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Lecture 18 Hydrological modelling

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Creating hydrologically correct DEMs. Modelling catchment ... The 'Golden Rule' of hydrology. 'water flows down hill' under ... web links. Hydrological ... – PowerPoint PPT presentation

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Title: Lecture 18 Hydrological modelling


1
Lecture 18Hydrological modelling
  • Outline
  • Basics of hydrology
  • Creating hydrologically correct DEMs
  • Modelling catchment variables

2
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

3
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.)

4
The hydrological cycle
atmosphere
interception
surface store (ground)
channel store
soil store
surface store (lake)
surface store (sea)
groundwater store
5
Catchment models
  • Catchment-based models
  • spatial representation
  • lumped
  • distributed
  • process representation
  • black-box
  • grey-box
  • white-box

6
Spatial representations
  • Lumped vs Distributed models...

Rf
A
Int
OVF1
Rf
ET
Ovf
S1
OVF2
TF
S2
C
TF1
OVFn
Sn
P1
TF2
DTM
Ro
TFn
etc.
P2
Q
Pn
Q
2D distributed
lumped
3D distributed
7
Process representations
  • Black-box vs White-box models...

I
I
A

C

o




Cn
S
Gw
i
O
O
Black-box
White-box
8
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

9
DEMs for hydrology
slope
altitude
aspect
drainage basins
stream networks
10
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

11
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

12
Catchment stores
Atmosphere
Interception store
Channel store
surface store
Soil store
Groundwater store
13
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

14
Question
  • Why do we need to correct DEM to be
    hydrologically correct?
  • What problems might occur if we use an
    uncorrected DEM?

15
Creating a hydrologically correct DEM
16
Calculating flow direction
  • ArcGRID...
  • flowdirection
  • determines direction of flow from every cell
  • based on DTM
  • uses D8 algorithm
  • finds sinks

17
Flow direction grid
18
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

19
Flow accumulation grids
Flow accumulation (upslope area gt 1000)
Flow accumulation (upslope area gt 100)
20
Flat area problems
low relief basin outpour areas poor channel
delineation
high relief head water areas good channel
delineation
21
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 lt 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

22
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

23
Wetness index
24
Calculating watersheds
  • ArcGRID...
  • watershed
  • calculates upslope area contributing flow at a
    given location
  • based on flowdirection_grid and pour points

25
Watersheds from specified outflow points
26
Defining stream networks
  • ArcGRID...
  • stream networks
  • use con or setnull functions to delineate stream
    networks, i.e.
  • streamnet con (flowacc gt 100, 1)
  • streamnet setnull (flowacc lt 100, 1)
  • based on flowaccumulation_grid and threshold value

27
Calculating stream order
  • ArcGRID...
  • streamorder
  • calculates stream order
  • based on either STRAHLER or SHREVE ordering

28
Stream order - Strahler
29
Stream order - Shreve
30
Conclusions
  • DEMs are important for modelling the hydrological
    cycle
  • water flows down hill
  • other variables
  • Need to create hydrologically correct DEMs for
    accurate modelling

31
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 (150,000)

32
Practical
  • Steps
  • Follow flow chart (supplied) to correct the DEM
    and derive a stream network
  • Compare derived stream network with 150,000
    stream network
  • Identify problem areas and possible causes

33
Learning outcomes
  • Experience with DEM correction and stream network
    derivation in ArcGRID
  • Familiarity with problems of deriving stream
    networks in GIS

34
Useful web links
  • Hydrological modelling
  • http//www.gisdevelopment.net/application/nrm/wate
    r/surface/watsw0004.htm
  • DEMs and watershed modelling
  • http//www.basic.org/projects/dtm/dtmdemo.html

35
Next week
  • Environmental assessment
  • Basics of EIA
  • Using GIS to perform EIA
  • Examples
  • Practical
  • Develop EIA for wind farm example
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