Lecture 18
Hydrological modelling
Outline:
– Basics of hydrology
– Creating hydrologically correct DEMs
– Modelling catchment variables
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Basics of Hydrology
• The “Golden Rule” of hydrology.....
“water flows down hill”
– under force of gravity
– BUT, may move up through system via:
capillary action in soil
hydraulic pressure in groundwater aquifers
evapotranspiration
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The hydrological cycle
• Representation of:
– flows
water
energy
suspended/dissolved materials
– inputs/outputs to/from sub-systems
catchment/watershed
atmosphere
water stores (soil, bedrock, channel, etc.)
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The hydrological cycle
evapotranspiration
atmosphere
interception
precipitation
surface store (ground)
overland flow
evaporation
infiltration channel store evaporation
soil store channel flow
percolation through flow
surface store (lake)
groundwater store surface store (sea)
return flow
groundwater flow
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Catchment models
• Catchment-based models:
– spatial representation
lumped
distributed
– process representation
black-box
grey-box
white-box
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Spatial representations
• Lumped vs Distributed models...
Rf
A
Int
OVF1
Rf ET Ovf
S1
OVF2
S2 TF
C TF1 OVFn
Sn
P1 TF2 DTM
Ro
P2 TFn etc.
Q
Pn Q
lumped 2D distributed 3D distributed
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Process representations
• Black-box vs White-box models...
I I
ET A
** Int Inf
* Ovf C
o * ****
**
Cn S TF P Gw
i
O O
Black-box White-box
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Role of DTMs
• Surface shape determines water behaviour
– characterise surface using DTM
slope
aspect
(altitude)
– delineate drainage system:
catchment boundary (watershed)
sub-catchments
stream network
– quantify catchment variables
soil moisture, etc.
flow times... catchment response
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DEMs for hydrology
slope
altitude
aspect
drainage basins stream networks
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More spatial variables
• Other key catchment variables:
– soils
type and association
derived characteristics
– geology
type
derived characteristics
– land use
vegetation cover
management practices
– artificial drainage
storm drains/sewers
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Catchment inputs/outputs
• Inputs:
– precipitation (rain or snow)
– suspended/dissolved load
– pollutants (point source/non-point source)
• Outputs:
– stream discharge
– water vapour (evapotranspiration)
– groundwater recharge/transfer
– suspended/dissolved load
– pollutants
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Catchment stores
Atmosphere
Interception store
Channel store
surface store
Soil store
Groundwater store
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GIS-based catchment models
• Use data layers to represent:
– catchment characteristics
– inputs and outputs
– water stored in system
– flows within system
• Calculations between layers used to:
– represent relationships
– model processes
– predict RESPONSE
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Question…
– Why do we need to correct DEM to be
hydrologically correct?
– What problems might occur if we use an
uncorrected DEM?
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Creating a hydrologically correct DEM
DEM
FLOWDIRECTION
SINK
Are there
Yes any sinks?
No
FILL
Delineate watersheds Delineate stream network
WATERSHED BASIN FLOWACCUMULATION
Threshold FLOWACCUMULATION output
streamnet = con (flowacc > 100, 1)
STREAMLINE STREAMLINK STREAMORDER
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Calculating flow direction
• ArcGRID...
– flowdirection
determines direction of flow from every cell
based on DTM
uses D8 algorithm
finds sinks
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Flow direction grid
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Flow accumulation
• ArcGRID...
– flowaccumulation
calculates accumulated weight of all cells flowing
into each downslope cell
based on flowdirection_grid
high values = channels, zero values = ridges
may specify weight_grid
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Flow accumulation grids
Flow accumulation
(upslope area > 1000)
Flow accumulation
(upslope area > 100)
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Flat area problems
high relief head water areas – good channel low relief basin outpour areas – poor
delineation channel delineation
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Handling convergent drainage
• The problem with pits…
– closed depressions in DEM
– real or artefacts of DEM data model?
– often found in narrow valley bottoms where width of
flood plain < cellsize of DEM
– also found in low relief areas due to interpolation errors
– disrupt drainage topology
• To remove or not remove?
– fill in to obtain continuous flow direction network
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Uses of local drain direction
• Flowaccumulation (local drain directions):
– useful for computing other properties because of
information on connectivity:
cumulative amount of material passing through a cell (e.g.
water, sediment, etc.)
basis of many hydrological models
– mass balance model
flow = cumulative Rf - Int - Inf - ET
– wetness index
ln(As/tanB) ...where As = upslope area, B = slope)
– stream power index
w = As.tanB
– sediment transport index
T = (As/22.13)0.6 (sinB/0.0896)1.3
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Wetness index
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Calculating watersheds
• ArcGRID...
– watershed
calculates upslope area contributing flow at a given
location
based on flowdirection_grid and ‘pour points’
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Watersheds from specified outflow points
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Defining stream networks
• ArcGRID...
– stream networks
use con or setnull functions to delineate stream
networks, i.e.
streamnet = con (flowacc > 100, 1)
streamnet = setnull (flowacc < 100, 1)
based on flowaccumulation_grid and threshold
value
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Calculating stream order
• ArcGRID...
– streamorder
calculates streamorder
based on either STRAHLER or SHREVE ordering
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Stream order - Strahler
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Stream order - Shreve
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Conclusions
• DEMs are important for modelling the
hydrological cycle
– water flows down hill
– other variables
• Need to create hydrologically correct DEMs
for accurate modelling
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Practical
• Catchment modelling
• Task: Derive a stream network from a DEM
• Data: The following datasets are
provided…
– Section of Upper Tyne Valley DEM (50m
resolution)
– River network (1:50,000)
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Practical
• Steps:
1. Follow flow chart (supplied) to correct the
DEM and derive a stream network
2. Compare derived stream network with
1:50,000 stream network
3. Identify problem areas and possible causes
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Learning outcomes
• Experience with DEM correction and
stream network derivation in ArcGRID
• Familiarity with problems of deriving
stream networks in GIS
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Useful web links
• Hydrological modelling
– http://www.gisdevelopment.net/application/nrm/water/s
urface/watsw0004.htm
• DEMs and watershed modelling
– http://www.basic.org/projects/dtm/dtmdemo.html
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Next week…
• Environmental assessment
– Basics of EIA
– Using GIS to perform EIA
– Examples
• Practical:
– Develop EIA for wind farm example
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