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| Titel |
A straightforward estimation of the spatial distribution of groundwater transit times in catchments. |
| VerfasserIn |
M. Van der Perk, D. S. J. Mourad, M. J. M. Vissers |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026108
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| Zusammenfassung |
| Quantification of the groundwater transit time distribution is fundamental for the prediction
of the fate of diffuse pollution in catchments. We propose a straightforward method to
estimate the spatial and statistical distribution groundwater transit time in lowland catchments
with homogeneous, horizontal aquifers. The method is based on the notion that in an
isotropic aquifer, the age of a water parcel increases logarithmically with depth. Under this
assumption, the groundwater transit time depends on groundwater recharge rate (N), aquifer
dimensions (thickness (D) and width (X)), aquifer porosity (n), and distance from the
divide. The mean transit time T of groundwater infiltrating in a model gridcell is given
by:
( ( ) ( ) )
¯ n- x2- X- x1- X-
T = D -
N Îx ln x2 - Îx ln x1 + 1
Where x1 and x2 are the distances from the divide to the respective upgradient and
downgradient cell boundaries and Îx is the gridcell size.
The method was tested and applied to the 901 km2 large Ahja catchment in south-eastern
Estonia (Mourad, 2008). Using spatial data on land cover, drainage network and
aquifer properties and a simple water balance model to estimate the groundwater
recharge rate, we calculated the spatial distribution of groundwater transit time at a
spatial resolution of 100 m. For the lower-order streams (Strahler stream order -¤
3), we related the in-stream logtransformed dissolved inorganic nitrogen (DIN)
concentration during baseflow conditions (summer 2002) to the calculated proportion of
groundwater from agricultural areas with transit times between 12 and 50 years. The
period between 12 and 50 years before 2002 represents the period that fertilizer
application in Estonia was considerably more extensive than in the periods before
and after. The relation was positive and statistically significant (n = 37; R2 = 0.49;
p-value: 0.000). This case study illustrated that it is not only important, but also
readily possible, to derive information on groundwater transit times and implement
this information in hydrology-based models of diffuse pollution at the catchment
scale.
References
Mourad, D.S.J. 2008. Patterns of nutrient transfer in lowland catchments – A case study
from northeastern Europe. Utrecht: KNAG, Netherlands Geographical Studies 370. |
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