Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment1 Lecture 18 Hydrological modelling Outline: – Basics of hydrology – Creating hydrologically correct DEMs – Modelling catchment variables

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment2 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment3 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.)

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment4 The hydrological cycle precipitation atmosphere interception evapotranspiration overland flow infiltration soil store through flow percolation groundwater store groundwater flow return flow channel store channel flow surface store (lake) surface store (ground) evaporation surface store (sea) evaporation

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment5 Catchment models Catchment-based models: – spatial representation  lumped  distributed – process representation  black-box  grey-box  white-box

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment6 Spatial representations Lumped vs Distributed models... lumped 2D distributed 3D distributed A C Q RfET Ro OVF2 TF1 TF2 TFn OVF1 OVFn S1 S2 Sn P1 P2 Pn etc. Rf Int Ovf TF DTM Q

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment7 Process representations Black-box White-box * *** ** * * i o I O I O Int Inf OvfP TF Cn S C Gw ET A Black-box vs White-box models...

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment8 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment9 DEMs for hydrology altitude slope aspect drainage basinsstream networks

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment10 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment11 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment12 Catchment stores Atmosphere Interception store Soil store Groundwater store Channel store surface store

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment13 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment14 Question… –Why do we need to correct DEM to be hydrologically correct? –What problems might occur if we use an uncorrected DEM?

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment15 DEM FLOWDIRECTION SINK FILL Yes Are there any sinks? No WATERSHEDBASINFLOWACCUMULATION Threshold FLOWACCUMULATION output streamnet = con (flowacc > 100, 1) STREAMLINESTREAMLINKSTREAMORDER Delineate watershedsDelineate stream network Creating a hydrologically correct DEM

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment16 Calculating flow direction ArcGRID... – flowdirection  determines direction of flow from every cell  based on DTM  uses D8 algorithm  finds sinks

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment17 Flow direction grid

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment18 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment19 Flow accumulation grids Flow accumulation (upslope area > 100) Flow accumulation (upslope area > 1000)

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment20 Flat area problems high relief head water areas – good channel delineation low relief basin outpour areas – poor channel delineation

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment21 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment22 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment23 Wetness index

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment24 Calculating watersheds ArcGRID... – watershed  calculates upslope area contributing flow at a given location  based on flowdirection_grid and ‘pour points’

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment25 Watersheds from specified outflow points

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment26 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment27 Calculating stream order ArcGRID... – streamorder  calculates stream order  based on either STRAHLER or SHREVE ordering

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment28 Stream order - Strahler

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment29 Stream order - Shreve

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment30 Conclusions DEMs are important for modelling the hydrological cycle –water flows down hill –other variables Need to create hydrologically correct DEMs for accurate modelling

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment31 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)

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment32 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

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment33 Learning outcomes Experience with DEM correction and stream network derivation in ArcGRID Familiarity with problems of deriving stream networks in GIS

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment34 Useful web links Hydrological modelling – urface/watsw0004.htmhttp:// urface/watsw0004.htm DEMs and watershed modelling –

Week 22GEOG2750 – Earth Observation and GIS of the Physical Environment35 Next week… Environmental assessment –Basics of EIA –Using GIS to perform EIA –Examples Practical: –Develop EIA for wind farm example