Lecture 18 Hydrological modelling

1. Lecture 18Hydrological modelling Outline: Basics of hydrology Creating hydrologically correct DEMs Modelling catchment variables GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

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.) GEOG2750 – Earth Observation and GIS of the Physical Environment

4. 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 GEOG2750 – Earth Observation and GIS of the Physical Environment

5. Catchment models • Catchment-based models: • spatial representation • lumped • distributed • process representation • black-box • grey-box • white-box GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

7. ET Inf Int Ovf TF P Processrepresentations • Black-box vs White-box models... I I A ** C * o *** * * ** Cn S Gw i O O Black-box White-box GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

9. DEMs for hydrology slope altitude aspect drainage basins stream networks GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

12. Catchment stores Atmosphere Interception store Channel store surface store Soil store Groundwater store GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

14. Question… • Why do we need to correct DEM to be hydrologically correct? • What problems might occur if we use an uncorrected DEM? GEOG2750 – Earth Observation and GIS of the Physical Environment

15. 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 GEOG2750 – Earth Observation and GIS of the Physical Environment

16. Calculating flow direction • ArcGRID... • flowdirection • determines direction of flow from every cell • based on DTM • uses D8 algorithm • finds sinks GEOG2750 – Earth Observation and GIS of the Physical Environment

17. Flow direction grid GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

19. Flow accumulation grids Flow accumulation (upslope area > 1000) Flow accumulation (upslope area > 100) GEOG2750 – Earth Observation and GIS of the Physical Environment

20. Flat area problems low relief basin outpour areas – poor channel delineation high relief head water areas – good channel delineation GEOG2750 – Earth Observation and GIS of the Physical Environment

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 < 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 GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

23. Wetness index GEOG2750 – Earth Observation and GIS of the Physical Environment

24. Calculating watersheds • ArcGRID... • watershed • calculates upslope area contributing flow at a given location • based on flowdirection_grid and ‘pour points’ GEOG2750 – Earth Observation and GIS of the Physical Environment

25. Watersheds from specified outflow points GEOG2750 – Earth Observation and GIS of the Physical Environment

26. 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 GEOG2750 – Earth Observation and GIS of the Physical Environment

27. Calculating stream order • ArcGRID... • streamorder • calculates stream order • based on either STRAHLER or SHREVE ordering GEOG2750 – Earth Observation and GIS of the Physical Environment

28. Stream order - Strahler GEOG2750 – Earth Observation and GIS of the Physical Environment

29. Stream order - Shreve GEOG2750 – Earth Observation and GIS of the Physical Environment

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 GEOG2750 – Earth Observation and GIS of the Physical Environment

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 (1:50,000) GEOG2750 – Earth Observation and GIS of the Physical Environment

32. 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 GEOG2750 – Earth Observation and GIS of the Physical Environment

33. Learning outcomes • Experience with DEM correction and stream network derivation in ArcGRID • Familiarity with problems of deriving stream networks in GIS GEOG2750 – Earth Observation and GIS of the Physical Environment

34. Useful web links • Hydrological modelling • http://www.gisdevelopment.net/application/nrm/water/surface/watsw0004.htm • DEMs and watershed modelling • http://www.basic.org/projects/dtm/dtmdemo.html GEOG2750 – Earth Observation and GIS of the Physical Environment

35. Next week… • Environmental assessment • Basics of EIA • Using GIS to perform EIA • Examples • Practical: • Develop EIA for wind farm example GEOG2750 – Earth Observation and GIS of the Physical Environment