Pilot description and assessment : Jurassic 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 Jurassic aquifer and at selected observation boreholes within this aquifer. Multiple tools, selected from the TACTIC toolbox that is developed under WP2 of the TACTIC project, have been used for this purpose. The Jurassic limestones are prominent aquifers in the Cotswold Hills, in eastern England and the North Yorkshire moors. They are relatively hard limestones with low specific yields but high permeability characteristics due to extensive fracture network and enlarged by solution. The topography of the Jurassic outcrop is characterised by vales and escarpments with arable being the dominant land use. While groundwater may remain under confined conditions during the year at some location, aquifer conditions switch between confined and unconfined at some other areas depending on the season of the year. 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. Groundwater data at three boreholes are used in this model. The differences between the 75th and 25th percentile recharge values are found to be between 26% and 33%, which indicates a relatively high degree of uncertainty. Thee recharge values estimated using the distributed recharge model are found to be close to those estimated using the lumped model noting that these models calculate potential recharge and actual recharge values respectively. The absolute recharge values calculated by the transfer function-noise model Metran are also different from those calculated by the lumped model with no consistent pattern that can be reported. Future recharge values have been calculated using the projected rainfall and potential evaporation values are -4.4 to 15.8% 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.