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GEOG5060 GIS & Environment

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  1. School of Geography FACULTY OF ENVIRONMENT GEOG5060 GIS & Environment Dr Steve Carver Email: S.J.Carver@leeds.ac.uk

  2. Lecture 8: Hydrological modelling 1: catchment models Outline: Basics of hydrology Creating hydrologically correct DEMs Modelling catchment variables

  3. Lecture 8.Hydrological modelling 1: catchment models Outline: Basics of hydrology Creating hydrologically correct DEMs Modelling catchment variables GEOG5060 - GIS and Environment

  4. 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

  5. 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.)

  6. evapotranspiration precipitation overland flow evaporation evaporation infiltration channel flow percolation through flow return flow groundwater flow The hydrological cycle atmosphere interception surface store (ground) channel store soil store surface store (lake) surface store (sea) groundwater store

  7. The catchment = primary unit of study

  8. Catchment-based models: • spatial representation • lumped • distributed • process representation • black-box • grey-box • white-box

  9. Lumped vs Distributed models... Rf A Int OVF1 Rf ET Ovf S1 OVF2 TF S1 C TF1 OVFn S1 P1 TF2 DTM Ro TFn etc. P2 Q Pn Q 2D distributed lumped 3D distributed

  10. ET Inf Int Ovf TF P • Black-box vs White-box models... I I A ** C * o *** * * ** Cn S Gw i O O Black-box White-box

  11. 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

  12. slope altitude aspect drainage basins stream networks

  13. 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

  14. 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

  15. Catchment stores Atmosphere Interception store Channel store surface store Soil store Groundwater store

  16. transport deposition destination Flows within the system • Movement of water from store to store • Movement of solid/dissolved loads… source erosion “the jerky conveyor-belt”

  17. 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

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

  19. DEM FLOWDIRECTION SINK Are there any sinks? Yes No FILL Delineate watersheds Delineate stream network WATERSHED BASIN FLOWACCUMULATION Threshold FLOWACCUMULATION output streamnet = con (flowacc > 100, 1) STREAMLINE STREAMLINK STREAMORDER Creating a hydrologically correct DEM

  20. Calculating flow direction • Arc/Info GRID... • flowdirection • determines direction of flow from every cell • based on DTM • uses D8 algorithm • finds sinks

  21. Flow direction grid

  22. Flow accumulation • Different algorithms: • D8: uses 3x3 filter to calculate direction of flow as steepest downhill slope • Rho8: statistical version of D8 adding a uniformly distributed stochastic element • Monte Carlo simulation of D8: repeat D8 x 100 and use most probable result • FD8 and FRho8: modifications allowing flow dispersion to more than one downhill cell on a slope weighted basis

  23. Flow accumulation grids Flow accumulation (upslope area > 1000) Flow accumulation (upslope area > 100)

  24. Flat area problems high relief head water areas – good channel delineation low relief basin outpour areas – poor channel delineation

  25. 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

  26. Question… • When should we not remove pits?

  27. Flow accumulation • Arc/Info GRID... • 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

  28. 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

  29. GEOG5060 - GIS and Environment

  30. Calculating watersheds • Arc/Info GRID... • watershed • calculates upslope area contributing flow at a given location • based on flowdirection_grid and ‘pour points’

  31. Watersheds from specified outflow points

  32. Defining stream networks • Arc/Info GRID... • 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

  33. Calculating stream order • Arc/Info GRID... • streamorder • calculates stream order • based on either STRAHLER or SHREVE ordering

  34. Stream order - Strahler

  35. Stream order - Shreve

  36. Converting to vector • Arc/Info GRID... • streamlink • assigns unique values to each link • useful for attaching related attribute data • streamline • creates vector line coverage • takes flow direction into account such that all arcs point downstream • doesn’t lump adjacent cells

  37. Conclusions • DEMs are important for modelling the hydrological cycle • water flows down hill • other variables • Need to create hydrologically correct DEMs for accurate modelling

  38. Workshop • Demonstration: • creating hydrologically correct DEM in Arc/Info GRID • deriving other catchment variables

  39. 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)

  40. Practical • Steps: • Follow flow chart (supplied) to correct the DEM and derive a stream network • Compare derived stream network with 1:50,000 stream network • Identify problem areas and possible causes

  41. Practical • Experience with DEM correction and stream network derivation in Arc/Info GRID • Familiarity with problems of deriving stream networks in GIS

  42. Next week… • Hydrological modelling 2: runoff models • Incorporating time • Distributed models • Other examples • Workshop: Running dynamic models • Practical: Running a GIS-based hydrological model