Water Resources Management Plan

Hi John 

These figures are not correct, but there is a large apparent drop in WAFU on a like for like basis. This is caused by the WRPG requirement that the benefits from Temporary Use Bans and Non-Essential Use Bans are included as an option (at 83Ml/d in total), rather than WAFU as was the case for WRMP19. We would recommend any comparison is carried out on final planning tables rather than baseline for this reason. You should also be aware that availability of supply side drought orders and permits ceases in 2024/25.

On a like for like basis (i.e. including TUBs and NEUBs) the DO for WRZs 1-5 actually increased by 11Ml/d compared to WRMP19. Outage has also reduced by just under 7Ml/d, so the supply situation in WRZs 1-5 is better than it was in WRMP19 following the water resources re-assessment described in Appendix 5.1. Similarly, on a like for like basis there is an increase of 6.5Ml/d (due to reduced outage and increase TUBs/NEUBs allowance) in WRZ6. In addition, we have negotiated a 10Ml/d reduction in the bulk supply to SEW, further increasing WWAFU in WRZ 6 by 10Ml/d in 2025/26.

The move to 1 in 500 year drought resilience has a very small impact for Affinity water, at 2Ml/d in 2039/40. This reflects the fact that rates of groundwater reduction are small at such extreme values and most of the impact on groundwater sources has already occurred. 

John you also asked another question via email to one of the team - please also find answer below :

Question - Please could you explain the DO increase of about 60 Ml/d in WRZ2 from 2049-50 onwards, as shown in Row 38 in the screenshot below from your WRMP tables. What is the source of the increase? Why does the amount gradually increase to 76 Ml/d throughout the rest of the plan period?

Answer - As noted in our dWRMP, this is a balancing figure that seeks to avoid double counting of impacts under the enhanced environmental destination. Because ED is set based on individual source DOs and prior to climate change, the DO of WRZ2 would fall below zero with the assigned ED under the enhanced scenario. The net impact of the enhanced scenario at 2050 is therefore 220Ml/d, rather than the 277Ml/d based on individual sources under current day (pre-climate change) conditions. This increases as climate change increases to ensure the WRZ level DO does no fall below zero.

John you also asked a further two clarifications - please find below:

Question - I don’t understand how a WRZ deployable output can fall below zero. Isn’t the zone DO the sum of all DOs of the sources supplying the zone? Therefore, even if all the zone sources are abandoned, the total zone DO would drop to zero, but not to below zero. If you are saying that loss of sources leads to negative zone DO, doesn’t that mean that the original zone DO was incorrectly estimated?

Answer - This only occurs under the ‘Enhanced’ environmental destination scenario, where all of the base year deployable output from WRZ2 is ceased. The adjustment was an automatic balancing figure to ensure some of the future changes did not effectively end up double counting – i.e. we did not want to overestimate the supply/demand impact of future environmental destination based reductions.

 As fully described in Appendix 5.1. and in accordance with the best practice ‘Manual of Source Yields’, system Annual Average Deployable Output (ADO) is not simply equal to the Minimum Deployable Output (MDO) capacity of all sources. It is calculated to be equivalent to the nature of the demand included in the DYAA supply/demand balance, so is equal to the level of unrestricted annual average demand that can be met by the system under the design event. Because Affinity has no raw water storage, in our case that means the summer rolling 30 day period represents the point of greatest system stress. This means that the benefits of TUBs and NEUBs are large, but even when they are taken into account the system ADO is around 4% lower than a simple sum of the MDO values. The ADO shown for WRZs 1-5 was calculated as an aggregate total in accordance with the procedure described in Appendix 5.1. The ADO splits contained in the table are calculated post modelling based on percentage allocations from that aggregate figure using source capacity data.

 As noted in our previous response, the target is set based on pre-climate change MDO made up from individual sources. WRZ2 is the only zone in Affinity Water that is strongly impacted by climate change. The balancing figure therefore includes 24Ml/d at 2050 to prevent double counting of climate change under this scenario, plus 4Ml/d from the difference between ‘bottom up’ total MDO and the system ADO.

 The remainder of the adjustment then relates to two further considerations.

 Firstly, the adjustment in the model was applied to base DO, without allowing for the benefits of TUBs and NEUBs. In accordance with the WRPG, the TUBs and NEUBs benefit (12.4Ml/d) was added into this WRZ. Logically the balancing factor should be applied after TUBs and NEUBs, but should account for process losses and outage (2.2Ml/d). This means the balancing factor could be 10.2Ml/d lower than shown. However, we were aware that the benefits of abstraction reductions included in the WRSE model assumed DO splits based on the ‘low’ and ‘high’ scenarios, and the proportion of sources affected under the enhanced scenarios that lie within the Thames catchment is higher, so we kept this additional adjustment to offset any under-estimation of that benefit.

The remaining 17Mld of the adjustment at 2050 relates to the way the overall system ADO was apportioned into WRZs. This was based on source capacity allocations, which we have reviewed and concluded that this does fully represent the relative contributions of the different WRZs in Pywr. As a result of your query, we will therefore adjust the allocation to reflect the baseline ADO splits contained in the WRMP19 tables for 2025/26. Overall, this means that for the revised draft WRMP the proportion of DO allocated to WRZ2 will increase by 17Ml/d, which will effectively reduce the balancing figure by that amount. This means the supply/demand balance will be 17Ml/d worse from 2050 onwards than currently included for the ‘enhanced’ scenario in our revised Draft Plan.

Question - Why does the need for the “balancing” figure start suddenly in 2050?

Answer - As noted above, the balancing figure is only needed for the ‘enhanced’ environmental destination. The 2049/50 reductions under this scenario are concentrated on WRZ 2, when 64.7 Ml/d of baseline DO reductions come into effect. 

Hi John

The tables reflect the modelled period in the WRSE decision making tool – i.e. 2025-2075. Options (including drought intervention measures – see other repone) are not selected in the AMP7 period and figures are intended to provide baseline trends only. 

Grafham is included as a raw water import, at 91Ml/d annual average. 

Hi Eleanor - you can also send us a response via email to WRMP.Consultation@affinitywater.co.uk -  if you would like me to send you the questions in word format do let me know - Many Thanks