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: h.mcmillan@niwa.co.nz

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