the user wants from his gauge and how best to provide it. Multipath ultrasonic

FIG.4 A multiple ultrasonic transducer array (courtesy of Adam Hilger Ltd).

gauges depend upon the precise sequencing of a complex series of actions by a controlling microprocessor, the digestion of a con­siderable data set and the mathematical manipulation of that data set, all to produce a single value of instantaneous flow. Greatly improved self-checking techniques help to reduce the likelihood of the user rejecting the data, but the ability to conduct post mortems on doubtful data is a great advantage. As a consequence, the secondary data logger has become a feature of the newer gauges. This device keeps a short-term detailed record of all the system parameters that have contributed to the computation of related values of flow. Where one or more of the latter are found to be suspect, the secondary record can be inspected for aberrations in individual values of path velocity or water depth.

ELECTROMAGNETIC FLOW MEASUREMENT

In this method a well known technology has been removed from the experimental category and made full (and economic) use of by a combination of engineering and electronic innovation, the combination

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492 R.W.Herschy

producing a simpler device. The introduction of electrical insulation of channel bed and

banks has been a major breakthrough while, at the same time, increasing by approximately an order of magnitude, the size of the signal available to be measured. Increasingly powerful micropro­cessors have eliminated the need for the microcomputer that was a distinctive feature of early electromagnetic flow gauges and has helped reduce total system cost. Lowered power requirements for the electromagnet itself, coupled with developing microelectronic component technology, have also greatly increased system reliability.

With the confirmation of electromagnetic flow gauging as an accepted technique for rivers, it was not long before a variant appeared that addressed the problem of full-range sewer flow measure­ment, and this technique is now well established (Fig.5). An ultrasonic water level sensor is normally added to the system to make it non-intrusive, and the various related considerations of safe operation in a confined space have also been considered in the design.

CURRENT METER GAUGING

It is the recent arrival of the field-portable microcomputer that has opened up the possibility of a "one pass" current meter gauging system, and versions of such a device are presently under evaluation

FIG.5 An electromagnetic sewer flow gauge (courtesy of Adam Hilger Ltd).

Advances in technology in hydrology 493

in both the United States and the United Kingdom. In the UK a program has been developed to run on a battery-powered, field-portable microcomputer, that puts a potentially very powerful tool in the hands of a gauging technician (Fig.6).

FIG.6 A field-portable microcomputer for current meter gauging.

The availability to the gauging technician of the result, without effort, while he is still on site, is a very great advantage allowing, as it does, the best possible form of quality control to be applied - early examination by a person knowledgeable about field conditions. The other advantages are that there are no transcrip­tion or computation effort, improved timeliness in availability of the result to data users and the opportunity to confirm the measure­ment if it chances to be an extraordinary one.

WEATHER RADAR

New technology now enables weather radar to detect all forms of precipitation measured to an acceptable precision, in near real-time, from a single location. Moreover, in association with a computer,

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data from the weather radar can be processed to provide areal totals of precipitation over sub-catchments or entire catchments. This information enables streamflow and levels to be predicted which is particularly useful in flood forecasting.

Data from weather radar are normally presented visually on a television screen in a number of different colours to indicate different intensities of rainfall, usually eight, which is about the maximum the eye can distinguish comfortably under operational conditions.

Provided the radar is calibrated against independent measurements such as a network of telemetering raingauges, it is able to make quantitative measurements of rainfall over areas with sizes ranging from a few square kilometres to many thousands of square kilometres and can also assess snowfall. Measurements obtained during the Dee Weather Radar Project in the UK indicated that to achieve the same precision for hourly totals of rainfall measurement in showery rain (i.e. very unevenly distributed in time and space) a radar calibra­ted by one gauge per 1000 km is equivalent to a gauge density of at least 35 per 1000 km2. Although in more uniform rain, fewer gauges would be required, radar is more flexible and virtually unaffected by areal distribution of rainfall.

Radar operates in real-time and can display up to the minute rainfall information and provides sub-area totals on, for instance, an hourly basis. Archiving procedures however enable detailed precipitation information to be collected and retained, so that the data may be used for many of the tasks to which data from manually observed or recording gauges are put.

REMOTE SENSING

The conventional measurement in hydrology is a point-time phenomenon, whereas hydrology itself is concerned with space-time phenomena. This suggests that conventional ground measurements in hydrology have inherent disadvantages. It is towards the rectification of these shortcomings that the main thrust of remote sensing applica­tions is directed, by supplying the essential, but missing, spatial element. There is an essential difference between the type of information provided by field measurement and that to be derived from remote sensing interpretation, and work has been concentrated on methods of correlating the two different kinds of information. Three broad categories of utilization of remote sensing data for hydrology have been recognized. First there is the simple qualita­tive kind of observation, such as difference of tone or hue in water bodies, suggesting some kind of pollution, suspended sediment or depth difference. Second there is the type of analysis related to measurement of size, pattern, location and distribution, such as would be performed in a quantitative analysis of basin drainage networks. Finally, there is the category dealing with correlations between point measurements on the ground and the relevant properties of remote sensing data such as reflectance and emittance levels in various EM energy bands. Such correlations then allow extrapola­tions of the point measured data into the realms of space and time by means of remote sensing imagery.

Advances in technology in hydrology 495

The role of remote sensing in hydrology

Investigations into the applications of remote sensing data to water resource assessment were largely a phenomenon of the late I960's, and although considerable success has been achieved in demonstrating that such applications have great potential, it is probably fair to say that the actual use of remote sensing data in hydrology is not very widespread, and that, in general, hydrologists and others involved in water resources do not, as a matter of course, take advantage of the opportunities offered by the use of remote sensing data. Practical hydrology is virtually synonymous with engineering hydrology, and whilst engineers are certainly familiar with the air photograph, their training in its use is rather limited and probably more inclined towards the measurement aspects of interpretation than the broader interpretive aspects. The more recent developments in remote sensing are probably not very well known or understood by hydrologists and there is an obvious need for training in this area.

Combination of satellite and weather radar data

Satellite data used in conjunction with weather radar imagery forms a powerful combination for watching the development of weather systems, tracking their movement and measuring the precipitation associated with them.

A network of five weather radars in England provides up-to-date information on the distribution of surface rainfall. Radar pictures are available at 15-minute intervals for a 5-km grid covering the country and replaying recent sequences reveals the movement of areas of rain. This leads to the prediction of their future movement and detailed forecasts for specific locations. A satellite ground station receives and processes Meteosat imagery every 30 minutes. These images allow rain clouds outside the radar coverage to be identified and included in the forecast. The radar and satellite pictures are on the same map projection so that they may be freely mixed on a display monitor. The forecaster can thus examine the relationship between the rain at the surface and the cloud above.

TELEMETRY

Six stages may be identified in a data acquisition system no matter how simple the system may be. These are:

(a) primary measurement, (b) encoding, (c) transmission, (d) decoding, (e) validation, (f) use.

The extent of a telemetry system encompasses stages (b) to (e) (Fig.7). An interface exists between each of these stages.

A comprehensive telemetry system

Remote monitoring of rainfall, river and groundwater levels and

496 R.W.Herschy

a.

Primary sensor

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Signal conditioning

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Coding

Control logic

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I On-site data j I record/ I I display/ | I storage j

Communications modem

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NOTE — Dashed blocks indicate optional extras to the basic system. 1

Data processing

Data storage/ retrieval

Data presentation

'' FIG.7 Data acquisition system - block diagram (courtesy of the International Organization for Standardization).

Advances in technology in hydrology 497

providing controls at key sites is an essential prerequisite for effective river basin management. If properly designed, the system should also be suitable for providing historic data, and providing a forecasting role.

A suitable system may comprise four main elements. (a) A single-shot telemetry system for automatically retrieving

current rainfall and river levels for operational purposes including flood warning.

(b) A hydrometric archiving system which will enable retrieved historic data to be automatically processed and archived.

(c) A flow forecasting system whereby flow forecasting models can be run on a real-time basis.

(d) A system of operating river management models on a real-time basis with or without associated control functions.

Means of transmission

The most readily available, and most widely used, energy form is electrical and sufficient power requires to be generated by the transmitter to guarantee the integrity of the received signal, allowing for energy losses along the transmission path. In a terrestrial environment, these losses can be significant whereas outside the Earth's atmosphere they become insignificant.

Transmission by line Lines may be "dedicated" or shared with general purpose communications. In some networks, lines may be dedicated to data transmission but shared between users. A dedica­ted line would generally have a high level of system integrity, but the installation and maintenance costs may be high and certainly higher than for a shared line.

Transmission by radio Radio waves are specified by frequency (in Hertz) and occupy the portion of the electromagnetic spectrum having frequencies less than visible light. For convenience, these frequencies are normally identified by band widths (Table 1).

TABLE 1 Radio frequencies

Bandwidth

Very low frequency (VLF) Low frequency (LF) Medium frequency (MF) High frequency (HF) Very high frequency (VHF) Ultra high frequency (UHF) Super high frequency (SHF) Extra high frequency (EHF)

up to 30 kHz from 30 kHz from 300 kHz from 3 MHz from 30 MHz from 300 MHz from 3 GHz from 30 GHz

to 300 kHz to 3 MHz to 30 MHz to 300 MHz to 3 GHz to 30 GHz to 300 GHz

Hydrological telemetry is generally restricted to the UHF and VHF bands; therefore, the transmitter and receiver antennae should be

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on a radio-line-of-sight for reliable reception.

Transmission by satellite Recent advances in electronic technology have enabled low cost radio transmitters ("Data Collec­tion Platforms") with built-in data acquisition and processing capabilities to be used for routine hydrological data collection.

Hydrological data may be collected with the use of either polar-orbiting or geostationary satellites. The latter are more suited to synoptic reporting and emergency (alert) applications, since they offer continuous availability compared with the few minutes every few hours offered by polar-orbiting satellites.

MICROCOMPUTER HYDROLOGICAL DATA PROCESSING

Many hydrological data acquisition systems have performed effectively over extended periods but the traditional data gathering, processing and archiving procedures have become increasingly less suited to the information requirements of water management. In particular, they have lacked the adaptability to exploit fully the data acquisi­tion potential associated with the introduction of modern data sensing, recording and transmission facilities. Fortunately, the advances made in data archiving techniques and the opportunities afforded by microprocessor technology allow all but the most demand­ing hydrological data processing problems to be addressed indepen­dently of monolithic mainframe computer installations. These have often tended to limit the flexibility of data processing options and commonly afford little priority for hydrological activities in general.

The power of modern microcomputers allied to their tolerance of a range of environmental conditions can provide a viable alternative to mainframe processing and can facilitate the introduction of a data processing and archiving capability into regions where computer usage has previously been impractical. Perhaps the most pressing problem remaining is how best to apply the new technology and coordinate its use with existing procedures and personnel (Fig.8).

Personal computers

New computer technology has enabled a spin off from microcomputers to still smaller, cheaper personal computers. Although these computers may not be suitable for data processing systems, they afford a facility for solving many hydrological problems such as flow forecast models, extension of rainfall or river flow records, algorithms etc. Further simple systems have been designed for the solution of these problems which require little previous experience in modelling or computer programming. Packages such as optimization routines based upon multiple regression procedures are now available at negligible cost for many personal computers. Normally a personal computer with at least 16K storage is recommended for hydrological use (Marsh, 1986).

Advances in technology in hydrology 499

Input Output

Keyboard devifes

Disc storage

Magnetic tape storage devices

Graphics tablet / digitizer

FIG. 8 (court

Program Irvférpreter/ £ompiler__

Operating system

Central processing unit CPU

Random access memory RAM Disc

storage devices

Magnetic r . tape ( J storage ^—devices

Cassette Tape drive Schematic outline of a microcomputer system

esy of Adam Hilger Ltd).

Visual display unit

Graph plotter

Printer

THE FUTURE

The pace of change over the last 10-15 years has been so fast that it Is difficult to predict what will happen to instruments and data collection devices in the near future. However, it seems likely that technology will continue to develop rapidly, to present the hydrologist with new opportunities in many directions and improvement in reliability, versatility and cost in most.

ACKNOWLEDGEMENT Acknowledgement is made to Adam Hilger Ltd of Bristol UK and Boston USA for permission to use extracts and figures from their publication New Technology in Hydrometry (1986)

REFERENCES

Herschy, R.W. (1985) Streamflow Measurement. Elsevier Applied Science Publishers, London.

Marsh, T.J. (1986) Microcomputer-based hydrometric data processing. In: New Technology in Hydrometry (ed.by R.W.Herschy). Adam Hilger Ltd, Bristok, UK.

Walker, S.T. (1986) Data collection instrumentation. In: New Technology in Hydrometry (ed.by R.E.Herschy). Adam Hilger Ltd, Bristol, UK.