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| Titel |
Physical interpretation of parameters in the gamma distribution: implications for time-variant transit time assessment in catchments. |
| VerfasserIn |
Markus Hrachowitz, Chris Soulsby, Doerthe Tetzlaff, Iain Malcolm |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035369
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| Zusammenfassung |
| Transit time distributions (TTD) are being increasingly used as tools for exploring catchment
hydrological function and a means of process conceptualization. Estimating the parameters of
TTDs usually involves relating the temporally varying input concentration of a conservative
tracer to the signal in the stream. To date, most studies have been confined to single sites or to
a small number of nested catchments with short (< 2 years) data collection periods. This may
constrain the significance and transferability of findings as only a narrow range of the
climatic variability in a spatially restricted area is captured. We used long-term (>
10 years) data sets of 18O and Cl- from 38 catchments (ranging in size from 0.5
- 1600km2), in 8 geomorphically and climatically distinct parts of the Scottish
Highlands. The most appropriate TTD to model the tracer signals in streams using
convolution integrals was the gamma distribution: The associated α and β parameters
allow more flexibility to parsimoniously account for non-linearities in the system
response than other distributions. The resulting mean transit times (MTT = αβ) ranged
between 40 and 1500 days for the individual catchments. Multiple linear regression
models suggested that catchment MTT is largely controlled by the proportion of
responsive (i.e. overland flow generating) soil cover, drainage density and precipitation
intensity (R2 = 0.90). Although calibrated only for catchments of up to 35km2, these
controls also hold for catchments of up to 1600km2 in other regions of the Scottish
Highlands.
An additional inter-annual analysis of TTDs in 3 selected catchments illustrated the
time-variant nature of TTDs and explored possible physical interpretations for α and β. The
individual annual MTTs were much more variable than the MTT for the entire observation
period. Using multiple precipitation indices, it was found that parameter β was systematically
related to changes in climatic conditions and therefore controlling the temporal dynamics
of MTTs. Consistent with previous analysis, it was shown that α varies spatially
between catchments, rather than temporally, and that it appears to be influenced by
aspects of catchment structure and organization that control the non-linearities in
catchment hydrological response. This was illustrated by low α values (α < 0.5) for
catchments with predominantly responsive soils, which exhibit high degrees of
non-linearity caused by rapid activation of preferential and overland flow paths, and higher
values (α -¥ 0.5), for catchments with more freely draining soils, dominated by
subsurface flows which move towards the theoretical response of a linear reservoir (α =
1). The relationship between β and precipitation characteristics was subsequently
used to express β in a given catchment as a climate-related function to estimate
time-variant TTDs, which should give more accurate characterization of the timing and
magnitude of peak solute fluxes. In the wet, cool climatic conditions of the Scottish
Highlands, the fluctuations in transit times from the time-variant TTD were shown to be
roughly consistent with the variations of MTTs revealed by inter-annual analysis. |
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