Abstract
Abstract. Groundwater drought is a spatially and temporally variable phenomenon. Here we describe the development of a method to regionally analyse and quantify groundwater drought. The method uses a cluster analysis technique (non-hierarchical k-means) to classify standardised groundwater level hydrographs (the standardised groundwater level index, SGI) prior to analysis of their groundwater drought characteristics, and has been tested using 74 groundwater level time series from Lincolnshire, UK. Using the test data set, six clusters of hydrographs have been identified. For each cluster a correlation can be established between the mean SGI and a mean standardised precipitation index (SPI), where each cluster is associated with a different SPI accumulation period. Based on a comparison of SPI time series for each cluster and for the study area as a whole, it is inferred that the clusters are independent of the driving meteorology and are primarily a function of catchment and hydrogeological factors. This inference is supported by the observation that the majority of sites in each cluster are associated with one of the principal aquifers in the study region. The groundwater drought characteristics of the three largest clusters, which constitute ~ 80 % of the sites, have been analysed. There are differences in the distributions of drought duration, magnitude and intensity of groundwater drought events between the three clusters as a function of autocorrelation of the mean SGI time series for each cluster. In addition, there are differences between the clusters in their response to three major multi-annual droughts that occurred during the analysis period. For example, sites in the cluster with the longest SGI autocorrelation experience the greatest-magnitude droughts and are the slowest to recover from major droughts, with groundwater drought conditions typically persisting at least 6 months longer than at sites in the other clusters. Membership of the clusters is shown to be related to unsaturated zone thickness at individual boreholes. This last observation emphasises the importance of catchment and aquifer characteristics as (non-trivial) controls on groundwater drought hydrographs. The method of analysis is flexible and can be adapted to a wide range of hydrogeological settings while enabling a consistent approach to the quantification of regional differences in response of groundwater to meteorological drought.
Highlights
Groundwater drought is a type of hydrological drought characterised by sustained low groundwater levels, reduced base flow and reduced flows to springs and groundwater-fed rivers and wetlands (Van Lanen and Peters, 2000; Tallaksen and Van Lanen, 2004; Mishra and Singh, 2010; Van Loon, 2015)
Three episodes of regionally significant groundwater drought associated with prolonged low groundwater levels from October 1988 to November 1993, May 1995 to February 1998, and from August 2010 to August 2012 correspond closely with episodes of meteorological drought in the SPI12 time series and are consistent with those identified by previous studies (Lloyd-Hughes and Saunders, 2002; Marsh et al, 2007, 2013; Kendon et al, 2013; Hannaford et al, 2011; Parry and Marsh, 2013; Folland et al, 2015)
It is inferred from these observations that the large-scale drought history of the study area is represented well by the average SPI12 and standardised groundwater level index (SGI) time series
Summary
Groundwater drought is a type of hydrological drought characterised by sustained low groundwater levels, reduced base flow and reduced flows to springs and groundwater-fed rivers and wetlands (Van Lanen and Peters, 2000; Tallaksen and Van Lanen, 2004; Mishra and Singh, 2010; Van Loon, 2015). The impact of droughts on regional groundwater resources can vary in space and time This is because the response of groundwater systems to meteorological droughts, through changes in groundwater levels and baseflow to groundwatersupported rivers, is influenced by spatial variations in intrinsic catchment and aquifer characteristics and processes. These include highly non-linear unsaturated zone processes, recharge, and saturated groundwater storage, flow and discharge over a range of spatial and temporal scales Bloomfield et al.: Regional analysis of groundwater droughts using hydrograph classification et al, 2009; Bloomfield and Marchant, 2013; Van Lanen et al, 2013; Van Loon and Laaha, 2015)
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