Supply and demand

The supply of water to a resident population is dependent on population size and rate of consumption. Additionally, water resources are consumed by industry and by agricultural users for irrigation, which is seasonal and weather dependent. Public water supply is the largest category of water use, accounting for an average of between 15 000 and 20 000 megalitres per day (MLday-1) over the period since 1971 (Fig. 17.5). Consumption by industry has declined in recent decades, reflecting changes in the industrial base in the UK. It can be seen that the largest water user after public supplies is the electricity-generating industry. Under normal circumstances much of this is non-consumptive abstraction that is returned to water-courses with limited

Fig. 17.5 Abstractions from freshwater in England and Wales (data source, Environment Agency).

Fig. 17.5 Abstractions from freshwater in England and Wales (data source, Environment Agency).

environmental impact. The proportion of the total abstraction from non-tidal surface waters decreased from 2000 to 2007, whereas abstraction from tidal waters increased, mostly for electricity generation.

Total annual freshwater abstractions in England and Wales are around 35 000 ML, against a licensed total of over 70 000 ML. However, about 70 per cent of the water taken for public water supply is ultimately returned to the environment as treated effluent. Consumption from public water supplies is approximately 52 per cent for domestic use, with around 23 per cent non-domestic users and a similar figure lost to leakage (Defra, 2008). Agricultural abstraction amounts to only a relatively small fraction of the total water use. Spray irrigation in particular accounts for only about 1 per cent of abstractions. However, it is most needed during hot, dry summers when it can then amount to 20 per cent of total water use, with almost all of that amount being lost as evapotranspiration.

In recent history, the general trend in demand for water has been a continual increase due to the growth in populations and to higher standards of living. This applies in most countries. In the UK, the population grew from 38.3 million in 1901 to 60.8 million in 2007, with projections of further increases to 67.2 million in 2021, 74.3 million in 2041, 79.8 million in 2061 and 85.3 million in 2081 (Hicks and Allen, 1999; Office for National Statistics, 2008, 2009).

The amount of water consumed is used as an indicator of demand. Estimates of per capita water consumption vary according to the assumptions made in the calculations. Both Water UK, which represents the UK water industry and Water-wise, a non-governmental organisation promoting water efficiency, estimate average domestic water use in the UK of around 150 L per person per day (Water UK, 2008; Waterwise, 2009). This figure is somewhere in the centre of a range estimated for other European countries, with Estonia and the Czech Republic estimated to consume only around 100 L per person per day, Germany 127, France 150, Italy and Ireland 190 and Spain 265 (Defra, 2008).

20000

18000

20000

18000

1999-00 2000-01 2001-02 2002-03 2003-04 2004-05 2005-06 2006-07 2007-08

^m Total non-household ■■ Leakage + other losses ^m Total household — Water available in dry year

1999-00 2000-01 2001-02 2002-03 2003-04 2004-05 2005-06 2006-07 2007-08

^m Total non-household ■■ Leakage + other losses ^m Total household — Water available in dry year

Fig. 17.6 Dry year 'supply demand balance' for water resources in England and Wales (Environment Agency, 2008, reproduced with permission). Vertical axis is in units of megalitres per day. The difference between the estimated demand total and available supply is known as the 'headroom'.

The water industry follows a 'twin-track' approach to ensuring security of supply. One track is demand-side water efficiency, which is promoted through educational and publicity campaigns, investment in infrastructure improvements, reducing leakage and use of water meters. The second track is enhancing supply where necessary, which may include planning new storage facilities or reservoirs, improving connections between different parts of supply areas and desalination plants.

One of the important considerations in managing water resources is the balance between supply and demand. This is assessed in water company plans making use of analysis of available resources contained in CAMS. Fig. 17.6 shows the balance between supply and demand for England and Wales for a 'dry year' (Environment Agency, 2008). The difference between the estimated available resource and the estimated demand is known as the 'headroom'. Whilst the headroom appears to offer security of supply for England and Wales as a whole, the total figures mask some important geographical differences. Fig. 17.7 shows water supply areas in England and Wales shaded according to whether or not the estimated supply meets the target headroom.

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