Journal cover Journal topic
Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/hess-2016-532
© Author(s) 2016. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
10 Oct 2016
Review status
A revision of this discussion paper is under review for the journal Hydrology and Earth System Sciences (HESS).
Aggregation effects on tritium-based mean transit times and young water fractions in spatially heterogeneous catchments and groundwater systems, and implications for past and future applications of tritium
Michael K. Stewart1, Uwe Morgenstern2, Maksym A. Gusyev3,4, and Piotr Maloszewski5 1Aquifer Dynamics & GNS Science, P.O. Box 30368, Lower Hutt, 5040, New Zealand
2GNS Science, Tritium & Water Dating Laboratory, Avalon, Lower Hutt, 5040, New Zealand
3nternational Centre for Water Hazard and Risk Management (ICHARM), Public Works Research Ins titute (PWRI), Tsukuba, Japan
4National Graduate Institute for Policy Studies (GRIPS), Tokyo, Japan
5AGH University of Science and Technology Cracow, Department of Hydrogeology and Engineering Geology, Al. Mickiewicza 30, 30-059 Cracow, Poland
Abstract. Applications of simple lumped parameter models to describe aspects of hydrological systems rest on assumptions of homogeneity that are rarely valid. The lumped parameters are supposed to represent the quantities within the system as well as those of the overall system, but such quantities will obviously vary greatly from place to place within heterogeneous systems. Less appreciated is the fact that aggregation errors will affect overall system parameters as well. Kirchner (2016a) recently demonstrated that aggregation errors due to heterogeneity in catchments could cause severe underestimation of the mean transit times (MTTs) of water travelling through catchments when simple lumped parameter models were applied to interpret seasonal tracer cycles. Here we examine the effects of such errors on the MTTs and young water fractions estimated using tritium concentrations. We find that MTTs derived from tritium concentrations in streamflow are just as susceptible to aggregation bias as those from seasonal tracer cycles. Likewise, groundwater wells or springs fed by two or more water sources with different MTTs will also show aggregation bias. However, the transit times over which the biases are manifested are very different; for seasonal tracer cycles it is 2–3 months up to about 5 years, while for tritium concentrations it is 6–12 years up to about 200 years. We also find that young water fractions derived from tritium are almost immune to aggregation errors as were those derived from seasonal tracer cycles.

To investigate the implications of these findings for past and future use of tritium for estimating MTTs in catchments and groundwater systems, we examined case studies from the literature in which simple and more complicated lumped parameter models had been used. We find that MTT aggregation errors are small when either component waters are young (less than 6–12 years, as found in many catchments), or component waters have similar MTTs to each other. On the other hand, aggregation errors are large when very young water components are mixed with old components. In general, well-chosen compound lumped parameter models should be used as they will eliminate potential aggregation errors due to the application of simple lumped parameter models. The choice of a suitable lumped parameter model can be assisted by matching simulations to time series of tritium measurements (underlining the value of long series of tritium measurements), but such results should also be finally validated to ensure that the parameters found by modelling correspond to reality.


Citation: Stewart, M. K., Morgenstern, U., Gusyev, M. A., and Maloszewski, P.: Aggregation effects on tritium-based mean transit times and young water fractions in spatially heterogeneous catchments and groundwater systems, and implications for past and future applications of tritium, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-532, in review, 2016.
Michael K. Stewart et al.
Michael K. Stewart et al.
Michael K. Stewart et al.

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Short summary
This paper presents for the first time the results of virtual experiments to examine the effects of aggregation errors due to heterogeneity in catchments on the mean transit times and young fractions estimated using tritium concentrations. Kirchner (2016a) had previously demonstrated such errors with seasonal tracer cycles. We found that mean transit times derived from tritium concentrations are just as susceptible to aggregation bias as those from seasonal tracer cycles.
This paper presents for the first time the results of virtual experiments to examine the effects...
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