Pilot description and assessment : Chalk aquifer (United Kingdom)

This report describes the work undertaken by the British Geological Survey (BGS/UKRI) as a part of TACTIC WP4 to calculate historical and future groundwater recharge across the outcrop of the Chalk aquifer and at selected observation boreholes within the chalk. Multiple tools, selected from the TACTIC toolbox that is developed under WP2 of the TACTIC project, have been used for this purpose. The Chalk aquifer is a major aquifer in England providing more than 70% of the public water supply in southern England (Foster and Sage, 2017). It is a microporous white limestone with low matrix permeability but with well-developed interconnected network of fractures and solution enhancement fractures. The Chalk outcrop is characterised by smooth rolling hills with a land use that includes enclosed fields, woodland, open land, and built-up areas. The central part of the aquifer is overlain by deposits of Palaeogene age and where the groundwater becomes under confined conditions. Groundwater within the outcrop is mostly under unconfined conditions, albeit the presence of patches of Clay and flints. Three tools have been used to estimate the recharge values. These are the lumped parameter computer model AquiMod (Mackay et al., 2014a), the transfer function-noise model Metran (Zaadnoordijk et al., 2019), and the distributed recharge model ZOODRM (Mansour and Hughes, 2004). Future climate scenarios are developed based on the ISIMIP (Inter Sectoral Impact Model Inter-comparison Project (www.isimip.org) datasets. The resolution of the data is 0.5°x0.5°C global grid and at daily time steps. As part of ISIMIP, much effort has been made to standardise the climate data (e.g. undertake bias correction). The estimation of the recharge model using the lumped model AquiMod is achieved by running the model in Monte Carlo mode. This produces many runs that are equally acceptable and consequently the uncertainty in the estimated recharge values can be assessed. The application of additional tools provides an additional mean to assess this uncertainty. Generally speaking, the differences between the 75th and 25th percentile recharge values are not significant when compared to the absolute recharge values calculated at the selected boreholes. In addition, the recharge values estimated using the distributed recharge model at these boreholes are very close to those obtained from the lumped model. This was expected as the two models use the same recharge calculation method; however, the former calculates potential recharge and the latter calculates actual recharge. The absolute recharge values calculated by the transfer function-noise model Metran are different from those calculated by the lumped model, but the pattern of spatial distribution is maintained. Future recharge values have been calculated using the projected rainfall and potential evaporation values are 5 to 20% different from historical values on average. The 3o Max scenario, the wettest used in this work, produces values that are very different from the historical ones. This is observed in the output of both the lumped and the distributed models. Finally, future estimates are discussed in this report using long term average recharge values. It is recommended to carry out further analysis to these output in order to understand the temporal changes in recharge values in future, especially over the different seasons. In addition, it is recommended that the values and conclusion produced from this work should be compared to those obtained from different studies that applies future climate data obtained from different climate models.