Post on 10-Mar-2020
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
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
A Model is an Abstraction of RealitySome are static…
� An elevation model is a representation of the earth’s surface
– Topographic contour map– Colour-shaded relief map– Physical terrain model
Source: http://www.ucmp.berkeley.edu/tectonics/globe1.gif
Source:http://www.solidterrainmodeling.com/
A Model is an Abstraction of RealitySome are static…
� An elevation model is a representation of the earth’s surface
– 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…
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?,...)?,,,( 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
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
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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
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
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
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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)
Time step 2 (after 50,000 years)
Time step 4 (after 100,000 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
Time step 1 (say after 25,000 years)
Time step 3 (after 75,000 years)
Time step 2 (after 50,000 years)
Time step 4 (after 100,000 years)
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
Time step 1 (say after 25,000 years)
Time step 3 (after 75,000 years)
Time step 2 (after 50,000 years)
Time step 4 (after 100,000 years)
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
Time step 1 (say after 25,000 years)
Time step 3 (after 75,000 years)
Time step 2 (after 50,000 years)
Time step 4 (after 100,000 years)
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
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…