Drought Planning and TDRA: some points about what drought means in the context of water resource planning and how “vital” TDRA is to Londons drought solution.
Drought Planning and TDRA
Thames Water market TDRA as a “vital drought resilience scheme for London”. But what does that mean?
How Thames Water has “explained” drought resilience is not useful. The phrases 1-in-200 and 1-in-500 are thrown around. There is talk of severe and extreme drought, of stand- pipes and hospitals closing and the loss of £300 or £500 million to the London economy – PER DAY!
It is a very easy public argument to throw out there and is very hard to push back at.
In simple terms a drought means an event that causes a reduction in the amount of water that is available. Climate change means that droughts are likely to happen more frequently and possibly be more severe or extreme – longer periods of no rain or longer periods of high temperature.
Water Companies have Drought Plans in place which describe the actions they will take to deal with drought situations. For longer term planning water resource management plans (WRMPs) include drought supply options to deal with the reduction in available water caused by droughts as they become more frequent or more extreme.
The instructions for the water resource management 2024 plans (WRMP24) were that water companies must have measures in place to deal with extreme drought situations by 2040.
The terms, “severe drought” and “extreme drought”, are also described as 1-in-200 and 1-in-500 events. A 1-in-200 event means an event which has a .05% chance of occurring every year, and 1-in-500 means an event that has a .02% chance of occurring every year. So, being resilient to 1-in-500 means being ready for that .02% chance.
In London, if a severe drought happened the impact on the water supply is assumed to be a reduction of available water of about 120mld. In an extreme drought it would be about 300mld.
Whenever a drought happens the first steps Water Companies take are to urge people to use less water, then fix leaks quicker, then maybe introduce hosepipes bans (TUBs). This means less water needed so any shortfall will not have such a big impact. If drought conditions continue then supply options have to be brought online, for example - aquifer storage systems, or, for those companies with working desalination plants, they get started up.
If the drought carries on the impact on available water really starts to kick in. This is the point when more intrusive supply options are needed. These are the Drought Permit options and the drought options such as standpipes. These need EA or Government permission to be started up as they have environmental and human consequences.
The National Framework for water resources The National Framework For Water Resources 2025 says this of the 1-in-500 resilience: “This level of resilience means that by 2040 the use of emergency drought options such as standpipes will not be needed unless there is an extreme drought”.
So, what does this mean in relation to TDRA? Thames Water said that they need a drought solution by 2033 in order for London to be resilient to the possibility of a severe (1-in-200) drought from that year.
The need to be resilient to severe drought was never given a specific date by the regulators in WRMP19 (unlike WRMP24). It was a “glide path” and a suggestion. The 2033 date in fact comes from that being the earliest that TDRA can be completed.
The sudden drop in available water supply in London 2032/33 (as shown in the supply demand data) and publicised by Thames Water arises because that is when Thames Water show the reduction.
So, which comes first, the chicken or the egg? Is TDRA selected impartially from the available options to suit 2033, which just happens to be the date that Thames Water plan for a reduction in available water or is the earliest date of construction of TDRA put in the IVM and then that date becomes the date when planning for reduction of available water happens?
As noted in the quote from the National Framework for Water Resources above the resilience level is meant to be to protect from the use of emergency drought options. It is definitely not to be used to avoid TUBs (hosepipe bans). So, how is a scheme to reduce the possibility of standpipes and emergency drought permits in a 1-in-200 situation (a .05% chance each year) end up having a proposed use of every second year? Are we to believe that every second year from 2033 there is likely to be a severe drought?
This issue of drought planning is very complex. What we are highlighting here is that TDRA becoming the “solution” to drought is not as straightforward a process as Thames Water present.