Pilot description and assessment : Permo-Triassic 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 Permo-Triassic sandstone aquifer and at selected observation boreholes within the aquifer. Groundwater levels and weather data at seven boreholes are examined in this study. Multiple tools, selected from the TACTIC toolbox that is developed undert WP2 of the TACTIC project, have been used for this purpose. The Permo-Triassic sandstone aquifer is the second major aquifer after the Chalk in the UK. These sandstone formations are mainly red sandstones that originated in a desert environment. Much of the sandstone is a soft, compact rock that is only weakly cemented. Groundwater flows through the matrix but the permeability of the aquifer is also considerably enhanced by the presence of fractures. The topography of the Permo-Triassic aquifer outcrop varies significantly nationally with a dominant landuse over the aquifer outcrop being mainly arable and improved grassland. the groundwater in the Permo-Triassic aquifer can be under confined or unconfined conditions or alternating between these conditions. 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 this study, the recharge values estimated using the distributed recharge model at these boreholes are different from those obtained from the lumped model. It is worth noting that the national recharge model calculates potential recharge, while the lumped model calculates actual recharge. In all cases the potential recharge values calculated by the national recharge model are higher than those calculated by the lumped model. 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 15% 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 that further analysis being carried out to extract additional information from the produced output to understand the temporal implications of the 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.