Evaluating hydrological model structure using tracer data within a
multi-model frameworkHilary McMillan1, Doerthe Tetzlaff2, Martyn Clark3, Chris Soulsby2 1 National Institute of Water and
Atmospheric Research, New Zealand.
2 School of Geosciences,University of Aberdeen
3 National Centre for Atmospheric Research, Boulder, Colorado
Aims1. Use tracer data to choose
between model structures with similar dynamics
2. Show how tracer response is affected by interaction of model structure, parameters and mixing assumptions
Case Study: Loch Ard, Scotland
Water-Tracking MethodFUSE multi-model framework was modified to track distributions of water age in each store and flux.
Outflow age distributions, transit time distributions and tracer dynamics can be derived.
Results
Differences in tracer response could be explained by differences in model transit time distribution
Model ‘virtual experiments’ allow transit time behaviour to be exploredWe tested which transit time characteristics lead to good model performance
At Loch Ard a FUSE model could be designed which simulated both runoff and tracer dynamicsKey choices were a structure with single upper zone variable and Topmodel lower zone architecture
Tracer Simulation Seasonality SensitivityStructure vs CalibrationSome parameters (e.g. upper soil zone depth) control transit times equally with model structure
Transit Time Simulation
Model simulates flow but not tracers
Model simulates flow and tracers
Models which perform well have strong seasonal variation in MTT
Transit time distributions for fast flow pathways (<30 days) depend strongly on catchment wetness. At these timescales we shouldn’t assume the TTD is stationary. At timescales >30 days, seasonality is less important.
Mixing AssumptionsDoes saturation excess flow mix with soil water?
Flow partitioning between surface and soil water was found to have only a small effect on transit times so the simplifying assumption of no mixing was acceptable
But very different model structures can produce similar hydrographs
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To choose between model structures we use diagnostic tests which target individual model components using data types such as flow, soil moisture, and here tracers
Models with physically realistic structures are needed to produce good forecasts under a wide range of conditions
FUSE (Framework for Understanding Structural Errors) allows modular testing of popular hydrological model components
Definitions:Transit Time Distribution (TTD)Histogram of time taken for water to exit the catchment,
i.e. the breakthrough curve
Mean Transit Time (MTT)Mean of the Transit Time Distribution
12 years of data was used: rainfall, flow and weekly samples of chloride.
Chloride in rainfall originates from sea-salt and the concentration varies seasonally due to wind speed and direction
-- T
he s
tory
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Loch Ard Burn 10 is a small (0.9km2) catchment forested with Sitka spruce.Soils are poorly drained gleys and storm runoff is dominated by the upper soil horizons. Tree roots and exposed bedrock allow deeper recharge.
Contact: [email protected]
Model is more sensitive to store depth when store response is more nonlinear
High MTT in summer
Low MTT in winter
Models with single upper zone variable
Time-varying Mean Transit Times
Tim
e (d
ays)
Flo
w (
mm
)
Linear Tank2 Linear TanksTopmodel
Measured Flow
Time
Flo
w (
mm
)
Time
Less mixingTime (days)
TTD with variable mixing
More mixing
Fre
quen
cy
i i
i D
Transit Time Distributions
Vary Lower Zone Size Vary Upper Zone Size
Nas
h S
core
Lower Zone Size (mm) Upper Zone Size (mm)
Fre
quen
cy
Time (days) Time (days)
Rai
n (m
m)
Flo
w (
mm
)C
hlor
ide
(mg/
L)
Modelled tracer series
Models with split upper zone variables
Chl
orid
e (m
g/L)
Date
Steady state transit time distributions
Fre
quen
cy
Baseflow only
Time (days)
All flow
Seasonal Transit Time Distributions
% o
f flo
w
Less mixing
More mixing
Loch Ard Catchment
H31F-1227
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