Potential recharge and how it may change in the future

Climate change and recharge

Climate change will affect temperatures and rainfall rates leading to modified patterns of groundwater recharge. Reduction in infiltration recharge leads to reduction in groundwater resources exploited for domestic and industrial use and to reduction in river flows affecting the ecology during dry seasons. The increase of infiltration recharge, may lead to groundwater flooding.

The availability of a national scale distributed potential recharge model (ZOODRM) offers the opportunity to study possible infiltration recharge under future climates. Using UKCP09 projections of climate change in the UK available from the Met Office Hadley Centre, 150 years simulations of potential recharge are produced. Future potential recharge values are compared to the historical values to estimate the spatial changes in potential recharge.

The project has been co-funded by the Environment Agency (EA) and the British Geological Survey (BGS). A non-technical note was published by the EA with supporting materials available in a BGS project report.

Climate projections

Based on the 11 variants of the Hadley Centre Regional Climate Model HadRM3-PPE, which underpins the UKCP09 scenarios, the Centre of Ecology and Hydrology (CEH) applied a bias-correction and downscaling procedure to produce 11 scenarios of Future Flow Climate data. The HadRM3 is used as a perturbed physics ensemble approach to produce the 11 ensembles (Prudhomme et al., 2012) with one unperturbed example (afgcx) and ten variants (aixfa – afixq). These data are 1 km gridded climate time variant projections of rainfall and potential evaporation and allow comparison of results across a range of scales and geographical regions. The data were produced as daily grids from 1st January 1950 to 30th November 2099.

Projections of changes in potential recharge values

Projections of potential recharge values have been produced for groundwater bodies in England and Wales as well as River Basin Management Districts (RBMDs). Selected outputs from the model are available via the NGDC: https://www.bgs.ac.uk/services/ngdc/accessions/index.html.

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. Generally, the recharge season is peakier in the future, with greater recharge occurring in fewer months. There appears to be agreement between the ensemble outputs on this feature of predicted change.

ECDF &  median recharge for the 2080s - Winter
ECDF &  median recharge for the 2080s - Spring
ECDF &  median recharge for the 2080s - Summer
ECDF &  median recharge for the 2080s - Autumn

When recharge volumes were produced for the RBMDs, the volumes tend to increase from the historical simulation to the 2020s/2050s, but more significantly in the 2080s. This suggests that recharge may increase as the climate change signal predominates, however there are a range of outcomes based on the ensemble members.

The recharge season appears to become shorter with a greater amount of recharge “squeezed” into fewer months. 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 improved in future work. Mainly, actual recharge rather than potential recharge need to be calculated, the current study does not 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.

Potential recharge - September
Potential recharge - October
Potential recharge - November
Potential recharge - December
Potential recharge - January
Potential recharge - February

References

Prudhomme, C, Dadson, S, Morris, D, Williamson, J, Goodsell, G, Crooks, S, Boelee, L, Davies, H, Buys, G, Lafon, T and Watts, G. 2012. Future Flows Climate: an ensemble of 1-km climate change projections for hydrological application in Great Britain. Earth System Science Data, 4(1), pp.143-148.