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Possible Interventions
Intervention Contribution (Ml/day) Financial cost Social contribution Environmental impact
Water reuse 46—100 Low Low Low
New reservoir 287 High Low High
Desalination 150 High Low Medium
Usage reductions 67 Low High Low
Leakage reductions 53 High Low Low
Plans (combinations of interventions)
Plan Water reuse New reservoir Desalination Usage reductions Leakage reductions Overall financial cost Overall social contribution Overall environmental impact
No change - - - - - Low Low Low
Reuse-only 100 - - - - Low Low Low
Reuse and Reservoir 46 287 - - - Medium Low Medium
Reuse, Reservoir and Desalination 100 287 150 - - High Low High
Reuse and Reduce 100 - - 67 - Low High Low
Reuse and Reduce, with Desalination 46 - 150 67 53 High High Medium
Comparing demand and future flow scenarios, under ‘no change’ plan (all realisations)
River flows (climate scenario) Demand Expected frequency of restrictions (per year) Meets regulations
Level 1 Level 2 Level 3 Level 4
Near future Low 0.252 0.121 0.043 0.002 Close
Near future Baseline 0.463 0.294 0.160 0.027 No
Near future High 0.997 0.986 0.991 0.922 No*
Far future Low 0.51 0.331 0.178 0.01 No
Far future Baseline 0.741 0.583 0.419 0.115 No
Far future High 0.991 0.999 0.989 0.844 No*
Comparing plans under high demand, near future flows scenario (all realisations)
Plan Expected frequency of restrictions (per year) Meets regulations
Level 1 Level 2 Level 3 Level 4
No change 0.997 0.986 0.991 0.922 No*
Reuse-only 0.409 0.252 0.130 0.015 No
Reuse and Reservoir 0.312 0.170 0.074 0.005 No
Reuse, Reservoir and Desalination 0.233 0.108 0.033 0.002 Close
Reuse and Reduce 0.379 0.230 0.112 0.011 No
Reuse and Reduce, with Desalination 0.321 0.175 0.078 0.005 No

River flows

Rain falling in the catchment area flows into rivers and eventually into the Thames.

How much water flowed into London in the past? How might this change in the future, as the climate changes too?

The chart shows water arriving in the London water system, near Windsor. This is used as an input to the Water Futures model.

Water Futures uses 100 different versions ("realisations") of how river flows might change in the near and far future. We need to look at lots of versions to make reasonable estimates of how the water system will behave.

Pick a scenario

Demand for water

How much water might we use in the future?

People use water for all sorts of things. If the population of London continues to grow, and if we all keep using more water, the amount of water that we take out of the water system will increase in the future.

Water Futures considers three scenarios of future water demand: low, central and high.

This is another important input to the model. River flows define the amount of water coming into the system. Demand defines the amount of water that we would like to take out of the system.


What could we do in response to how the world might change?

We could build or change different things in the water system. These are the interventions in the first table.

For example, we could build a new desalination plant, which would supply more water to the system. Or we could reduce how much water we use, which would put less demand on the system.

Then we need to make a plan which includes some of these interventions.

For example, in ‘Reuse and Reduce’ we plan to reuse 100 Ml of water per day, and reduce demand by 67 Ml per day.

The plans described in the second table can all be tested in the Water Futures model with near future flows and high demand – otherwise we assume no change from the current system.


Select a flows scenario, a demand scenario and a plan to run the model.

River flows

Demand for water


The model will calculate:

  • storage: how much water is in all the reservoirs combined
  • shortfall: if demand is more than we can supply
  • restriction levels: to reduce water usage for short periods. They should only be used if absolutely necessary.
    • level 1: at most once in 5 years
    • level 2: at most once in 10 years
    • level 3: at most once in 20 years
    • level 4: never


Do we need to do anything different in the future? If so, what should we do?

After running the model hundreds of times, we can compare the various scenarios and plans.

The expected frequency of restrictions is calculated as the number of years in each scenario combination where a restriction level is met divided by the number of years simulated in all realisations of the scenario combination.

For example, we run 100 realisations of the near future, low demand, no change plan. Each realisation models 30 years, making 3000 modelled years. Level 4 restrictions only occur in 6 of those modelled years. So the expected annual frequency is 6 / 3000 = 0.002.

If we compare the no change plan across all other scenarios, we can see that only the low demand, near future scenario is close to meeting regulations. So we may need to do something different in the future.

If we compare how each of our plans perform under the high demand, near future scenario, we can see some variation in performance, with only Reuse, Reservoir and Desalination coming close to meeting regulations. So this plan might be worth exploring in more detail.