Aggregation effects on tritium-based mean transit times and young water fractions in spatially heterogeneous catchments and groundwater systems
2017
Kirchner (2016a) demonstrated that aggregation errors due
to spatial heterogeneity, represented by two homogeneous subcatchments, 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 cycle data. Here we examine the effects
of such errors on the MTTs and young water fractions estimated using tritium
concentrations in two-part hydrological systems. 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 have aggregation bias. However, the transit times over which the biases
are manifested are different because the two methods are applicable over
different time ranges, up to 5 years for seasonal tracer cycles and up to 200 years for tritium concentrations. Our virtual experiments with two water
components show that the aggregation errors are larger when the MTT
differences between the components are larger and the amounts of the
components are each close to 50 % of the mixture. We also find that young
water fractions derived from tritium (based on a young water threshold of 18 years) are almost immune to aggregation errors as were those derived from
seasonal tracer cycles with a threshold of about 2 months.
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