Calculation of recharge at boreholes in the UK
Over the past month, the British Geological Survey team was busy calculating groundwater recharge values at boreholes in major aquifers in the UK using the lumped-catchment groundwater model AquiMod. While the main purpose of this model is to simulate the fluctuations of groundwater levels to study extreme events both historically and under future climate projections, it produces recharge values as a by-product. We compare these recharge values to those obtained from a national scale recharge model developed earlier (NR_LTA; blue bars in the above picture). The national scale recharge model calculates potential recharge, which is the water volume leaving the soil zone. AquiMod, has the advantage of calculating all terms of the groundwater balance and gives the actual recharge, which is the volume of water reaching the water table.
AquiMod is calibrated using a Monte Carlo approach. This gives many likely realisations, i.e. simulations with efficiency measures equal to or above a given threshold. We use all acceptable models to set uncertainty bounds for the recharge values. The figure above shows the 25 percentile, mean, and 75 percentile (red, green, and orange respectively) of the historical recharge values calculated by AquiMod (AM) for each boreholes.
We also started applying tools produced by other European Geological Surveys to assess the recharge values at these boreholes. Here we present historical recharge values derived from time series models created by METRAN, which is a transfer noise model developed by the Geological Survey of the Netherlands (TNO-GSN) (https://ngwa.onlinelibrary.wiley.com/doi/full/10.1111/gwat.12819).
The UK study is part of the TACTIC work of applying multiple models in selected pilots across Europe. The analysis allows us to compare estimates and assess the performance of different models and approaches. For the individual pilots the analyses will improve our estimate of the uncertainty in the estimated recharge values. Looking across the pilots, the analysis will aid in identifying hydrogeological settings for which some tools may be less applicable, and learn how assessments by different models are comparable for the generation of harmonised recharge estimates across Europe.