Regional yield assessment

Recommended methods for assessing water resources yield and the deployable output of water resources systems in the UK are discussed in guidance published by UK Water Industry Research (UKWIR2) and the Environment Agency (2000) and in the Environment Agency's periodically updated Water Resources Planning Guideline and related documents. The guidance requires water companies to determine the deployable output from water resource systems that could be sustained through the worst historic drought conditions. Typically the analysis involves complex modelling of the water resources system using software packages such as MOSPA and Aquator.

The assessment of deployable output is often carried out for discrete water resources zones. A resource zone is the largest possible area in which all water resources can be shared and hence all customers or a water company could experience the same risk of supply failure from a shortfall in resources. Fig. 17.8 shows resource zones in the northwest of England (previous versions of this map have been published showing major water transfer routes, however these may may no longer be shown owing to security concerns). United Utilities Water Plc supplies water to the four water resource zones, with the integrated resource zone serving a population of 6.5 million, the Carlisle resource zone, serving 106 000, the North Eden resource zone, serving 14 000 and the West Cumbria resource zone, serving 152 000. The supply network within the integrated zone has a high degree of inter-connection, and serves 95 per cent of the region's population. The other three zones are relatively small, and are remote from the regional network.

The yield from each resource zone is shown in Table 17.5, which is data published in the United Utilities WRMP of September 2009.3 The figures give a projection of future yield incorporating the effects of reductions caused by changes in abstractions licence conditions ('sustainability reductions') and by climate change (see Chapter 19). Whilst the great majority of the abstractions are not thought to impact adversely on the environment, it is expected that the amount of water that can be abstracted from certain water bodies during dry-weather conditions will be reduced to help protect salmon and other aquatic species from potential harmful effects due to low-flow conditions.

The models used to assess yield in each resource zone are based on complex networks of components used to represent physical features such as abstractions, links (pipelines, aqueducts or channels), reservoirs and lakes, flow regulators, diversions, pumping stations and water treatment works. In addition, limits and constraints can be applied to represent abstraction licences or behavioural features such as reservoir release control curves.

Fig. 17.8 United Utilities water resource zones, major lakes and reservoirs.
Table 17.5 Yield assessment for United Utilities water resources zones (in megalitres per day). (Source: United Utilities Water Resources Management Plan, September 2009)

Resource zone

Yield 2007-08

Impact of sustainability

Impact of climate

Yield 2034-35

reductions

change at 2034-35

Integrated

1908

-32.9

-28.1

1863.6

Carlisle

36.5

-3.8

-0.3

32.4

North Eden

10.3

0

0

10.3

West Cumbria

57.9

-9.4

-0.2

48.2

Region

2012.6

-46.1

-28.6

1954.7

In the West Cumbria resource zone, a model was applied to investigate operational changes that may be required to meet requirements of the EU Habitats Directive (see Chapter 19) in the River Eden SAC. This is downstream of Ennerdale Water, where outflows are controlled via a weir, fish pass and valve structures seen in Fig. 17.9. The diagram in Fig. 17.10 is a schematic of the model, illustrating the complexity of events in this comparatively small water resources system. The model was particularly useful to test alternative scenarios for the operational use of various links and controls within the system, allowing these scenarios to include conditions that had not previously been experienced and hence could not be assessed using past observations. Based on assessments of deployable output and water demand, the WRMP identified a likely 8 MLday-1 deficit in resources by 2014-15. It also set out measures to address this,

Fig. 17.9 Ennerdale Water (image courtesy of United Utilities).
Fig. 17.10 Water resource system model for the West Cumbria resource zone (data used courtesy of United Utilities).

including reductions in leakage, helping customers to use water more efficiently and develop a new groundwater source in the region.

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