Water & Wastewater Treatment

8 | MARCH 2016 | WWT | www.wwtonline.co.uk Comment Seeing the Big Picture on Drought noted that weather generators are calibrated using historical data and it is therefore di cult to validate the drought events that are more severe than the historical record. Synthetic drought scenarios allow a water supply planner to decide what type of drought event they would like to test their system against. The drought events are created by resampling the historical record in order to produce the desired drought characteristics. The droughts events can be viewed as a stress test placed on a water supply system and can also be used to modify the characteristics of historical drought events. HR Wallingford is currently applying these techniques for a number of water companies to assess their system vulnerability to drought and to test the ability of future infrastructure development options to reduce the risk and impact of future droughts. Climate models are typically used to understand the impacts of climate change on mean climate throughout the twenty- • rst century. The latest climate models also provide time series of climate for the twentieth century with multiple versions provided to re• ect di• erent sequences of natural climate variability. The climate model outputs can therefore be analysed to identify drought sequences. However, climate models are generally considered to have limited skill in simulating some climate variables relevant to drought formation and persistence and so at the current time should only be used to identify drought signals which can then be combined with resampling of the historical record. Hydrological and water supply system simulation models are typically used to assess a design drought scenario's impact on the water available for supply. Where complex water supply system models - which may be computational intensive - are necessary, a screening approach can be used to help identify the key design droughts of interest before using employing the more complex models. In order to understand the impacts of the design drought scenarios on the water supply system a modelling study is usually required. Catchment hydrological models, such as Hysim, CatchMod, PDM and SIMFLOW are oˆ en used to simulate the impacts of the drought scenarios on river • ows. Testing a water supply system with droughts that are more severe than those to which it has been originally designed allows a water supply planner to understand whether their system would be robust when exposed to drought events that are di• erent. It can help to identify water supply systems that may be more vulnerable to drought which could require additional drought measures in the immediate term or further options development to mitigate the risk of drought over the longer term planning process. Read more from Dr Ralph Ledbetter and his colleagues at www.hrwallingford.com DR RALPH LEDBETTER SENIOR HYDROLOGIST, HR WALLINGFORD Water companies need to look beyond their historical records if they want to understand the full picture on drought planning, argues Dr Ralph Ledbetter T he draˆ Environment Agency planning guidelines for PR19 outline that water companies "need to know how much water is reliably available to supply your customers in each of your water resource zones through a range of challenging droughts." Traditionally for water supply planning, a company uses the historic record to identify the worst drought events that have a• ected their area and then uses these drought events to test the performance of their contemporary water supply system. In England and Wales the historical record typically goes back to the early twentieth century, although with greater uncertainty as data records are less widespread further back through time. This will typically provide up to • ve or six historical events, with di• ering degrees of magnitude and impacts, in order to design and plan a water supply system. Drought events are rare and their characteristics are both spatially and temporally variable between di• erent drought events. Planning to the events in the historical record alone does not re• ect the fact that droughts could occur in the future with di• erent characteristics to those that we have already experienced. The concern therefore is that a water supply system may appear robust to the historical events to which it is designed but it may not perform to the same standards when exposed to a previously unseen drought. A number of methods and evidence sources exist for creating droughts with di• erent characteristics from those in the historical record. Understanding the di• erences between methods can help to identify which is most suitable for testing a speci• c water supply system. Stochastic weather generators are used to generate long sequences of arti• cial weather and are typically trained to do this based on historical data. The weather sequences are generated for many hundreds of years in which it is possible to identify drought periods for testing a water supply system. A weather generator can be used to produce drought events with a range of di• erent characteristics to those from the historic record. However, it should be

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