Water & Wastewater Treatment Magazine
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Databank www.wwtonline.co.uk | WWT | MARCH 2019 | 27 GOAL 9 DROUGHT RESILIENCE Resilience against drought is an issue of growing impor- tance for water utilities given the twin challenges of popu- lation growth and climate change. A commonly used definition of drought (used by the National Infrastructure Commission in its assessment published in summer 2018) is a period of such low rainfall that water companies are forced to impose restrictions on the water use of house- holds. This does not mean temporary usage restrictions such as hosepipe bans, but so-called 'level 4' restrictions, such as rota cuts (where water is turned off at certain times of day), or where water is only made available through standpipes in the street. A drought this bad, equal to the worst drought in recorded history, is a highly unlikely event – 1 in 100 years or 1% probability. However, this still means that the chance of it happening by 2050 is 1 in 4. Through their long-term resource planning, water com- panies are therefore seeking to achieve resilience against the effect of a 'severe' drought – de- fined as a 1-in-200 year drought or 0.5% probability – and they have been asked to report on their ability to withstand this as part of PR19. A tougher hurdle to meet is resilience against an 'extreme' drought, defined as a 1-in-500-year drought or 0.2% probability. Statistics available for the preparedness of water com- panies include the percentage chance of temporary use bans in any given year, and the percentage chance of a water company having to apply for a drought permit to make ad- ditional abstractions. Drought is a greater threat in the south and east of England than it is in the rest of the coun- try, but some of the solutions involve water transfers between regions, and the National Infra- structure Commission has called on the Government to adopt a national approach to the issue. It recommended a triple-headed approach of reducing consump- tion, cutting leakage and infra- structure to enhance supplies; each would provide roughly a third of the required resilience. DROUGHT RESILIENCE Percentage chance in a single year of applying for a temporary use ban Percentage chance in a single year of applying for a drought permit Source: Water UK Source: National Infrastructure Commission (source: Water UK long term planning framework/Discover Water) 1% The chance of the 'worst historic' drought occurring in any given year (probability 1 in 100 years) 0.5% The chance of a 'severe' drought (probability 1 in 200 years) 0.2% The chance of an 'extreme' drought (probability 1 in 500 years) Drought probabilities Plans for increasing drought resilience Notable droughts in the last 50 years 1975-6: The most severe drought in living memory – many water restrictions put in place, including localised use of rota cuts and standpipes in England and Wales 1989-1992: Large rainfall deficits led to a prolonged drought in the East of England, with spray irrigation restrictions implemented and drought orders granted 3000 ML/day = current resilience 4000 ML/day = required resilience by 2050 32% 34% 34% building supply infrastructure leakage reduction demand management 1995-6: A dry winter led to very low reservoir levels in parts of Yorkshire, resulting in road tankers having to be used to bring water from Northumbrian Water's supply area to Yorkshire Water's. 2004-6: Two dry winters led to a drought in the south-east of England which at its worst saw hosepipe bans imposed for 15 million people. Four water companies applied for drought orders. 2010-12: The effect of two dry winters meant that 7 water companies in south and east England imposed temporary use bans on 20 million people in April 2012. The drought ended abruptly with the wettest April to September on record, resulting in widespread flooding Affinity 10% Anglian 10% Bristol 5% Cambridge 5% Dwr Cymru 5% Essex & Suffolk 4% Northumbrian <1% Portsmouth 2% Severn Trent 3% South East 10% South Staffs 2% South West 5% Southern 10% SES Water 5% Thames 5% United Utilities 5% Wessex <1% Yorkshire 4% Affinity 2.5% Anglian 2.5% Bristol 3% Cambridge 2% Dwr Cymru 2.5% Essex & Suffolk 2% Northumbrian <1% Portsmouth <1% Severn Trent 3% South East 2.5% South Staffs 2.5% South West 2.5% Southern 5% SES Water 2.5% Thames 5% United Utilities 5% Wessex <1% Yorkshire 1.3%