aggregation in environmental systems – part 1: seasonal tracer cycles quantify young water fractions, but not mean transit times, in spatially heterogeneous catchments
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ID: 182365
2016
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Abstract
Environmental heterogeneity is ubiquitous, but environmental systems are
often analyzed as if they were homogeneous instead, resulting in aggregation
errors that are rarely explored and almost never quantified. Here I use
simple benchmark tests to explore this general problem in one specific
context: the use of seasonal cycles in chemical or isotopic tracers (such as
Cl−, δ18O, or δ2H) to estimate timescales of
storage in catchments. Timescales of catchment storage are typically
quantified by the mean transit time, meaning the average time that elapses
between parcels of water entering as precipitation and leaving again as
streamflow. Longer mean transit times imply greater damping of seasonal
tracer cycles. Thus, the amplitudes of tracer cycles in precipitation and
streamflow are commonly used to calculate catchment mean transit times. Here
I show that these calculations will typically be wrong by several hundred
percent, when applied to catchments with realistic degrees of spatial
heterogeneity. This aggregation bias arises from the strong nonlinearity in
the relationship between tracer cycle amplitude and mean travel time. I
propose an alternative storage metric, the young water fraction in
streamflow, defined as the fraction of runoff with transit times of less
than roughly 0.2 years. I show that this young water fraction (not to be
confused with event-based "new water" in hydrograph separations) is
accurately predicted by seasonal tracer cycles within a precision of a few
percent, across the entire range of mean transit times from almost zero to
almost infinity. Importantly, this relationship is also virtually free from
aggregation error. That is, seasonal tracer cycles also accurately predict
the young water fraction in runoff from highly heterogeneous mixtures of
subcatchments with strongly contrasting transit-time distributions. Thus,
although tracer cycle amplitudes yield biased and unreliable estimates of
catchment mean travel times in heterogeneous catchments, they can be used
to reliably estimate the fraction of young water in runoff.
| Reference Key |
kirchner2016hydrologyaggregation
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|---|---|
| Authors | ;J. W. Kirchner |
| Journal | materials research bulletin |
| Year | 2016 |
| DOI |
10.5194/hess-20-279-2016
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| URL | |
| Keywords |
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