School of Geography FACULTY OF ENVIRONMENT School of Geography FACULTY OF ENVIRONMENT GEOG5060 GIS & Environment Dr Steve Carver

School of Geography FACULTY OF ENVIRONMENT Outline: –Basics of hydrology – Creating hydrologically correct DEMs – Modelling catchment variables Lecture 8: Hydrological modelling 1: catchment models

School of Geography FACULTY OF ENVIRONMENT Lecture 8 GEOG GIS and Environment3 Lecture 8. Hydrological modelling 1: catchment models Outline: – Basics of hydrology – Creating hydrologically correct DEMs – Modelling catchment variables

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT 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.)

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT The catchment = primary unit of study

School of Geography FACULTY OF ENVIRONMENT Catchment-based models: spatial representation lumped distributed process representation black-box grey-box white-box

School of Geography FACULTY OF ENVIRONMENT Lumped vs Distributed models... lumped 2D distributed 3D distributed A C Q RfET Ro OVF2 TF1 TF2 TFn OVF1 OVFn S1 P1 P2 Pn etc. Rf Int Ovf TF DTM Q

School of Geography FACULTY OF ENVIRONMENT 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...

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT altitude slope aspect drainage basinsstream networks

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Catchment stores Atmosphere Interception store Soil store Groundwater store Channel store surface store

School of Geography FACULTY OF ENVIRONMENT Flows within the system Movement of water from store to store Movement of solid/dissolved loads… sourceerosion transportdeposition “the jerky conveyor-belt” destination

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Question… Why do we need to correct DEM to be hydrologically correct? What problems might occur if we use an uncorrected DEM?

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Calculating flow direction Arc/Info GRID... flowdirection determines direction of flow from every cell based on DTM uses D8 algorithm finds sinks

School of Geography FACULTY OF ENVIRONMENT Flow direction grid

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Flow accumulation grids Flow accumulation (upslope area > 100) Flow accumulation (upslope area > 1000)

School of Geography FACULTY OF ENVIRONMENT Flat area problems high relief head water areas – good channel delineation low relief basin outpour areas – poor channel delineation

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Question… When should we not remove pits?

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Lecture 8 GEOG GIS and Environment29

School of Geography FACULTY OF ENVIRONMENT

School of Geography FACULTY OF ENVIRONMENT Calculating watersheds Arc/Info GRID... watershed calculates upslope area contributing flow at a given location based on flowdirection_grid and ‘pour points’

School of Geography FACULTY OF ENVIRONMENT Watersheds from specified outflow points

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Calculating stream order Arc/Info GRID... streamorder calculates stream order based on either STRAHLER or SHREVE ordering

School of Geography FACULTY OF ENVIRONMENT Stream order - Strahler

School of Geography FACULTY OF ENVIRONMENT Stream order - Shreve

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Conclusions DEMs are important for modelling the hydrological cycle water flows down hill other variables Need to create hydrologically correct DEMs for accurate modelling

School of Geography FACULTY OF ENVIRONMENT Workshop Demonstration: creating hydrologically correct DEM in Arc/Info GRID deriving other catchment variables

School of Geography FACULTY OF ENVIRONMENT 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)

School of Geography FACULTY OF ENVIRONMENT 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

School of Geography FACULTY OF ENVIRONMENT Practical Experience with DEM correction and stream network derivation in Arc/Info GRID Familiarity with problems of deriving stream networks in GIS

School of Geography FACULTY OF ENVIRONMENT Next week… Hydrological modelling 2: runoff models Incorporating time Distributed models Other examples Workshop: Running dynamic models Practical: Running a GIS-based hydrological model