Identifying Groundwater Droughts using standardized Water Levels

Abstract:

Drought indices are frequently used to compare the occurrence and characteristics of droughts at different sites as well as to characterize different hydrometeorological aspects of drought. The existing indices have been mostly focused on precipitation, soil moisture, and surface waters though. To enable a comparison of groundwater drought with other hydrometeorological aspects of drought, the Standardized Groundwater level Index SGI was proposed by Bloomfield and Marchant (2013). So far, the SGI has been applied only to consolidated aquifers in the UK. The purpose of this work is to assess the applicability and performance of the SGI in unconsolidated, porous aquifers situated in valleys, which represent the main sources of drinking water in many regions. For this purpose, long-term time series of groundwater levels both in wet and dry regions of Austria are analyzed and compared with time series of precipitation, evapotranspiration and river stages. It is shown that large drought events, such as 2003 with only 79% of the long-term average precipitation, but also less severe events are reflected by negative SGI anomalies. To identify and classify time periods with a groundwater deficit, such benchmark years are used to propose a threshold value of the SGI defining the onset of a drought. Time periods where a clear drop in SGI does not correspond to a significant anomaly in precipitation are also visible in the data. More detailed investigations into small valley fill aquifers in the south-east of Austria reveal that the SGI may closely correlate with river stage fluctuations. Also, effects of the geographic setting (mountainous area vs. lowland) and the impacts of human activities (hydropower, drinking water extraction) are shown. Bloomfield, J. P., Marchant, B. P., Analysis of groundwater drought building on the standardised precipitation index approach, Hydrology and Earth System Sciences, 17, 4769–4787, 2013.