How can models be used to a nswer …...Lecture Outline 1. Recall what you already know about –...

Drainage Basin GeomorphologyHow can models be used to a nswergeomorphological questions?

Steve Darby

Source: http://www.ucmp.berkeley.edu/tectonics/globe1.gif

Lecture Outline

1. Recall what you already know about– Geomorphic Models– Geomorphological Questions

2. How models are used3. Some problems with models4. What do we mean by ‘answer’?

How can models be used to answer g eomorphological questions?

Source:Tucker and Whipple (2002)

Landscape Evolution Modelling

DRAINAGE BASINCODE

(Software)

INPUT DATA

OUTPUT DATA

CLIMATE TECTONICS GEOLOGY

SOILS LANDCOVER

MO

DE

L

Inside the Black Box…

DRAINAGE BASINCODE

(Software)

A Model is an Abstraction of RealitySome are static…

� An elevation model is a representation of the earth’s surface

– Topographic contour map

© Joseph Wheaton: Source USGS 7.5 Minute Series Topographic Map



– Topographic contour map– Colour-shaded relief map– Physical terrain model

Source: http://www.ucmp.berkeley.edu/tectonics/globe1.gif

Source:http://www.solidterrainmodeling.com/



– Topographic contour map– Colour-shaded relief map– Digital Elevation Model (DEM)

),( yxfz =

Source: © Joseph Wheaton; Sulphur Creek WatershedWith data from http://gis.napa.ca.gov/

LEMs are Dynamic

� All LEMs are time dependent

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zs=

∂∂

elevation location

Why do we ‘need’ models to answer Geomorphological Questions?

� Geomorphology is interested in changes of the landscape through time.

– Rates of change are often longer than the time we have to observe.

� Many of our conceptual models span time scales beyond our lifetime

� Models are good at keeping track of many interacting phenomena concurrently… I am not…

),( yxfz =

?,...)?,,,( yxft

z =∂∂

One observation… a snapshot of the landscape

LEMs can span any time-span at any time-step (in principle anyway)

Review: Geomorphological Questions

� What is ___? – The sediment production (sediment

budget)– The likelihood of ______

� When did ___ happen?� What if ___ happened? � Why does this landscape look like

this? � How does ____ happen?� Build and Test Hypotheses and/or

Conceptual ModelsSource: © Frantisek Staud

Broken River, New Zealand

YOUR TURN: What is going on here?

© USGS Source: (To be revealed during OLA 8)

What kind of questions can you think of?

YOUR QUESTIONS:

1. Your question here…2. Your question here…

© USGS Source: (To be revealed during OLA 8)

Q TYPE: (e.g. What is, When did, What if, Why, How…. Hypothesis)

e.g. what if

Can’t conceptual models answer these questions?

Source: Caption

Remember, a conceptual model is really just a hypothesis

So what features does the LEM need to answer the questions you posed?

Real or Analogue?

Spatial Resolution and Range?

Temporal Resolution and Range?•Time step (seconds, hours, years, decades, centuries)

Landscape Parameterization?•Geology, soils, vegetation, land cover

Runoff Production?Hillslope Processes?•Creep•Mass Wasting (different types?)•Gullying/ Rilling

Fluvial Processes?•Lateral migration or just vertical scour?

Tectonics?•Uplift, subsidence, drift

Climate Representation?•Steady sate? Variable?

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zs=

∂∂

Howard LEM: The Importance of Bedrock Controls

Source: Alan Howard’s Drainage Basin Model. From course slides:http://erode.evsc.virginia.edu/frlect/drainmodel/index.htm

Start FinishTime

Question: How does bedrock of variable erodibility and spatial distribution influence landscape evolution?

Source: Alan Howard’s Drainage Basin Model. From course slides:http://erode.evsc.virginia.edu/frlect/drainmodel/index.htm

Start FinishTime

Howard LEM: The Importance of Bedrock Controls

How can YOU use a LEM to answer geomorphological questions?

Source: http://www.niu.edu/landform/tutorial.html

WILSIM

What happens if _______?

Source: Caption

Source: http://www.niu.edu/landform/tutorial.htmlTutorial Scenario One (default)

•The entire landscape has the same erodibility

•Climate stays the same

•There is no uplift

Time step 1 (say after 25,000 years)

Time step 3 (after 75,000 years)



What about if I ____?

Source: http://www.niu.edu/landform/tutorial.htmlTutorial Scenario Two

•Change the erodibility of the upper 85% of the catchment so it is more erodible

•Keep climate constant

•Keep no uplift





How about when I _____?

Source: http://www.niu.edu/landform/tutorial.htmlTutorial Scenario Three

•Let the entire landscape have the same erodibility

•Climate stays the same

•Add uplift to the upper 40% of the basin





Let’s just change that a little:

Source: http://www.niu.edu/landform/tutorial.htmlTutorial Scenario Four

•The entire landscape has the same erodibility

•Have climate change from wetter to dryer

•Keep uplift in the upper 40% of the basin





Validation: Is it right?

Source: Coulthard and Macklin (2003)

The first step is recognizing your LEM is NOT reality

Model Outcomes

YNN8

YYN7

NYN6

YNY5

NNY4

NYY3

NNN2

YYY1

ParameterisationProcess lawsDiscretisationID

MODEL STATE MATRIX OUTCOMES

Can only replicate reality(Convergence)

Replicates reality incorrectly

OR

Correctly does not replicateReality (Divergence)

All of this is further complicated by uncertainty , especially parameter uncertainty

TYPE I

TYPE II

TYPE III

Source: Van Asselt and Rotmans (2002)

Modelling Uncertainties

Parametric Uncertainty

Every knob you turn, every input you specify has some uncertainty about it.

We’ll deal with this in the OLA.

What should f(x,y,…) be ?

•Which processes and how they are represented in the model represent structural uncertainties in the models.

•The suitability of applying a LEM to a specific site depends on whether or not the LEM suitably represents the processes of interest.

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zs=

∂∂

What can we learn from a model when it is wrong?

To be wrong and not know why is a problem…

© Joseph Wheaton: Sulphur Creek Watershed, California. PreliminaryCAESAR simulations showing excessive landslide production.Source: http://www.geog.soton.ac.uk/users/WheatonJ/SulphurCreek.asp

…or worse, to be wrong andnot even know it

What can we learn from a model that is wrong?

Source: USGS Map in progress (P. Comm RusellGraymer)

To be wrong and to know why is very powerful (type III outcome).

© Joseph WheatonView of rock outcrop in area that LEM is predicting slides.

Greenstone

River Feshie, Scotland

© Joseph Wheaton

� LEMs are powerful exploration tools in geomorphology

� The type of LEM and its features heavily constrain what types of geomorphological questions you can ask of them

� Models can not answer questions, simply provide insight into questions

� The only certainty in models is that what they predict will not actually happen…

How can models be used to a nswer …...Lecture Outline 1. Recall what you already know about –...

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