OR/17/026 Summary and recommendations for further work

Mansour, M M, and Hughes, A G. 2017. Summary of results for national scale recharge modelling under conditions of predicted climate change. British Geological Survey Internal Report, OR/17/026.

Summary

Work undertaken

This report has described the application of the BGS distributed recharge model ZOODRM to produce recharge values (potential recharge) for Great Britain (England, Scotland and Wales). Detailed analysis has been completed for England and Wales as part of this project. This model has been run with the rainfall and potential evaporation for the Future Flows Climate datasets (11 ensembles). The following results have been produced:

• For the groundwater bodies in England and Wales:

• The mean, standard deviation and the following percentiles: 10, 25, 50, 75, 90 have been produced for the Long Term Average (LTA) annual recharge totals of each period; simulated historic (1950–2009), 2020s (2010–2039), 2050s (2040–2069) and 2080s (2070–2099).
• The LTA 25th percentile and 75th percentile for the simulated historic for each month has been calculated as mm/d. The daily recharge values calculated by the recharge model were aggregated to monthly values first and the analysis was undertaken using these monthly values (as mm/d). A proportion of recharge values above and below these values for the future climate has been calculated.
• The LTA mean monthly recharge values were calculated for each month for the simulated historic period. The change in monthly average recharge values (mm/d) in absolute terms was calculated for the 2020s (2010–2039), 2050s (2040–2069) and 2080s (2070–2099).
• Monthly change factors (percentage difference between monthly long-term average recharge for historic simulation and future climate 20s, 50s and 80s) for each groundwater body for each ensemble were produced. These have been summarised in plots which illustrate for each month the minimum, maximum and median monthly change factor from all the ensembles for each groundwater body.

• River Basin Management Districts in England and Wales:

• The LTA mean monthly recharge value was calculated for each month. The change in recharge value in absolute terms was calculated for the 2020s (2010–2039), 2050s (2040–2069) and 2080s (2070–2099).
• The long-term average total recharge volume as x106 Ml was calculated for 1961–90, 1971–00 and for the 2020s (2010–2039), 2050s (2040–2069) and 2080s (2070–2099).
• Empirical cumulative distribution functions (ECDF) have been produced for seasonal (spring, summer, autumn and winter) as well as monthly averages for historic simulation (both 1961–1990 and 1971–2000) as well as for the 2020s, 2050s and 2080s.

Summary of findings

The results confirm the dynamic between climate variability and climate change with a stronger climate signal being observed in the 2080s than either of the 2020s or 2050s. This is evidenced by the increasing sign of climate change for the 2080s over the 2020s or 2050s demonstrated by the ECDF plots in Total recharge for River Basin Management Districts. Generally the recharge season is peakier in the future, with greater recharge occurring in fewer months. Typically the recharge season is between five to seven months each year (September to April) during the historical simulation. It appears that this is shortened by one or two months for the future climate predictions. This is seen in both the changes in 25%/75% recharge values (Appendix 2 - Occurrence under 25 per cent and exceedance of 75 per cent recharge values) and the monthly differences (Seasonal and monthly empricial distribution functions and Appendix 3 - Mean monthly change). There appears to be agreement between the ensemble outputs on this feature of predicted change.

When recharge volumes were produced for the RBMDs (Total recharge for River Basin Management Districts and Appendix 5 - River basin management districts), the volumes tend to increase from the historical simulation to the 2020s/2050s, but more significantly in the 2080s. However, the range of possible outcomes also increases and so one possible outcome is that recharge volumes reduce.

The recharge season appears to be forecast to become shorter with a greater amount of recharge ‘squeezed’ into fewer months (e.g. Figure 14 and Total recharge for River Basin Management Districts). This could result in greater ‘lumpiness’ of the recharge signal leading to flashier groundwater level response and potentially greater drought vulnerability. The latter might be the case if rainfall ‘fails’ for one month, since rainfall totals are reliant on fewer months. Furthermore, if potential recharge took place over fewer months the lead in time for reaching drought status could also be reduced. These findings could have implications for water resources managers planning and responding to droughts in future. The increased vulnerability to drought could have knock on impacts for groundwater users and for groundwater dependent rivers, lakes and wetlands. Further groundwater hydrographs may become spikier which may lead to increased risk of groundwater flooding.

Whilst this work offers concrete conclusions, there are limiting assumptions and caveats that need to be observed. These caveats include: the current study has calculated potential recharge as opposed to what actually reaches the water table, it doesn’t take into account change in nature of rainfall, i.e. increase intensity and there may be increased amounts of rejected recharge due to a higher water table due to ‘spikier’ groundwater response.

File:OR17026fig16.jpg

Figure 16    Indicative change in monthly recharge under conditions of climate change.

Recommendations for further analysis

Given the amount of model outputs produced, a more detailed examination of the results for both groundwater bodies and those produced for the RBMDs would be beneficial. The summary plots produced for the groundwater bodies should be used as a basis for further work. Four issues in particular need to be addressed:

1. Integration of recharge volumes for the River Basin Management Districts — One issue that is clear is that whilst the 2050s and 2080s demonstrate a shorter recharge season, the volumes from the RBMD show an increase. However, the plots for the summaries of ensembles (Total recharge for River Basin Management Districts) show variation between the groundwater bodies. Further work should be undertaken to examine the impact of changing recharge on water resources and in particular groundwater bodies associated with the outcrops of the primary aquifers: Chalk, Permo-Triassic Sandstone and Jurassic Limestone. Alongside this the results for each time slice (2020s, 2050s and 2080s) for the groundwater bodies should be ranked. This will enable the areas where potential recharge may decrease to be identified.
2. Shortening of recharge season and vulnerability to drought — given the indication that more recharge is occurring in fewer months then the question is ‘does this make groundwater resources more vulnerable to drought?’. This question needs to be addressed to consolidate the underlying assumption that recharge is predicted to increase.
3. Range of ensembles and likely worse cases — examining the range of recharge volumes for each RBMD for the full set of ensembles show that recharge could decrease under some climate scenarios. The likelihood of this outcome and its implications needs to be examined in more detail.
4. Implications for water resources — Marrying the outputs of the model with either a water balance, e.g. CAMS ledger or producing change factors for recharge. The latter could be used with regional groundwater models or with the current qualitative status of the groundwater bodies and examining how they may change under future projected climate.

Finally whilst the initial analysis has focussed in how recharge will change for water resources, no consideration of groundwater flooding has been included. It is recommended that work on how the frequency of groundwater flooding is affected by climate change be examined.