Monthly Standardised Groundwater level Index (SGI) for observation boreholes across the UK from 1891 to 2015, based on reconstructed groundwater level time series (Bloomfield et al., 2018; https://doi.org/10.5285/ccfded8f-c8dc-4a24-8338-5af94dbfcc16). Standardised groundwater levels have been estimated using a non-parametric normal scores transform of groundwater level data for each calendar month. Probability estimates of an SGI being less than 0, -1, -1.5 and -2 are also provided. Full details about this dataset can be found at https://doi.org/10.5285/d92c91ec-2f96-4ab2-8549-37d520dbd5fc
