The use of predefined drought indices for the assessment of groundwater drought episodes in the Baltic States over the period 1989–2018

Abstract

Study regionThe Baltic region (Estonia, Latvia, Lithuania) located at the Eastern Shore of the Baltic Sea, Northern Europe. Study focusThe spatial and temporal distribution of groundwater drought events over the period 1989–2018 was examined. Standardized Groundwater level Index (SGI) was calculated for six monitoring wells installed into shallow unconfined or semiconfined aquifers evenly distributed across the region. A set of meteorological and hydrological drought indices were used to characterize drought events using data from EAR-5 reanalysis. New hydrological insightsFour major drought events affecting all six monitoring wells were identified: 1992–1994, 1996–1997, 2002–2004 and 2005–2007 with another episode apparently starting in 2018. Meteorological drought indices - Standardize Precipitation Index (SPI), Standardized Precipitation Evapotranspiration Index (SPEI), and Reconnaissance Drought Index (RDI) - showed the highest correlation with groundwater drought events. The ERA-5 parametrization of surface and subsurface runoff used to calculate the hydrological indices - Standardized Runoff Index (SRI) and Standardized Sub-surface Runoff Index (SSRI) - most likely did not adequately represent the hill-slope scale processes affecting the unconfined groundwater dynamics, thus indices SRI and SSRI showed less clear relationship with groundwater drought events. Different patterns of SGI fluctuations and correlation with other drought indices highlight the importance of localized meteorological, land use, geomorphological or geological factors in controlling the groundwater response to the atmospheric forcing.