Spatio-temporal modelling of the status of groundwater droughts

B.P Marchant,J.P Bloomfield

doi:10.1016/j.jhydrol.2018.07.009

Abstract

An empirical (geo)statistical modelling scheme is developed to address the challenges of modelling the severity and distribution of groundwater droughts given their spatially and temporally heterogeneous nature and given typically highly irregular groundwater level observations in space and time. The scheme is tested using GWL measurements from 948 observation boreholes across the Chalk aquifer (UK) to estimate monthly groundwater drought status from 1960 to 2013. For each borehole, monthly GWLs are simulated using empirical mixed models where the fixed effects are based on applying an impulse response function to the local monthly precipitation. Modelled GWLs are standardised using the Standardised Groundwater Index (SGI) and the monthly SGI values interpolated across the aquifer to produce spatially distributed monthly maps of SGI drought status for 54 years for the Chalk. The mixed models include fewer parameters than comparable lumped parameter groundwater models while explaining a similar proportion (more than 65%) of GWL variation. In addition, the empirical modelling approach enables confidence bounds on the predicted GWLs and SGI values to be estimated without the need for prior information about catchment or aquifer parameters. The results of the modelling scheme are illustrated for three major episodes of multi-annual drought (1975–1976; 1988–1992; 2011–2012). They agree with previous documented analyses of the groundwater droughts while providing for the first time a systematic, spatially coherent characterisation of the events. The scheme is amenable to use in near real time to provide short term forecasts of groundwater drought status if suitable forecasts of the driving meteorology are available.

Highlights

Groundwater drought is a type of hydrological drought defined as a period of below-normal groundwater level or reduced spring discharge (Tallaksen and Van Lanen 2004; Mishra and Singh, 2010; Van Loon, 2015)
This paper describes the first modelling method to take account of both the irregular and uncertain nature of groundwater level data and the spatial and temporally varying nature of droughts as they propagate through the terrestrial water cycle to estimate the monthly status of groundwater droughts including confidence bands on those estimates
Building on the mixed model approach of Marchant et al (2016), this paper reports on a simple approach to model groundwater drought status that both accounts for issues related to the availability and nature of groundwater level (GWL) data and that takes account of catchment and aquifer heterogeneity and the heterogeneous propagation of droughts

Summary

Introduction

Groundwater drought is a type of hydrological drought defined as a period of below-normal groundwater level or reduced spring discharge (Tallaksen and Van Lanen 2004; Mishra and Singh, 2010; Van Loon, 2015). Assessment of the severity and distribution of groundwater droughts is challenging due to a combination of issues primarily related to the availability and nature of groundwater level (GWL) data (Bachmair et al, 2016; Van Loon et al, 2017), and because groundwater droughts are commonly spatially and temporally heterogeneous in character (Peters et al, 2006; Mendicino et al, 2008; Tallaksen et al, 2009; Bloomfield et al, 2015). The spatio-temporal characteristics of the drought signal changes as it passes through the terrestrial water cycle so that groundwater droughts are typically lagged and attenuated compared

Methods

Results

Conclusion