2d - Drainage Analysis & Spatial Data

2d‐ Drainage Analysis & Spatial Data

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Spatial Data:Drainage Analysis, Basin delineation

CIVE 781: Principles of Hydrologic Modelling

University of Waterloo

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Summary

• Drainage analysis / basin delineation– How it works

– Tips & strategy

– Issues

Model Evaluation Against data

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Spatial Delineation

• Definitions– Spatial delineation : the process of separating a watershed into homogeneous computational units for use in a hydrologic model

– Subbasin: • the drainage area of a location on a stream network minus the drainage areas of one or more upstream subbasins which flow directly into that subbbasin.

– Hydrological response unit (HRU) • an area with hydrologically unique response to meteorological events.

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Spatial Delineation

• Standard approach:

– Delineate subbasins via terrain analysis (GIS) [TODAY]

– Delineate HRUs via map overlay analysis (GIS) [NEXT DAY]

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Catchment: Definition• Common hydrologic modelling task is to

delineate drainage basins upstream of a point of interest

• This is a fundamental model discretization step

• Watershed/basin/subbasin/catchment/drainage basin

• Drainage basin (Water Survey of Canada) : – The area enclosed by a topographic divide such

that surface runoff drains by gravity into a river, lake or other water body Corresponds to a single outlet point on 2D landscape

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Data Source: Catchments

• ECCC product: WSC catchments– Unlike U.S., not finely discretized– Outlets don’t correspond to stream gauge

locations– Simply organizational

• Some provinces (e.g., BC) have finer resolutioncatchment product

• NRCan/ECCC working on better national product, – CHyF – Canadian Hydrologic Features– Part of National Hydrographic Network (NHN)

• UW group has developed a North American discretization product (more later!)

• Unfortunately, we will usually have to do the work of catchment delineation ourselves…

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Drainage Analysis

• Basis of catchment delineation, stream network creation, and runoff accumulation models– The topography of a watershed controls the distribution of water drainage

• Based upon 1 premise:– ‘Water flows downhill’– i.e., water will follow the path of steepest descent

• Also used to track soil, sediment, or contaminants carried by water

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Manual Drainage Analysis

• (1) Identify outlet point

• (2) Trace a line perpendicular to the topographic map contours

– Look for ridges

–May intersect peaks

–Watch out for saddle points

– Look at neighboring streams

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1010

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DEMs

• Digital Elevation models

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Canadian DEM Source Data

• Geobase – CDED (Canadian Digital Elevation Data:– Complete coverage of Canada– Compatible with translators for USGS DTED– Grid resolution of 8 to 23 m (1:50,000), 32 to 93 m (1:250,000)– http://geogratis.gc.ca/api/en/nrcan‐rncan/ess‐sst/

• WWF HydroSHEDS– Hydrologically Conditioned DEMs and Flow directions– Grid resolution of 3 arc‐seconds (<60‐90m) above 49°N– Lower quality (and not publically available) above 60°N– Great starting point for basin delineation in rugged terrain– Developed by B. Lehner at McGill University– http://www.hydrosheds.org/

• ArcticDEM– High quality product north of 60N ‐2m resolution!!!

– Links at http://cshs‐cwra.ca/greyjay/

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1 degree DEMs

• 1‐degree ‐ A.K.A. 1:250,000 scale DEMs

• Digitized from a 1:250,000 scale map

– 1201x1201 grids • Each grid cell is 3x3 arc‐seconds (3x6 or 3x12 towards north pole)

• 15‐minute ‐ A.K.A. 1:50,000 scale DEMs

– 1201x1201 grids • Each grid cell is 0.75x0.75 arc‐seconds (0.75x1.5 or 0.75x3 towards north pole)

• Horizontal resolution increases as we go north

3”

3”

N

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Understanding DEM Scales

1 deg.

1 deg.

15 min DEM0.75 arc‐sec cells

7.5 min DEM30 m3 m1 m…

(UTM, NAD1983)

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Finer Resolutions

• Both Provincial and Federal governments provide more detailed DEMs, typically in UTM coordinate systems

– Typically referred to by resolution:

• 30 meter (1:10,000) scale

• 10 meter

• <1 meter (if you are lucky) (LIDAR)

• 2 meter (ArcticDEM)

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Lidar

https://www.e‐education.psu.edu/lidar/book/

Lidar is useful for some applications, but usually overkill (and sometimes problematic) for regional scale watershed modelling

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CDED‐ known issues

• It is not seamless and certain files present certain deficiencies such as edge matching, holes‐spikes and unknown accuracy

• Inconsistent: created over a large time span using many different technologies and from various sources

• 1‐m vertical resolution insufficient in many areas of the country

• Not hydrologically conditioned – Automatically generated catchments may be erroneous

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Automated Drainage Analysis

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8‐direction pour‐point model

• 8 Slopes calculated from adjacent cells:

– maximum slope determines flow direction

67 56 49

53 44 37

58 55 22

1m

Slope = (44‐22)/2 = 15.56

67 56 49

53 44 37

58 55 22

1m

Slope = (44‐37)/ 1 = 7

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Flow Direction Grid

• Calculated from 8‐pour point model

67 56 49

53 44 37

58 55 22

46 50

38 48

31 24

61 47 21 16 19

53 34 12 11 12

2 2 4

1 2 4

128 1 2

4 8

8 4

4 8

2 1 4 4 4

1 1 1 2 16

DEM Flow Direction

Grid

Flow Direction Grid(meaning)

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Alternate: D‐infinity model

Flow direction is defined as steepest downward slope on planar triangular facets on a block centered grid.

Each cell can/will have two outlets

Tarboton, 1997, "A New Method for the Determination of Flow Directions and Contributing Areas in Grid Digital Elevation Models," Water Resources Research, 33(2): 309‐319

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Flow Direction Grid

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Flow Network

• Derived directly from flow direction grid, by connecting cell centers:

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Calculating Catchment Boundaries

• Catchment Defined as all cells draining into a given cell or stream node (drainage outlet point)

Drainage Outlet“Pour Point”

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Flow Accumulation Grid

• Used to calculate net volumetric flow to surface water features

• Simple flow accumulation Grid– Each cell contains the number of cells draining into it

– A proxy for drainage area of a point

• Weighted flow accumulation Grid– Each cell contains the quantity of runoff draining into it (from another dataset)

• weighting grid in units of m3/cell/day – usually annual avg. runoff

– Can be used to track other parameters (e.g., contaminant, soil) • weighting grid in units of kg/cell/day

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• Simple flow grid:

– each cell contains # of inputs

Cumulative flow (# of cells)

0 0 0

0 3 2

0 0 11

0 0

2 0

0 1

0 0 1 15 0

0 2 5 24 1

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• Once the flow accumulation grid is calculated, we can obtain

– Catchment boundaries

• Defined by cells that contribute to downstream point

– Stream delineation

– Estimate of surface water/mass fluxes

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Automated Stream Delineation

• Streams are defined by a threshold number of cells draining into them

Streams defined as having a threshold > 2 Cells

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Watershed Delineation

Requires specification of “pour points” (outlet points)

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Pour point snapping

• Must be pretty careful about pour point placement on rasterized stream

• Snapping algorithms move outlet to nearest (or most appropriate) stream cell

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Drainage Analysis: Problems

• Pits:– There may be cells (or groups of cells) with no downslope (pits)

• They can be errors in the DEM or natural groundwater recharge areas/ponds, prairie sloughs…

– These typically must be corrected:• Raise the elevation of all the cells in a pit to the minimum elevation of the surrounding cells (filling)

• Remove the edge cells around a pit (breaching)

• Most drainage analysis requires a “sink‐less” DEM– A.k.a. a ‘hydrologically conditioned’ DEM

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Automated Filling of DEM sinks

• To calculate realistic drainage basins, “sinks” in the DEM must be removed using “fillling” algorithms

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Filling a DEM

Obvious pit –one cell, no outflows

Less obvious pit –many cells, no outflows

Iterative Process!!!

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Breaching a DEM

Lawrence W Martz, Jurgen Garbrecht, An outlet breaching algorithm for the treatment of closed depressions in a raster DEM, Computers & Geosciences, Volume 25, Issue 7, 1999

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Breaching/carving a DEM

Blockage removed

Iterative Process!!!

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From S. Šamanović et al., 2016, Analysis of the pit removal methods in digital terrain models of variousresolutions, XXIII ISPRS Congress, 12–19 July 2016, Prague, Czech Republic

Original DEMBreached DEM Filled DEM

Filling a DEM is much more invasive, especially in pockholed landscapes. Breaching algorithms are more complex than filling algorithms and aren’t present in some GIS software

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Drainage Analysis: Overview

Real Elevations DEM Flow Direction

Weighted Flow/mass Accumulation

Drainage Watersheds

Fill*

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Delineation dangers: Porous landscapes

From Simon Lin, 2020 PAGE 43

“The Lake Panasoffkee watershed differs from the Lake Panasoffkeebasin, which is a much larger area that could theoretically contribute surface‐water flow to Lake Panasoffkee based on physiography but does not because of the karst terrain and its well‐developed internal drainage system.” (USGS Scientific Investigations Report, 2010)

Toolbox Delineation USGS Reference Delineation

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Delineation dangers: Urban watersheds

From Simon Lin, 2020 PAGE 44

Toolbox Delineation Reference Delineation

Source: Peace River – Saddle Creek watershed outline (Lake County Water Atlas)• Flat

• Stormwater system dominated

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Automatic Delineation: Symptoms

• Getting catchment areas wrong is similar to getting precipwrong in a hydrologic model

–Wrong contributing area=wrong amount of water entering the basin

• Can also lead to some secondary errors

– Truncate/miss critical land use (glacier cover)

– Can miss relevant flow paths (contaminant/isotope transport)

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Automatic Delineation: Thoughts

• Comparing automatically generated streams/catchments to mapped streams is a useful exercise– Don’t trust automated catchment implicitly– Verify against surface water feature map– May have to manually adjust boundaries/areas– Plotting DEM differences pre‐/post‐filling is enlightening

• Resolution Issues:– Need for lidar in some domains– Poor coverage of small basins problematic

• Sometimes we *want* sinks in our landscape (e.g., Prairie Potholes)– Actually correspond to non‐contributing area– Not a lot of automated tricks for this

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Data Source: National Hydro Network

• National Hydro Network

– Useful basemap information –Rivers, water bodies, impoundments

– Organized by major watersheds

– Useful reference for verifying calculated watershed boundaries

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Spatial Delineation Strategy

• Fundamental decisions:– How many subbasins are desirable for proper routing?

• # of stream gauges

• Model purpose (simulating ungauged subbasins?)

– What level of land use/soil/aspect detail is desirable in HRU?• Tempting access to “map overlay overkill”

– Thousands of unnecessary HRUs

• Soils data are often not particularly useful for hydrologic applications– Influence subtle to evaluate; local impacts smooth out over watershed

• South/north aspect can be critical– Volumetric flows correct

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Subbasin Delineation Strategy

• Many ways to break up the landscape

Source: Choi et al. (2011), A new algorithm for grid‐based hydrologic analysis by incorporating stormwater infrastructure. Computers & Geosciences. Volume 37, Issue 8, Pages 1035–1044

Subbasin Delineation Ground rules

• Gauge locations correspond to subbbasin outlets

– No need for fancy corrections

– Output::Observations : Apples::Apples

– Often pointless to use “standard” hydrologic unit catchments

– don’t do it

• Subbasins generally similar in size

– Generally as subbbasin size decreases, routing skill increases

• Subbasins at major branches in river network

• Dams/lakes/reservoirs correspond to subbbasin outlets (for reservoir treatment in Raven)

– Small reservoirs/hydraulic controls treated a bit differently from large ones

• Self draining (isolated) subbasins handled carefully

• Ideally, some subbasins will have dominant land use/soil type

– Useful for calibration

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Subbasin Delineation Ground rules

• Other reasons to break into smaller subbasins

– Landscape changes (e.g. mountains to hills to flatlands)

– Groundwater characteristics (if known)

– Trade‐off between modelling event timing with time of concentration vs in‐channel routing

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Subbasin Delineation: TINs/Grids?

I don’t like low‐resolution grids in hydrological modelsThey throw away / smears out one of the few things that we actually know.

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Subbasin Delineation: Challenges

Source: Ehsanzadeh, E., van der Kamp, G. and Spence, C. (2012), The impact of climatic variability and change in the hydroclimatology of Lake Winnipeg watershed. Hydrol. Process., 26: 2802–2813.

Non‐contributing areas

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Subbasin Delineation: Challenges

• Must verify consistency with river network

• Very wary of lakes• Topology must be properly encoded

• Circular networks sometimes problematic

• Larger networks mandate more subbasins to explicitly represent channel travel time

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Take Home

• Terrain analysis methods

• Basin delineation strategies

• Today – trying out some Terrain analysis tools

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Documents

2d - Drainage Analysis & Spatial Data