Catchment-based water resources

decision-support tool in the United Kingdom 12.3.1 Introduction

The current policy on low-flow estimation in England and Wales has the objective of making information on water resources availability and abstraction licensing more accessible to the public and of providing a trans- parent, consistent and structured approach to this aspect of water resources management. This is being achieved through a regulatory process called the Catchment Abstraction Management Strategy (www.environment- agency.gov.uk), which is an implementation compo- nent of the European Union Water Framework Direc- tive (WFD) within England and Wales. In Scotland, the WFD is being implemented by the Scottish Environ- ment Protection Agency through the Controlled Activi- ties Regulations (www.sepa.org.uk). Within both regu- latory jurisdictions, a basic hydrological management tool is the flow-duration curve (FDC). The evaluation of modifications to the FDC arising from water use and the consequences of such modifications for aquatic ecosystems are key management issues within both jurisdictions.

12.3.2 European Union Water Framework Directive Low-flow information is used to implement national and international law and policy directives. In Europe, the most important of these is the WFD, which was adopted by the European Parliament and the Council of the European Union in September 2000. This has es- tablished a strategic framework for the sustainable management of both surface water and groundwater resources. Each country must set up a competent au- thority to implement the Directive and log every signi- ficant body of water, above ground and below it, inland and on the coast. Most of the provisions of existing water-related European Directives, covering issues such as water abstraction, fisheries, shellfish waters and groundwater, will be combined, with past legislation being repealed or modified as the new regulations in- corporate them.

The WFD requires that water management be based on river basins, rather than on administrative issues, political boundaries or water sectors. River basin ma- nagement plans are being developed for each river basin district. The objectives are to provide protection for the basin in terms of aquatic ecology, unique and valuable habitats, drinking water resources and bathing waters. To achieve these ambitions, all those who have an impact on a river must be identified and involved in

planning how to meet the Directive’s requirements. In England and Wales, the Environment Agency has de- veloped a number of initiatives to implement the Directive which include the Catchment Abstraction Management Strategy (CAMS). This strategy is a sustainable, catch- ment-specific approach to water resources management which aims to balance human and environmental water requirements both in the present and in the future. 12.3.3 Managing catchment abstractions

in England and Wales

The key objective of CAMS implementation in England and Wales is to provide a consistent and structured ap- proach to local water resources management, recog- nizing both the reasonable water requirements of ab- stractors and environmental needs. It has made information on water resources publicly available and provides an opportunity for greater public involvement in the process of managing abstraction at the catchment level. A key element of CAMS is to assess the low-flow resources available. This provides information on whether there is a surplus of water available to meet current licensed abstractions, or a deficit. It also enables time-limited (normally 12-year) abstraction licences to be issued, in contrast to the historical practice of issuing perpetual licences. The CAMS approach is to produce FDCs for a range of situations, including the natural, current and future demand scenarios. This makes it possible to identify river reaches that have the potential for further development, that are overabstracted or overlicensed or that have insufficient water for further development. FDCs are being derived from over 800 continuously recording flow gauges in England and Wales. However, despite this gauging network, which is dense by international standards, over 95 per cent of reaches in England and Wales are located far from a flow-measuring station. These sites use the LowFlows methods and structure, which are implemented by the regulatory agencies in the United Kingdom. The balance between the available resources and the environmental needs of the river, current abstractions and licensed abstractions determines the status of a specific location (Figure 12.5). By developing water resources in a sustai- nable manner, the CAMS strategy will reduce conflict at the basin level between competing water users. The need to develop a rapid, nationally consistent ap- proach to estimating natural and artificially influenced FDCs within ungauged catchments led to the develop- ment of the LowFlows software system (Young and others, 2003). The system is underpinned by regiona- lized hydrological models that enable the natural, Case stuDIes

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long-term FDCs to be estimated for any river reach in the United Kingdom, mapped at a 1:50 000 scale. Both long-term “annual” statistics (considering variability within a year) and “monthly” statistics (considering variability within a calendar month) are provided. The impact of artificial influences is simulated using a geo- graphically referenced database that quantifies seasonal water use associated with individual features.

12.3.4 Region-of-influence model

The regionalized models employed within LowFlows are based on a region-of-influence approach, which removes the need for a priori identification of regions and instead develops a “region” of catchments similar to the ungauged catchment (section 9.5). The approach is founded on the dynamic construction of a region, based upon the similarity of the characteristics of the gauged catchments to those of the ungauged catchment. The application of this approach for estimating “annu- al” and “monthly” flow-duration statistics is described fully by Holmes and Young (2002) and Holmes and others (2002a, 2002b). In summary, the similarity bet-

ween the ungauged catchment and other catchments is assessed based on the distribution of soils and parent geology classes using an Euclidean distance metric. A region of the 10 most similar catchments is then identi- fied from a good quality dataset of catchments with na- tural flow regimes. Estimates of the flow statistics for the ungauged catchment are then calculated as a weigh- ted combination of the observed (standardized) flow- duration statistics for the 10 catchments in the region. The standardized annual FDC is re-scaled by multiplying by an estimate of annual mean flow from a national run- off grid derived from a daily soil moisture accounting model (Holmes and others, 2002b). A similar approach is used to determine the FDC for any month based on a distribution of annual runoff within the year (Holmes and Young, 2002).

12.3.5 The LowFlows software system

LowFlows (Young and others, 2003) incorporates the regionalized hydrological models within a PC-based software framework using contemporary programming

0.1 1 10 100 1000

Percent of time discharge exceeded

Fl ow ( % m ea n f lo w ) 10 20 30 405060 70 80 90 95 99 1 5 0.1

Percent of time discharge exceeded

Natural Influenced

Water use scenario A

Abstraction point Discharge point Case stuDIes

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tools. A geographic information system-based graphical interface provides access to the spatial datasets of catchment characteristics and the climatic variables re- quired for the application of the regionalized models. These are defined for the entire United Kingdom at a 1 km × 1 km resolution. A 1:50 000 scale vectored digi- tal river network and a set of digitized catchment bound-aries are used in conjunction with a digital terrain model to define catchment boundaries.

In practice, natural flow duration statistics are obtained by first selecting a point on the digital river network which defines the catchment outlet. A boundary is auto- matically generated by a digital terrain model. This boundary is overlaid onto the spatial datasets to obtain the catchment characteristics, such as the distributions of soil classes within the catchment, and the other va- riables required by the underlying hydrological models described above. The required natural flow duration statistics are returned at a “monthly” and “annual” resolution and are displayed in tabular and graphical form. Examples of output from the software are shown in Figure 12.6.

Water use within the catchment is simulated by utilizing data stored in a flexible database system. Seasonal water- use patterns associated with point influences, including abstractions, discharges and impounding reservoirs, are geographically referenced to enable catchment-based data retrievals. The net influences acting within a catch- ment are calculated by summing the individual influen- ces, where discharges and releases from reservoirs are positive and abstractions are negative. The “influ- enced” flow-duration statistics are presented, together with the natural statistics, at a “monthly” and “annual” resolution. Hence, practitioners can make a comparison between the natural regime and associated river-flow objectives, and the regime as modified by current water use within the catchment. Furthermore, the software can be used for scenario analysis, for ex- ample, to investigate the effect of increasing the abstrac- tion rates across a particular catchment for simulating a future water-use strategy.

The Environment Agency in England and Wales and the Scottish Environment Protection Agency have both adopted the LowFlows system as the standard decision- support tool for low-flow estimation in ungauged catch- ments. More than 170 trained staff use the system on a weekly basis at over 95 installations across the United Kingdom. The system is routinely used as an operational tool in the process associated with granting new ab-

straction licences and discharge consents, as well as for reviewing licence renewal applications. Furthermore, the results obtained using LowFlows are being used ex- tensively in the implementation of the WFD.

12.4 Low-flow management issues