Publisher: Copernicus Publications
Journal: Hydrology and Earth System Sciences
Subjects: T, G, GE1-350, Geography. Anthropology. Recreation, Environmental technology. Sanitary engineering, Environmental sciences, Technology, TD1-1066
To improve the understanding of how aquifers in different alluvial
settings respond to extreme events in a changing environment, we analyze
standardized time series of groundwater levels (Standardized Groundwater
level Index – SGI), precipitation (Standardized Precipitation Index – SPI),
and river stages of three subregions within the catchment of the river Mur
(Austria). Using correlation matrices, differences and similarities between
the subregions, ranging from the Alpine upstream part of the catchment to its
shallow foreland basin, are identified and visualized.
Generally, river stages exhibit the highest correlations with groundwater
levels, frequently affecting not only the wells closest to the river, but
also more distant parts of the alluvial aquifer. As a result, human impacts
on the river are transferred to the aquifer, thus affecting the behavior of
groundwater levels. Hence, to avoid misinterpretation of groundwater levels
in this type of setting, it is important to account for the river and human
impacts on it.
While the river is a controlling factor in all of the subregions, an
influence of precipitation is evident too. Except for deep wells found in an
upstream Alpine basin, groundwater levels show the highest correlation with a
precipitation accumulation period of 6 months (SPI6). The correlation in the
foreland is generally higher than that in the Alpine subregions, thus
corresponding to a trend from deeper wells in the Alpine parts of the
catchment towards more shallow wells in the foreland.
Extreme events are found to affect the aquifer in different ways. As shown
with the well-known European 2003 drought and the local 2009 floods,
correlations are reduced under flood conditions, but increased under drought.
Thus, precipitation, groundwater levels and river stages tend to exhibit
uniform behavior under drought conditions, whereas they may show irregular
behavior during floods. Similarly, correlations are found to be weaker in
years with little snow as compared with those with much snow. This is in
agreement with typical aquifer response times over 1 month, suggesting that
short events such as floods will not affect much of the aquifer, whereas a
long-term event such as a drought or snow-rich winter will.
Splitting the time series into periods of 12 years reveals a tendency towards
higher correlations in the most recent time period from 1999 to 2010. This
time period also shows the highest number of events with SPI values below
−2. The SGI values behave in a similar way only in the foreland aquifer,
whereas the investigated Alpine aquifers exhibit a contrasting behavior with
the highest number of low SGI events in the time before 1986. This is a
result of overlying trends and suggests that the groundwater levels within
these subregions are more strongly influenced by direct human impacts, e.g.,
on the river, than by changes in precipitation. Thus, direct human impacts
must not be ignored when assessing climate change impacts on alluvial
aquifers situated in populated valleys.
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