- Email: [email protected]
Capitalising on river flow data to meet changing national needs — a UK perspective
Flow Measurement and Instrumentation 13 (2002) 291–298 www.elsevier.com/locate/flowmeasinst
Capitalising on river flow data to meet changing national needs — a UK perspective Terry J. Marsh∗ Centre for Ecology and Hydrology, Wallingford, OX10 8BB, UK
Abstract Ongoing or anticipated changes in climate, land cover, and patterns of water utilization, together with a changing legislation framework and an increasing public engagement in water issues, will ensure a continuing growth in the demand for river flow data. This paper examines several initiatives aimed at addressing contemporary data needs and maximizing the strategic utility of nationally archived river flow data in the UK. The need for network evolution to take account of both operational and strategic information needs is examined. Emphasis is placed on the importance of a continuing dialog between information users and those responsible for hydrometric data acquisition in order to capitalize fully on the network infrastructure and data processing capabilities. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hydrometric networks; River flow data; Hydrological data dissemination
1. Introduction River flow data are the foundation of water management; data are required for resource assessments, regulatory purposes, river management, and, in a digested form, to direct policy development and help draft legislation. As with much environmental monitoring, the need for river flow data becomes particularly compelling during periods of change, or anticipated change. The droughts of the early and mid-1990s together with the very extensive flooding experienced in 2000/01 [1,2] demonstrated the UK’s continuing vulnerability to extreme weather conditions. As yet, there is limited evidence of flow regime changes attributable to global warming [3] but most scenario-based climate change projections suggest that the broad hydrological stability which characterized the twentieth century may not continue through the twenty-first [4,5]. However, climate change is but one of a number of driving forces which can affect river flow patterns; others include changes in land cover, agricultural practices, and patterns of water use. Similarly, the information needs of an increasingly broad community of river flow data users evolve through time in response to changes in, for instance, the legislat-
*
E-mail address: [email protected] (T.J. Marsh).
ive framework, national policy objectives, and the need to engage the community more fully in water issues. In the face of change, hydrometric data collection and archiving systems need to adapt to ensure that data provision matches contemporary user requirement. This paper examines recent UK initiatives to address strategic river flow information needs with particular reference to the UK gauging station network and the National River Flow Archive (NRFA).
2. The UK gauging station network The UK has a dense drainage river network — some 200,000 km of watercourses in 1500 discrete river systems draining to the sea through over 100 estuaries [6]. In a global context, the gauging station network is also very dense — a necessary response to both the drainage density and the diversity of the UK in terms of its climate, geology, land use, and patterns of water utilization. In total there are around 1400 primary gauging stations representing a public investment of more than £200 million. A distinguishing characteristic of the UK network is the variety of flow measurement techniques used — see Table 1. A very wide range of purpose-built structures are deployed, constituting around 70% of the network in England and Wales. An enterprising
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T.J. Marsh / Flow Measurement and Instrumentation 13 (2002) 291–298
Table 1 Types of gauging stations in the UK Number Velocity area Flat V weir Crump weir Compound Crump weir Flume Compound broad-crested weir Thin-plate weir Essex weir Broad-crested weir V Notch weir Flat V/Velocity area Crump/Velocity area Broad crested/Velocity area Flume/Velocity area Thin-plate/Flume Ultrasonic Electromagnetic Miscellaneous
365 261 136 89 65 32 23 19 13 2 51 20 12 7 4 48 20 101
% 28.8 20.6 10.7 7.0 5.1 2.5 1.8 1.5 1.0 0.2 4.0 1.6 0.9 0.6 0.3 3.8 1.6 8.0
Agency (SEPA), and, in Northern Ireland, the Rivers Agency (Department of Agriculture and Rural Development). For the great majority of gauging stations, river levels are recorded at 15-minute intervals and converted to flows in local or regional offices. Daily flow data are routinely forwarded to the National River Flow Archive maintained by the Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology). After validation, the data are archived and made available through a comprehensive data retrieval service — see below. Data usage, analysis, and ultimately the associated decision-making, provide endpoints to the hydrometric information flow chart (see Fig. 1); in an effective system they should also provide continuous feedback influencing the overall design and structure of the information system. The NRFA now contains over 44,000 station years of flow data, 15 million daily flow values and 0.5 million monthly catchment rainfall totals. The utility of the basic flow data are increased by the availability of reference and spatial information (e.g., catchment characteristics, details of gauging station performance), which assist the
approach to the exploitation of new technology gauging options (e.g., ultrasonic, electromagnetic, acoustic doppler) has allowed flow measurement to be undertaken in situations that preclude the use of more traditional methods. An increasing proportion of UK gauging stations are ‘hybrid’ (exploiting different measurement techniques for different flow ranges) and a significant minority are multi-site configurations (e.g., where high and low flow measurement is undertaken at different locations). Such gauging arrangements normally require more complex data processing procedures to derive flows for archiving purposes. Flow measurement in the UK rarely presents the difficulties of access, large velocity ranges, inadequate hydraulic conditions, and paucity of hydrometric equipment that are common throughout the developing world. Nonetheless, the relatively modest flow and limited depth of UK rivers, combined with the technical and logistical difficulties of refining the stage–discharge relation above bankfull, implies that the accuracy bands which characterize the medium flow ranges can seldom be approached in the extreme flow ranges. In addition, the increasingly pervasive impact of man on flow regimes — in the English Lowlands especially — limits the representativeness of many river flow time series and underlines the need for user guidance to ensure that river flow datasets are used appropriately.
3. The National River Flow Archive Responsibility for flow measurement in the UK resides primarily with the Environment Agency (EA) England and Wales, the Scottish Environment Protection
Fig. 1.
Flow chart for a hydrometric information system.
T.J. Marsh / Flow Measurement and Instrumentation 13 (2002) 291–298
selection of data for particular studies and in the interpretation of analyses based on the raw data. In 1999, a strategic review of the activities of the NRFA1 was undertaken with the aim of redefining its objectives in anticipation of the data/information needs of the twenty-first century [7]. A meeting of the principal stakeholders — including representatives of government departments, data suppliers, and major archive users — was followed by a wider consultation exercise to identify the primary information drivers and their implications for future information needs. An updated set of objectives was formulated to guide the future development of the NRFA and establish the capability to: 앫 Assess national and regional resources and monitor variability (including servicing the needs of the National Hydrological Monitoring Programme). 앫 Establish regional (and flow regime) benchmark hydrological conditions. 앫 Identify and interpret regional trends in runoff, in particular distinguish natural, direct, and indirect anthropogenic effects. 앫 Service the data requirements of a range of publications and official reports, including sites that illustrate issues of public concern and the effect of policy initiatives. 앫 Meet national and international obligations for data dissemination and exchange, and contribute to the designation of international monitoring programmes. 앫 Provide the continuous daily flow data required to complement water quality data archived under the Harmonized Monitoring Scheme (national water quality database). 앫 Constitute a national database to meet strategic research requirements, including the development of hydrological models. 앫 Increase public awareness and understanding of water-related issues, provide appropriate educational material, and respond to media requests. Strengthening the national capability to meet these strategic information objectives may be considered to involve three inter-related dimensions: 1. the utility of the gauging station network 2. the utility of nationally archived river flow data 3. access to, and retrieval/dissemination of, river flow information These are considered in turn.
1
The review also embraced the National Groundwater Level Archived maintained by the British Geological Survey, also at Wallingford, UK.
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3.1. The utility of the gauging station network The UK gauging station network grew most rapidly in the 1960s and 1970s [8] when systematic national coverage was encouraged by the Hydrometric Schemes (and the associated provision of grant-aid) administered by the Water Resources Board [9]. Around 800 geographically well-distributed gauging stations were operational by 1980; these allowed river flow patterns to be broadly characterized across Great Britain — but a few regions (e.g., western Scotland) remained poorly monitored and, generally, small undisturbed catchments were under-represented in the network. In ideal circumstances, the number and disposition of gauging stations should be kept under continuous review to match changing information needs and maximize synergistic benefits (e.g., by harmonizing monitoring effort across a range of environmental monitoring programmes). In strategic terms the relatively remote, natural catchments — often with limited operational justification — can be the most valuable for understanding hydrological processes, trend detection, and the development of regionalization procedures. From the late 1970s, however, network growth primarily reflected the operational needs of the measuring authorities. Several regionally focused network reviews led to the decommissioning of a number of small catchments with sensibly natural flow regimes. Identifying strategic network inadequacies, e.g., the dearth of well-monitored impermeable catchments with near-natural flow patterns in the English Lowlands, is an important precursor for rational network evolution. In practice, however, considerable network inertia is an unavoidable reality and the challenge is normally to capitalize most effectively on the existing network. As in any monitoring programme, data from individual gauging stations contribute unevenly to the overall information output from the UK network. Formal identification of those catchments with the greatest strategic utility allows the national dimension to assume greater importance in future network rationalization exercises. Since the demise of the Water Resources Board in 1974 there has been no comprehensive gauging station network review to ascertain whether the network is meeting the wider national strategic requirements. However, a number of appraisals of gauging station performance have been undertaken as part of major research programmes aimed at developing improved engineering design procedures or water management tools [10,11,12]. These appraisals require updating, but provide a foundation for a contemporary categorization of stations targeted on the national information needs listed above. Four national network categories have been defined to meet these objectives: Benchmark, Artificial Impacts, Rationalization, and Integrated Monitoring (see Table 2).
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Table 2 UK Gauging Station Network — strategic categories regionalization Category
Main objectives
Benchmark Artificial impacts
Identify and interpret hydrological trends — principally climate-driven Monitor heavily impacted catchments to establish the scale of disturbance to the natural flow regime. Demonstrate the effectiveness of water management strategies and remedial measures Underpin the development of regionalization techniques and modeling procedures. (Not all ‘regionalization’ stations will be suitable for both high and low flow analyses) Provide a focus for the improved understanding of hydrological processes from the sub-catchment to the basin scale
Regionalization Integrated monitoring
The categories are not mutually exclusive — many wellgauged catchments will qualify for selection in several categories — but they do provide an important guide to the range of applications to which individual gauging station records are suited. 3.1.1. Benchmark catchments The initial priority in the gauging station categorization exercise was to strengthen the UK capability to detect climate-driven trends. This priority reflects the enhanced political and public profile of climate change impacts and the major economic implications of any increase in the frequency of floods or droughts. It also recognizes the potential danger of excessive precautionary expenditure based on climate change scenarios which remain very uncertain in relation to future rainfall patterns at the catchment scale. Identifying and indexing climate-driven trends in river flows is hampered by a lack of long, good quality, time series data for rivers with relatively undisturbed regimes. This is a global problem compounded by the difficulties of galvanizing support for essential long-term monitoring programmes [13]. In the UK the proportion of gauged catchments unaffected by man is notably low — less than 15% — and filtering out any climatic change signal from both the general climatic variability and the impact of more immediate anthropogenic causes is a considerable scientific challenge. The principal criteria for categorization as a Benchmark catchment were: consistency of the gauging station’s hydrometric performance (in the extreme ranges especially), the degree of artificial disturbance to the flow regime, the homogeneity of the time series, and record length [14]. A measure of the contribution of groundwater to overall flow — the Base Flow Index [12] — was used to help to select a hydrogeologically representative mix of catchment types. The scale of the artificial influences and the hydrometric performance, a pivotal criteria for all catchment categories, was reviewed in collaboration with the regional measuring authorities. In order to achieve a representative national benchmark network some compromises were necessary especially in the English Lowlands where, for instance, limited net impacts of abstractions/returns on average
runoff was tolerated. A provisional national network of around 100 Benchmark catchments was identified; this number will reduce as homogeneity tests are carried on the time series. Their distribution is shown in Fig. 2. The average length of river flow records held on the NRFA is around 23 years and relatively few stations are able to provide a lengthy historical context in which to assess contemporary runoff variability. Thus the 45 stations having daily flow time series exceeding 50 years, including two with continuous records extending back into the nineteenth century, are especially valuable. However, many of the older records lack the accuracy that can be achieved today — flow measurement and
Fig. 2. tain.
The distribution of Benchmark gauging stations in Great Bri-
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data validation techniques have improved through time and some of the longest records are also from catchments with significant, and changing, artificial disturbance to runoff patterns. Notwithstanding these limitations, these longer-term records provide an indispensable insight into river flow patterns extending back over 150 years — an essential perspective within which to assess the significance of any emerging hydrological trends. In recognition of their importance, around 35 long-term records have been selected to complement the national network of benchmark gauging stations. 3.2. The utility of nationally archived river flow data Hydrometric data processing and archiving systems provide the bridge between the gauging station network and the information user. As with most databases, a river flow archive depends for its success on the ready availability of suitable datasets of appropriate accuracy. The presence of large volumes of erroneous data can easily undermine the confidence of users in any archiving enterprise. Many factors — including network design, instrument performance, the stability of the stage–discharge relation, staff education, training and motivation — play a part in determining the quality and continuity of archived river flow data. 3.2.1. Data accuracy and utility Over the last 30 years major developments in flow measurement, water level sensing, data recording, and data communication technologies have had a beneficial impact on the way river flow data are handled and processed. The performance characteristics of the great majority of gauging structures have been thoroughly investigated, both in the laboratory and the field [15]. They have proved robust, reliable, and capable of measuring river flow to an accuracy of better than 5% [16]. However, many factors can combine to reduce the accuracy of processed or archived flow data. Generally of most significance is the limited water depth in UK streams and rivers — stage values corresponding to low flows are commonly less than 100 mm, often much less. This places a premium on reliable water level sensing and recording. The conventional 15-minute recording interval implies that random errors in computed daily mean flows tend to be very low [16]. By contrast, systematic bias in measured river levels — caused, for example, by algal growth on weir crests or datum errors — can be substantial and difficult to eliminate. The most vulnerable gauging stations are those where small head changes correspond to substantial changes in flow — stations with insensitive controls. Fig. 3 is based on over 1000 gauging stations throughout the UK and shows the change in river flow associated with a 5-mm change in stage at the Q95 flow. A systematic error of 5 mm translates into a 10% flow error for more than 35%
Fig. 3. The sensitivity of UK gauging stations (darker shading relates to stations in the English Lowlands).
of the gauging station network. Stations in the English Lowlands are disproportionately represented in the higher error bands; a 5-mm stage error corresponding to a flow change of 15% or more at nearly 30% of the gauging stations. Serious systematic errors can also result from the lack of adjustment of computed flows to account for weir operation in the non-modular range or through the excessive extrapolation of stage–discharge relations. Instrumentation and data-processing procedures exist to address most of these problems but, with many competing demands on the time of hydrometric personnel, their application is patchy both spatially and through time; this can significantly impact on the homogeneity of the high flow time series in particular. The utility or ‘fitness for purpose’ of river flow data reflects their accuracy but is also strongly influenced by a number of other factors. Even a small proportion of missing data can greatly reduce the ability to derive meaningful summary statistics (e.g., annual runoff totals or 30-day minima). As importantly, the nature of hydrometric measurement determines that missing data tends to cluster disproportionately in the extreme flow ranges. There has been an increase in the proportion of missing data submitted to the NRFA over recent years. In part, this reflects the very unusual flow conditions experienced (with protracted periods of above-bankfull flow during 2000/01 [17], but also underlines the need for effective procedures to derive estimates of missing flows. Judgment needs to be exercised in applying such procedures to avoid archiving misleading flow estimates. In most circumstances however, the inclusion of an auditable and flagged estimate rather than a gap in the record will produce significant benefits in relation to the overall utility of the time series.
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3.2.2. Service-level agreements To improve the overall fitness for purpose of river flow data held on national and regional databases, and to promote good data processing practice, Service-Level Agreements (SLAs) are being introduced between the NRFA and the major UK data suppliers. These agreements cover routine data provision from around 65% of the current network — an estimated proportion based on the expected number of gauging stations to be included in the national categorization exercise. These stations contribute most to meeting national information needs and, as such, will be subject to more vigorous data validation procedures and homogeneity appraisals2. The SLA procedures are applied to each annual river flow dataset submitted to the NRFA. Data quality assessments are based on four criteria: 1. 2. 3. 4.
The proportion of missing daily flow data The proportion of anomalous flows The timeliness of data provision to the NRFA Responsiveness to queries raised by the NRFA validation procedures
The scores for individual stations are aggregated to enable objective regional comparisons to be undertaken. The SLA’s scoring mechanisms form part of the NRFA’s data acquisition, auditing, and validation suite. A central component of the system is a versatile hydrograph plotting capability to examine daily flow time series. Experience demonstrates that hydrograph appraisal undertaken by personnel familiar with the behavior of individual rivers is the most effective means of quality controlling daily flow time series in the UK. In normal usage, an annual dataset for a target gauging station is featured within an envelope of the corresponding longterm daily maximum and minimum flows (see Fig. 4) — the envelope helps to direct attention to the most hydrologically significant flows — those which approach, or exceed, the normal range of variability. The addition of one, or more, time series for nearby (or analogous) gauging stations, together with daily rainfall totals for a representative rain gauge within the catchment, helps to identify anomalous flows sequences, derive revised flows estimates where required, or infill gaps in the record. On-line access is provided to reference information and descriptive material (relating to station performance and artificial influences) — this material can be critical in determining the credibility or otherwise of apparently anomalous flows. Queried flow sequences are automatically logged and dispatched electronically (together with copies of the appropriate hydrograph plots) for consideration and action by the measuring authorities.
2 Note: The NRFA will continue to offer systematic storage for all submitted river flow data.
A danger with many SLAs and data quality appraisal schemes is that they operate too mechanistically, concentrating on the separate components within the scoring mechanism rather than contributing to an overall increase in information utility. Superficially impressive data capture rates can co-exist with time series of low utility, e.g., where high flows are routinely truncated, or where data users do not have ready access to information outlining the degree of artificial disturbance to the natural flow regime. The SLAs will therefore be complemented by regular Hydrometric Audits — undertaken jointly with the measuring authorities — providing a forum to help assess the degree to which user needs are being met. 3.3. Access to, and dissemination of, river flow data The value of river flow data is reflected in the volume of its usage and the breadth of its application. Demand for river flow data has never been greater; this demand, together with the need to match the requirements of a widening community of users, can create significant data management challenges. Fortunately, information technology developments over the last 15 years have revolutionized the accessibility to, and dissemination of, hydrometric data. Electronic data acquisition and telemetry systems allow a rapid linkage between the initial field sensing of water levels and the provision of data to the end user. This is commendable, but in circumstances where there is unmoderated access to the raw flow data, the potential exists for misapplication of raw data, or misinterpretation of analyses based on those data. For example, a substantial proportion of datasets are inadequate for flood analysis purposes and, at many gauging stations, low flow patterns are dominated by artificial influences. A professional stewardship of the data requires that they be accompanied by sufficient reference, spatial, and descriptive information for the user to judge its suitability. 4. The NRFA Website To increase accessibility to UK river flow data and information, the National River Flow Archive website was launched in 1996. The site provides a gateway to a range of data retrieval services and information services and currently comprises five main components — these are listed in Table 3. The NRFA website may be accessed via http://www.nwl.ac.uk/ih/nrfa/index.htm 5. Publications in the Hydrological data: UK series Systematic publication of river flow data in the UK began with the 1935–36 Surface Water Yearbook:3. With 3 Earlier data from a number of Scottish catchments were published by Captain W.N. McLean [18].
T.J. Marsh / Flow Measurement and Instrumentation 13 (2002) 291–298
Fig. 4.
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The NRFA River Flow Time series plotting facility.
Table 3 The National River Flow Archive website — main components Component
Contents
River Flow Data
Daily flow data for 200 index gauging stations throughout the UK. Introduction to the data retrieval facilities of the NRFA Annual and monthly summaries of hydrological and water resources variability throughout the UK. Output from the National Hydrological Monitoring programme Location maps, reference information, hydrographs, flow duration curves, together with data availability and descriptive material relating to all primary gauging stations Details of publications in the Hydrological data UK series and other associated publications and reports Regional and site specific trends for a range of river flow variables, including UK Climate change indicators
Water Watch UK Gauging Network Publications Hydrological Trends
the transfer of the NRFA to the Institute of Hydrology in 1982 a new series — the Hydrological data UK series — was initiated which incorporated both river flow and groundwater level data4 (some water quality data were also featured). Following a readership consultation exercise in 1995, the decision was taken to publish future Yearbooks via the NRFA website. This allowed data to be placed in the public domain more rapidly and provided the flexibility to feature an increased amount of data (and range of supporting information); the ability to update and revise material was an additional advantage.
4
Collated by the National Groundwater Level Archive.
Currently, daily flow data for 200 index gauging stations can be accessed via clickable maps. The annual hydrological reviews previously published in the Yearbooks are now posted on the ‘Water Watch’ pages which serve as the primary information dissemination route for the National Hydrological Monitoring Programme. This programme has been operational since 1988 and relies on the active cooperation of the UK measuring authorities. It provides routine monthly reports — and supporting hydrometric statistics — on rainfall, river flows, groundwater levels, and reservoir stocks throughout the UK. The descriptive text is aimed at a wide audience and forms the basis of a range of policy and media briefing material, and input to technical reports and scientific papers.
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5.1. Hydrometric Register and Statistics volumes The NRFA has a responsibility to provide data retrieval services to all users, and access to the internet is not universal. Consequently, descriptions of the retrieval facilities are included in the ‘Hydrometric Register and Statistics’ reports which are published on a five-year cycle. Their purpose is to provide an overview of water resource and hydrological variability, and serve as a reference source to help data users select gauging stations appropriate to their needs. As a working document designed primarily for desktop use, the Hydrometric Register and Statistics volumes are less suited to electronic publication and five-year editions will continue to be issued in traditional hardback format. However, much of the published information — including the essential user guidance material — will also be featured on the Website.
6. Conclusion The volume of data generated by hydrometric monitoring activities across the UK is increasing rapidly in response to a burgeoning need for river flow data across a range of applications. Only limited resources can be devoted to hydrometric data acquisition and processing and there is a heavy responsibility on system designers to ensure the fitness for purpose of the river flow data available to users. This requires a critical review of all the links in the data acquisition–decision making chain, a recognition of the strategic as well as operational value of the data, and a continuing dialog with stakeholders and data users to ensure that changing requirements are being addressed.
Acknowledgements The author acknowledges the advice and guidance provided by colleagues at the Centre for Ecology and Hydrology — Jamie Honnaford, Oliver Swain and Mark Robinson in particular — during the preparation of this article.
References [1] T.J. Marsh, The 1995 drought — evidence for climatic instability?, Jour. ICE Water, Maritime and Energy 118, Sept. (1996) 189–195. [2] CEH Wallingford/British Geological Survey, Hydrological Review of 2000. (2001) http://www.nwl.ac.uk/ih/nrfa/yb/yb2000/ hydrological review of 2000.htm. [3] A.J. Robson, T.K. Jones, D.W. Reed, A.C. Bayliss, A study of national trend and variation in UK floods, Int. J. Climat. 18 (2) (1998) 165. [4] N.W. Arnell, N.S. Reynard, Climate Change and UK Hydrology, In: M. Acreman (Ed.), The Hydrology of the UK — A Study of Change, Routledge, London, 2000, pp. 3–29. [5] M. Hulme, J.R. Turnpenny, G. Jenkins, Climate Change Scenarios for the United Kingdom: The UKCIP02 Briefing Report, Tyndall Centre for Climate Change Research, 2002. [6] Natural Environment Research Council Water Quality in the Environment, NERC, 1990. [7] Anon National Monitoring for the New Millennium — a Consultation Paper, Surface and Groundwater Archive Steering Committee (SAGA), CEH-Wallingford, 1999. [8] M.L. Lees, Inland Water Surveying in the United Kingdom - A Short History. 1985 Yearbook. Hydrological data UK series, Institute of Hydrology/British Geological Survey (1987) 35–48. [9] Water Resources Board Seventh Annual Report, HMSO, London, 1972. [10] Natural Environment Research Council, Flood Studies Report. NERC Research Council (5 Vols) ( (1975 — reprinted 1993). [11] Institute of Hydrology Flood Estimation Handbook, 5 volumes and associated software, Institute of Hydrology, Wallingford, 1999. [12] A. Gustard, A. Bullock, J.M. Dixon, Low flow estimation in the United Kingdom. Institute of Hydrology Report No. 108. (1992). [13] J.C. Rodda, Hydrological networks need improving! In: Proceedings International Conference on World Water Resources at the beginning of the 21st century, pp. 91-102. IHP-V Technical Documents in Hydrology No. 18 UNESCO, (1998) 91–102. [14] R.B. Bradford, T.J. Marsh, Defining a network of natural benchmark catchments for the UK. Jour. ICE Water, Maritime and Energy, (2003, In Press). [15] P. Ackers, W.R. White, J.A. Perkins, A.J.M. Harrison, Weirs and Flumes for Flow Measurement, John Wiley and Sons, Chichester, 1978. [16] R.W. Herschy, Streamflow measurement (second edition), E & FN Spon, London, 1995. [17] T.J. Marsh, M. Dale, The UK floods of 2000/01 - a hydrometeorological appraisal, CIWEM Jour 16 (3) (2002) 180–188. [18] A. Werritty, The McClean Hydrometric Data Collection. 1985 Yearbook, Hydrological data UK series. (1987) 49–54.
Capitalising on river flow data to meet changing national needs — a UK perspective Terry J. Marsh∗ Centre for Ecology and Hydrology, Wallingford, OX10 8BB, UK
Abstract Ongoing or anticipated changes in climate, land cover, and patterns of water utilization, together with a changing legislation framework and an increasing public engagement in water issues, will ensure a continuing growth in the demand for river flow data. This paper examines several initiatives aimed at addressing contemporary data needs and maximizing the strategic utility of nationally archived river flow data in the UK. The need for network evolution to take account of both operational and strategic information needs is examined. Emphasis is placed on the importance of a continuing dialog between information users and those responsible for hydrometric data acquisition in order to capitalize fully on the network infrastructure and data processing capabilities. 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hydrometric networks; River flow data; Hydrological data dissemination
1. Introduction River flow data are the foundation of water management; data are required for resource assessments, regulatory purposes, river management, and, in a digested form, to direct policy development and help draft legislation. As with much environmental monitoring, the need for river flow data becomes particularly compelling during periods of change, or anticipated change. The droughts of the early and mid-1990s together with the very extensive flooding experienced in 2000/01 [1,2] demonstrated the UK’s continuing vulnerability to extreme weather conditions. As yet, there is limited evidence of flow regime changes attributable to global warming [3] but most scenario-based climate change projections suggest that the broad hydrological stability which characterized the twentieth century may not continue through the twenty-first [4,5]. However, climate change is but one of a number of driving forces which can affect river flow patterns; others include changes in land cover, agricultural practices, and patterns of water use. Similarly, the information needs of an increasingly broad community of river flow data users evolve through time in response to changes in, for instance, the legislat-
*
E-mail address: [email protected] (T.J. Marsh).
ive framework, national policy objectives, and the need to engage the community more fully in water issues. In the face of change, hydrometric data collection and archiving systems need to adapt to ensure that data provision matches contemporary user requirement. This paper examines recent UK initiatives to address strategic river flow information needs with particular reference to the UK gauging station network and the National River Flow Archive (NRFA).
2. The UK gauging station network The UK has a dense drainage river network — some 200,000 km of watercourses in 1500 discrete river systems draining to the sea through over 100 estuaries [6]. In a global context, the gauging station network is also very dense — a necessary response to both the drainage density and the diversity of the UK in terms of its climate, geology, land use, and patterns of water utilization. In total there are around 1400 primary gauging stations representing a public investment of more than £200 million. A distinguishing characteristic of the UK network is the variety of flow measurement techniques used — see Table 1. A very wide range of purpose-built structures are deployed, constituting around 70% of the network in England and Wales. An enterprising
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T.J. Marsh / Flow Measurement and Instrumentation 13 (2002) 291–298
Table 1 Types of gauging stations in the UK Number Velocity area Flat V weir Crump weir Compound Crump weir Flume Compound broad-crested weir Thin-plate weir Essex weir Broad-crested weir V Notch weir Flat V/Velocity area Crump/Velocity area Broad crested/Velocity area Flume/Velocity area Thin-plate/Flume Ultrasonic Electromagnetic Miscellaneous
365 261 136 89 65 32 23 19 13 2 51 20 12 7 4 48 20 101
% 28.8 20.6 10.7 7.0 5.1 2.5 1.8 1.5 1.0 0.2 4.0 1.6 0.9 0.6 0.3 3.8 1.6 8.0
Agency (SEPA), and, in Northern Ireland, the Rivers Agency (Department of Agriculture and Rural Development). For the great majority of gauging stations, river levels are recorded at 15-minute intervals and converted to flows in local or regional offices. Daily flow data are routinely forwarded to the National River Flow Archive maintained by the Centre for Ecology and Hydrology, Wallingford (formerly the Institute of Hydrology). After validation, the data are archived and made available through a comprehensive data retrieval service — see below. Data usage, analysis, and ultimately the associated decision-making, provide endpoints to the hydrometric information flow chart (see Fig. 1); in an effective system they should also provide continuous feedback influencing the overall design and structure of the information system. The NRFA now contains over 44,000 station years of flow data, 15 million daily flow values and 0.5 million monthly catchment rainfall totals. The utility of the basic flow data are increased by the availability of reference and spatial information (e.g., catchment characteristics, details of gauging station performance), which assist the
approach to the exploitation of new technology gauging options (e.g., ultrasonic, electromagnetic, acoustic doppler) has allowed flow measurement to be undertaken in situations that preclude the use of more traditional methods. An increasing proportion of UK gauging stations are ‘hybrid’ (exploiting different measurement techniques for different flow ranges) and a significant minority are multi-site configurations (e.g., where high and low flow measurement is undertaken at different locations). Such gauging arrangements normally require more complex data processing procedures to derive flows for archiving purposes. Flow measurement in the UK rarely presents the difficulties of access, large velocity ranges, inadequate hydraulic conditions, and paucity of hydrometric equipment that are common throughout the developing world. Nonetheless, the relatively modest flow and limited depth of UK rivers, combined with the technical and logistical difficulties of refining the stage–discharge relation above bankfull, implies that the accuracy bands which characterize the medium flow ranges can seldom be approached in the extreme flow ranges. In addition, the increasingly pervasive impact of man on flow regimes — in the English Lowlands especially — limits the representativeness of many river flow time series and underlines the need for user guidance to ensure that river flow datasets are used appropriately.
3. The National River Flow Archive Responsibility for flow measurement in the UK resides primarily with the Environment Agency (EA) England and Wales, the Scottish Environment Protection
Fig. 1.
Flow chart for a hydrometric information system.
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selection of data for particular studies and in the interpretation of analyses based on the raw data. In 1999, a strategic review of the activities of the NRFA1 was undertaken with the aim of redefining its objectives in anticipation of the data/information needs of the twenty-first century [7]. A meeting of the principal stakeholders — including representatives of government departments, data suppliers, and major archive users — was followed by a wider consultation exercise to identify the primary information drivers and their implications for future information needs. An updated set of objectives was formulated to guide the future development of the NRFA and establish the capability to: 앫 Assess national and regional resources and monitor variability (including servicing the needs of the National Hydrological Monitoring Programme). 앫 Establish regional (and flow regime) benchmark hydrological conditions. 앫 Identify and interpret regional trends in runoff, in particular distinguish natural, direct, and indirect anthropogenic effects. 앫 Service the data requirements of a range of publications and official reports, including sites that illustrate issues of public concern and the effect of policy initiatives. 앫 Meet national and international obligations for data dissemination and exchange, and contribute to the designation of international monitoring programmes. 앫 Provide the continuous daily flow data required to complement water quality data archived under the Harmonized Monitoring Scheme (national water quality database). 앫 Constitute a national database to meet strategic research requirements, including the development of hydrological models. 앫 Increase public awareness and understanding of water-related issues, provide appropriate educational material, and respond to media requests. Strengthening the national capability to meet these strategic information objectives may be considered to involve three inter-related dimensions: 1. the utility of the gauging station network 2. the utility of nationally archived river flow data 3. access to, and retrieval/dissemination of, river flow information These are considered in turn.
1
The review also embraced the National Groundwater Level Archived maintained by the British Geological Survey, also at Wallingford, UK.
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3.1. The utility of the gauging station network The UK gauging station network grew most rapidly in the 1960s and 1970s [8] when systematic national coverage was encouraged by the Hydrometric Schemes (and the associated provision of grant-aid) administered by the Water Resources Board [9]. Around 800 geographically well-distributed gauging stations were operational by 1980; these allowed river flow patterns to be broadly characterized across Great Britain — but a few regions (e.g., western Scotland) remained poorly monitored and, generally, small undisturbed catchments were under-represented in the network. In ideal circumstances, the number and disposition of gauging stations should be kept under continuous review to match changing information needs and maximize synergistic benefits (e.g., by harmonizing monitoring effort across a range of environmental monitoring programmes). In strategic terms the relatively remote, natural catchments — often with limited operational justification — can be the most valuable for understanding hydrological processes, trend detection, and the development of regionalization procedures. From the late 1970s, however, network growth primarily reflected the operational needs of the measuring authorities. Several regionally focused network reviews led to the decommissioning of a number of small catchments with sensibly natural flow regimes. Identifying strategic network inadequacies, e.g., the dearth of well-monitored impermeable catchments with near-natural flow patterns in the English Lowlands, is an important precursor for rational network evolution. In practice, however, considerable network inertia is an unavoidable reality and the challenge is normally to capitalize most effectively on the existing network. As in any monitoring programme, data from individual gauging stations contribute unevenly to the overall information output from the UK network. Formal identification of those catchments with the greatest strategic utility allows the national dimension to assume greater importance in future network rationalization exercises. Since the demise of the Water Resources Board in 1974 there has been no comprehensive gauging station network review to ascertain whether the network is meeting the wider national strategic requirements. However, a number of appraisals of gauging station performance have been undertaken as part of major research programmes aimed at developing improved engineering design procedures or water management tools [10,11,12]. These appraisals require updating, but provide a foundation for a contemporary categorization of stations targeted on the national information needs listed above. Four national network categories have been defined to meet these objectives: Benchmark, Artificial Impacts, Rationalization, and Integrated Monitoring (see Table 2).
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Table 2 UK Gauging Station Network — strategic categories regionalization Category
Main objectives
Benchmark Artificial impacts
Identify and interpret hydrological trends — principally climate-driven Monitor heavily impacted catchments to establish the scale of disturbance to the natural flow regime. Demonstrate the effectiveness of water management strategies and remedial measures Underpin the development of regionalization techniques and modeling procedures. (Not all ‘regionalization’ stations will be suitable for both high and low flow analyses) Provide a focus for the improved understanding of hydrological processes from the sub-catchment to the basin scale
Regionalization Integrated monitoring
The categories are not mutually exclusive — many wellgauged catchments will qualify for selection in several categories — but they do provide an important guide to the range of applications to which individual gauging station records are suited. 3.1.1. Benchmark catchments The initial priority in the gauging station categorization exercise was to strengthen the UK capability to detect climate-driven trends. This priority reflects the enhanced political and public profile of climate change impacts and the major economic implications of any increase in the frequency of floods or droughts. It also recognizes the potential danger of excessive precautionary expenditure based on climate change scenarios which remain very uncertain in relation to future rainfall patterns at the catchment scale. Identifying and indexing climate-driven trends in river flows is hampered by a lack of long, good quality, time series data for rivers with relatively undisturbed regimes. This is a global problem compounded by the difficulties of galvanizing support for essential long-term monitoring programmes [13]. In the UK the proportion of gauged catchments unaffected by man is notably low — less than 15% — and filtering out any climatic change signal from both the general climatic variability and the impact of more immediate anthropogenic causes is a considerable scientific challenge. The principal criteria for categorization as a Benchmark catchment were: consistency of the gauging station’s hydrometric performance (in the extreme ranges especially), the degree of artificial disturbance to the flow regime, the homogeneity of the time series, and record length [14]. A measure of the contribution of groundwater to overall flow — the Base Flow Index [12] — was used to help to select a hydrogeologically representative mix of catchment types. The scale of the artificial influences and the hydrometric performance, a pivotal criteria for all catchment categories, was reviewed in collaboration with the regional measuring authorities. In order to achieve a representative national benchmark network some compromises were necessary especially in the English Lowlands where, for instance, limited net impacts of abstractions/returns on average
runoff was tolerated. A provisional national network of around 100 Benchmark catchments was identified; this number will reduce as homogeneity tests are carried on the time series. Their distribution is shown in Fig. 2. The average length of river flow records held on the NRFA is around 23 years and relatively few stations are able to provide a lengthy historical context in which to assess contemporary runoff variability. Thus the 45 stations having daily flow time series exceeding 50 years, including two with continuous records extending back into the nineteenth century, are especially valuable. However, many of the older records lack the accuracy that can be achieved today — flow measurement and
Fig. 2. tain.
The distribution of Benchmark gauging stations in Great Bri-
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data validation techniques have improved through time and some of the longest records are also from catchments with significant, and changing, artificial disturbance to runoff patterns. Notwithstanding these limitations, these longer-term records provide an indispensable insight into river flow patterns extending back over 150 years — an essential perspective within which to assess the significance of any emerging hydrological trends. In recognition of their importance, around 35 long-term records have been selected to complement the national network of benchmark gauging stations. 3.2. The utility of nationally archived river flow data Hydrometric data processing and archiving systems provide the bridge between the gauging station network and the information user. As with most databases, a river flow archive depends for its success on the ready availability of suitable datasets of appropriate accuracy. The presence of large volumes of erroneous data can easily undermine the confidence of users in any archiving enterprise. Many factors — including network design, instrument performance, the stability of the stage–discharge relation, staff education, training and motivation — play a part in determining the quality and continuity of archived river flow data. 3.2.1. Data accuracy and utility Over the last 30 years major developments in flow measurement, water level sensing, data recording, and data communication technologies have had a beneficial impact on the way river flow data are handled and processed. The performance characteristics of the great majority of gauging structures have been thoroughly investigated, both in the laboratory and the field [15]. They have proved robust, reliable, and capable of measuring river flow to an accuracy of better than 5% [16]. However, many factors can combine to reduce the accuracy of processed or archived flow data. Generally of most significance is the limited water depth in UK streams and rivers — stage values corresponding to low flows are commonly less than 100 mm, often much less. This places a premium on reliable water level sensing and recording. The conventional 15-minute recording interval implies that random errors in computed daily mean flows tend to be very low [16]. By contrast, systematic bias in measured river levels — caused, for example, by algal growth on weir crests or datum errors — can be substantial and difficult to eliminate. The most vulnerable gauging stations are those where small head changes correspond to substantial changes in flow — stations with insensitive controls. Fig. 3 is based on over 1000 gauging stations throughout the UK and shows the change in river flow associated with a 5-mm change in stage at the Q95 flow. A systematic error of 5 mm translates into a 10% flow error for more than 35%
Fig. 3. The sensitivity of UK gauging stations (darker shading relates to stations in the English Lowlands).
of the gauging station network. Stations in the English Lowlands are disproportionately represented in the higher error bands; a 5-mm stage error corresponding to a flow change of 15% or more at nearly 30% of the gauging stations. Serious systematic errors can also result from the lack of adjustment of computed flows to account for weir operation in the non-modular range or through the excessive extrapolation of stage–discharge relations. Instrumentation and data-processing procedures exist to address most of these problems but, with many competing demands on the time of hydrometric personnel, their application is patchy both spatially and through time; this can significantly impact on the homogeneity of the high flow time series in particular. The utility or ‘fitness for purpose’ of river flow data reflects their accuracy but is also strongly influenced by a number of other factors. Even a small proportion of missing data can greatly reduce the ability to derive meaningful summary statistics (e.g., annual runoff totals or 30-day minima). As importantly, the nature of hydrometric measurement determines that missing data tends to cluster disproportionately in the extreme flow ranges. There has been an increase in the proportion of missing data submitted to the NRFA over recent years. In part, this reflects the very unusual flow conditions experienced (with protracted periods of above-bankfull flow during 2000/01 [17], but also underlines the need for effective procedures to derive estimates of missing flows. Judgment needs to be exercised in applying such procedures to avoid archiving misleading flow estimates. In most circumstances however, the inclusion of an auditable and flagged estimate rather than a gap in the record will produce significant benefits in relation to the overall utility of the time series.
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3.2.2. Service-level agreements To improve the overall fitness for purpose of river flow data held on national and regional databases, and to promote good data processing practice, Service-Level Agreements (SLAs) are being introduced between the NRFA and the major UK data suppliers. These agreements cover routine data provision from around 65% of the current network — an estimated proportion based on the expected number of gauging stations to be included in the national categorization exercise. These stations contribute most to meeting national information needs and, as such, will be subject to more vigorous data validation procedures and homogeneity appraisals2. The SLA procedures are applied to each annual river flow dataset submitted to the NRFA. Data quality assessments are based on four criteria: 1. 2. 3. 4.
The proportion of missing daily flow data The proportion of anomalous flows The timeliness of data provision to the NRFA Responsiveness to queries raised by the NRFA validation procedures
The scores for individual stations are aggregated to enable objective regional comparisons to be undertaken. The SLA’s scoring mechanisms form part of the NRFA’s data acquisition, auditing, and validation suite. A central component of the system is a versatile hydrograph plotting capability to examine daily flow time series. Experience demonstrates that hydrograph appraisal undertaken by personnel familiar with the behavior of individual rivers is the most effective means of quality controlling daily flow time series in the UK. In normal usage, an annual dataset for a target gauging station is featured within an envelope of the corresponding longterm daily maximum and minimum flows (see Fig. 4) — the envelope helps to direct attention to the most hydrologically significant flows — those which approach, or exceed, the normal range of variability. The addition of one, or more, time series for nearby (or analogous) gauging stations, together with daily rainfall totals for a representative rain gauge within the catchment, helps to identify anomalous flows sequences, derive revised flows estimates where required, or infill gaps in the record. On-line access is provided to reference information and descriptive material (relating to station performance and artificial influences) — this material can be critical in determining the credibility or otherwise of apparently anomalous flows. Queried flow sequences are automatically logged and dispatched electronically (together with copies of the appropriate hydrograph plots) for consideration and action by the measuring authorities.
2 Note: The NRFA will continue to offer systematic storage for all submitted river flow data.
A danger with many SLAs and data quality appraisal schemes is that they operate too mechanistically, concentrating on the separate components within the scoring mechanism rather than contributing to an overall increase in information utility. Superficially impressive data capture rates can co-exist with time series of low utility, e.g., where high flows are routinely truncated, or where data users do not have ready access to information outlining the degree of artificial disturbance to the natural flow regime. The SLAs will therefore be complemented by regular Hydrometric Audits — undertaken jointly with the measuring authorities — providing a forum to help assess the degree to which user needs are being met. 3.3. Access to, and dissemination of, river flow data The value of river flow data is reflected in the volume of its usage and the breadth of its application. Demand for river flow data has never been greater; this demand, together with the need to match the requirements of a widening community of users, can create significant data management challenges. Fortunately, information technology developments over the last 15 years have revolutionized the accessibility to, and dissemination of, hydrometric data. Electronic data acquisition and telemetry systems allow a rapid linkage between the initial field sensing of water levels and the provision of data to the end user. This is commendable, but in circumstances where there is unmoderated access to the raw flow data, the potential exists for misapplication of raw data, or misinterpretation of analyses based on those data. For example, a substantial proportion of datasets are inadequate for flood analysis purposes and, at many gauging stations, low flow patterns are dominated by artificial influences. A professional stewardship of the data requires that they be accompanied by sufficient reference, spatial, and descriptive information for the user to judge its suitability. 4. The NRFA Website To increase accessibility to UK river flow data and information, the National River Flow Archive website was launched in 1996. The site provides a gateway to a range of data retrieval services and information services and currently comprises five main components — these are listed in Table 3. The NRFA website may be accessed via http://www.nwl.ac.uk/ih/nrfa/index.htm 5. Publications in the Hydrological data: UK series Systematic publication of river flow data in the UK began with the 1935–36 Surface Water Yearbook:3. With 3 Earlier data from a number of Scottish catchments were published by Captain W.N. McLean [18].
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Fig. 4.
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The NRFA River Flow Time series plotting facility.
Table 3 The National River Flow Archive website — main components Component
Contents
River Flow Data
Daily flow data for 200 index gauging stations throughout the UK. Introduction to the data retrieval facilities of the NRFA Annual and monthly summaries of hydrological and water resources variability throughout the UK. Output from the National Hydrological Monitoring programme Location maps, reference information, hydrographs, flow duration curves, together with data availability and descriptive material relating to all primary gauging stations Details of publications in the Hydrological data UK series and other associated publications and reports Regional and site specific trends for a range of river flow variables, including UK Climate change indicators
Water Watch UK Gauging Network Publications Hydrological Trends
the transfer of the NRFA to the Institute of Hydrology in 1982 a new series — the Hydrological data UK series — was initiated which incorporated both river flow and groundwater level data4 (some water quality data were also featured). Following a readership consultation exercise in 1995, the decision was taken to publish future Yearbooks via the NRFA website. This allowed data to be placed in the public domain more rapidly and provided the flexibility to feature an increased amount of data (and range of supporting information); the ability to update and revise material was an additional advantage.
4
Collated by the National Groundwater Level Archive.
Currently, daily flow data for 200 index gauging stations can be accessed via clickable maps. The annual hydrological reviews previously published in the Yearbooks are now posted on the ‘Water Watch’ pages which serve as the primary information dissemination route for the National Hydrological Monitoring Programme. This programme has been operational since 1988 and relies on the active cooperation of the UK measuring authorities. It provides routine monthly reports — and supporting hydrometric statistics — on rainfall, river flows, groundwater levels, and reservoir stocks throughout the UK. The descriptive text is aimed at a wide audience and forms the basis of a range of policy and media briefing material, and input to technical reports and scientific papers.
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5.1. Hydrometric Register and Statistics volumes The NRFA has a responsibility to provide data retrieval services to all users, and access to the internet is not universal. Consequently, descriptions of the retrieval facilities are included in the ‘Hydrometric Register and Statistics’ reports which are published on a five-year cycle. Their purpose is to provide an overview of water resource and hydrological variability, and serve as a reference source to help data users select gauging stations appropriate to their needs. As a working document designed primarily for desktop use, the Hydrometric Register and Statistics volumes are less suited to electronic publication and five-year editions will continue to be issued in traditional hardback format. However, much of the published information — including the essential user guidance material — will also be featured on the Website.
6. Conclusion The volume of data generated by hydrometric monitoring activities across the UK is increasing rapidly in response to a burgeoning need for river flow data across a range of applications. Only limited resources can be devoted to hydrometric data acquisition and processing and there is a heavy responsibility on system designers to ensure the fitness for purpose of the river flow data available to users. This requires a critical review of all the links in the data acquisition–decision making chain, a recognition of the strategic as well as operational value of the data, and a continuing dialog with stakeholders and data users to ensure that changing requirements are being addressed.
Acknowledgements The author acknowledges the advice and guidance provided by colleagues at the Centre for Ecology and Hydrology — Jamie Honnaford, Oliver Swain and Mark Robinson in particular — during the preparation of this article.
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