Quantitative Morphometric Analysis in Geomorphology
Spatial
Analysis
(Q
u
a
nt
i
tative
Mo
r
p
h
o
m
e
t
r
ic
An
a
l
y
s
i
s
)
and
key
terms
regarding
drainage
systems
and
surface
processe
s
,
how
the
tools
can
be
63
Dr.
Aurass
Muhi
Taha
Quantitative
Morphometric
Analysis:
Morphometry
is
the
measurement
and
mathematical
analysis
of
the
configuration
of
the
earth's
surface,
shape
and
dimension
of
its
landforms.
A
major
emphasis
in
geomorphology
over
the
past
several
decades
has
been
on
the
development
of
quantitative
physiographic
methods
to
describe
the
evolution
and
behavior
of
surface
drainage
networks.
The
morphometric
characteristics
at
the
watershed
scale
may
contain
important
information
regarding
its
formation
and
development
because
all
hydrologic
and
geomorphic
processes
occur
within
the
watershed.
Morphometric
analysis
of
a
watershed
provides
a
quantitative
description of
the
drainage
system,
which
is
an
important
aspect
of
the
characterization
of
watersheds.
GIS
techniques
are
now
a
day
used
for
assessing
various
terrain
and
morphometric
parameters
of
the
drainage
basins
and
watersheds
,
as
they
provide
a
flexible
environment
and
a
powerful
tool
for
the
manipulation
and
analysis
of
spatial
information.
The
measurement
and
mathematical
analysis
of
the
configuration
ofthe
earth's
surface
and
of
the
shape
and
dimensions
of
its
landform
provides
the
basis
of
the
investigation
of
maps
for
a
geomorphological
survey.
This
approach
has
recently
been
termed
as
Morphometry.
The
area,
altitude,
volume,
slop
e
,
profile
and
texture
of
landforms
comprise
principal
parameters
of
investigation.
Hydrology
tools:
The
Hydrology
tools
are
used
to
model
the
flow
of
water
across
a
surface.
Information
about
the
shape
of
the
earth's
surface
is
useful
for
many
field
s
,
such
as
regional
planning,
agriculture
,
and
forestry.
These
fields
require
an
understanding
of
how
water
flows
across
an
area
and
how
changes
in
that
area
may
affect
that
flow.
When
modeling
the
flow
of
water,
you
may
want
to
know
where
the
water
came
from
and
where
it
is
going.
The
following
topics
explain
how
to
use
the
hydrologic
analysis
functions
to
help
model
the
movement
of
water
across
a
surface,
the
concepts
focus
on
the
movement
of
water
across
a
surface.
6
4
Spatial
Analysis
(
Q
u
a
nt
i
tative
Mo
r
p
h
o
m
e
t
r
ic
An
a
l
y
s
i
s
)
Dr.
Aurass
Muhi
Taha
used
to
extract
hydrologic
information
from
a
digital
elevation
model
(DEM),
and
sample
hydrologic
analysis
applications.
The
Hydrology
tools
can
be
applied
individually
or
used
in
sequence
to
create
a
stream
network
or
delineate
watersheds.
I
Understanding
Drainage
Systems:
The
area
upon
which
waterfalls
and
the
network
through
which
it
travels
to
an
outlet
are
referred
to
as
a
drainage
system.
The
flow
of
water
through
a
drainage
system
is
only
a
subset
of
what
is
commonly
referred
to
as the
hydrologic
cycle,
which
also
includes
precipitation,
evapotranspiration,
and
groundwater
flow.
The
hydrology
tools
Spatial
Analysis
(
Q
u
antitative
M
o
rp
h
ometric
Analvsi
s
)
Dr.
Aurass
Muhi
Taha
A
drainage
basin
is
an
area
that
drains
water
and
other
substances
to
a
common
outlet.
Other
common
terms
for
a
drainage
basin
are
watershed,
basin,
catchment,
or
contributing
area.
This
area
is
normally
defined
as
the
total
area
flowing
to
a
given
outlet,
or
pour
point.
A
pour
point
is
the
point
at
which
water
flows
out
of
an
area. This
is
usually
the
lowest
point
along
the
boundary
of
the
drainage
basin.
The
boundary
between
two
basins
is
referred
to
as
a
drainage
divide
or
watershed
boundary.
data
type
(integer
or
floating
point
)
and
the
actual
sampling
of
the
surface
when
creating
the
original
D
E
M.
65
W
atershed
{Basin,C
a
t
chmen
t
,
Contributing
Are
a
)
W
ater
s
h
ed
bo
u
n
daries
(Drainage
divi
d
)
P
o
u
r
Po
i
n
ts
(
OUtlet
s
)
fAmp
o
n
enrs
of
dr
a
i
n
age
ln
s
i
n
The
network
through
which
water
travels
to
the
outlet
can
be
visualized
as
a
tree,
with
the
base
of
the
tree
being
the
outlet.
The
branches
of
the
tree
are
stream
channels.
The
intersection
of
two
stream
channels
is
referred
to
as
a
node
or
junction.
The
sections
of
a
stream
channel
connecting
two
successive
junctions
or
a
junction
and
the
outlet
are
referred
to
as
stream
links.
Exploring
Digital
Elevation
Iodels:
The
most
common
digital
data
of
the
shape
of
the
earth's
surface
is
cell-based
digital
elevation
models
(
DEMs).
This
data
is
used
as
input
to
quantify
the
characteristics
of
the
land
surface.
A
DEM
is
a
raster
representation
of
a
continuous
surface,
usually
referencing
the
surfac
e
of
the
e
a
r
th.
T
h
e
accuracy
of
this
data
is
d
e
t
e
rmined
primarily
by
the
resolution
(
the
distance
between
sample
points).
Other
factors
affecting
accuracy
are
Spatial
Analysis
sufficient
vertical
relief
that
a
flow
path
can
be
determined.
The
tools
operate
on
thc
66
Dr.
Aurass
Muhi
Taha
Errors
in
DEMs
are
usually
classified
as
either
sinks
or
peaks.
A
sink
is
an
area
surrounded
by
higher
elevation
values
and
is
also
referred
to
as
a
depression
or
pit.
This
is
an
area
of
internal
drainage.
Some
of
these
may
be
natural,
particularly
in
glacial
or
karst
areas
(
Mark
1988),
although
many
sinks
are
imperfections
in
the
DEM.
Likewise,
a
spike,
or
peak,
is
an
area
surrounded
by
cells
of
lower
value.
These
are
more
commonly
natural
features
and
are
less
detrimental
to
the
calculation
of
flow
direction.
Errors
such
as
these,
especially
sinks,
should
be
removed
before
attempting
to
derive
any
surface
information.
Sinks,
being
areas
of
internal
drainage,
prevent
downslope
flow
routing
of
water.
The
number
of
sinks
in
a
given
DEM
is
normally
higher
for
coarser-resolution
DEMs.
Another
common
cause
of
sinks
results
from
storing
the
elevation
data
as
an
integer
number.
This
can
be
particularly
troublesome
in
areas
of
low
vertical
relief.
It
is
not
uncommon
to
find
1
percent
of
the
cells
in
a
30-
meter-resolution
DEM
to
be
sinks.
This
can
increase
as
much
as
5
percent
for
a
3-arc
secondDEM.
DEMs
may
also
contain
noticeable
striping
artifacts,
a
result
of
systematic
sampling
errors
when
creating
the
DEM.
Agai
n
,
this
is
most
noticeable
on
integer
data
in
flat
areas.
The
hydrologic
analysis
tools
are
designed
to
model
the
convergence
of
flow
across
a
natural
terrain
surface.
There
is
an
assumption
that
the
surface
contains
Spatial
Analysis
(
Q
u
antitative
M
o
rp
h
ometric
Analysi
s
)
Dr.
Aurass
Muhi
Taha
assumption
that
for
any
single
cell,
water
can
flow
in
from
many
adjacent
cells
but
out
through
only
one
cell.
Deriving
Runoff
Characteristics:
When
delineating
watersheds
or
defining
stream
networks,
you
proceed
through
a
series
of
steps.
Some
steps
are
required,
while
others
are
optional
depending
on
the
characteristics
of
the
input
data.
Flow
across
a
surface
will
always
be
in
the
steepest
downslope
direction.
Once
the
direction
of
flow
out
of
each
cell
is
known,
it
is
possible
to
determine
which
and
how
many
cells
flow
into
any
given
cell.
This
information
can
be
used
to
define
watershed
boundaries
and stream
networks.
The
following
flowchart
shows
the
process
of
extracting
hydrologic
information,
such
as
watershed
boundaries
and
stream networks,
from
a
digital elevation
model
(
DEM
).
hydrologic
analysis
tools
allow
you
to
identify
the
sinks
and
give
you tools
to
fill
them.
67
Flow
A
ccumulation
Stream
Order
I
Stream
To
Feature
Ap
p
l
y
thresho
l
d
Stream
U
nk
Flow
l
e
ngth
Fill
J
Yes
.----------
.---------''--------'------,1
H
y
dr
o
l
o
g
i
a
l
mod
e
l
i
ng
fl
o
w
charr
Regardless
of
your
goal,
start
with
an
elevation
model.
The
elevation
model
is
used
to
determine
which
cells
flow
into
other
cells
(the
flow
direction).
However,
if
there
are
errors
in
the
elevation
model
or
if
you
are
modeling
karst
geology,
there
may
be
some
cell
locations
that
are
lower
than
the
surrounding
cells.
If
this
is
the
case,
all
water
traveling
into
the
cell
will
not
travel
out.
These
depressions
are
called
sinks.
The
68
Spatial
Analysis
(Q
u
a
nt
i
tative
Mo
r
p
h
o
m
e
t
r
ic
An
a
l
y
s
i
s
)
Dr.
Aurass
Muhi
Taha
The
result
is
a
depressionless
elevation model.
You
can
then determine
the
flow
direction
on
this
depressionless
elevation
model.
If
you
are
delineating
watersheds,
you
need
to
identify
pour
points
(locations
for
which
you
want
to
know
the
contributing
watershed
).
Usually
these locations
are
mouths
of
streams
or
other
hydrologic
points
of
interest,
such
as
a
gauging
station.
Using
the
hydrologic
analysis
tools,
you
can
specify
the
pour
points,
or
you
can
use
the
stream
network
as
the
pour
points.
This
creates
watersheds
for
each
stream
segment
between
stream
junctions.
To
create
the
stream
network,
you
must
first
calculate
the
flow
accumulation
for
each
cell
location.
If
you
are
defining
stream
network
s
,
you
not
only
need
to
know
the
direction
water
flows
from
cell
to
cell
but
also
how
much
water
flows
through
a
cell,
or
how
many
cells
flow
into
another
cell.
When
enough
water
flows
through
a
cell,
the
location
is
considered
to
have
a
stream
passing
through
it.
Creating
a
depressionless
DEM:
A
digital
elevation
model
(
DEM
)
free
of
sinks-a
depressionless
DEM-is
the
desired
input
to
the
flow
direction
process.
The
presence
of
sinks
may
result
in
an
erroneous
flow-direction
raster.
In
some
cases,
there
may
be
legitimate
sinks
in
the
data.
It
is
important
to
understand
the
morphology
of
the
area
well
enough
to
know
what
features
may
truly
be
sinks
on
the
surface
ofthe
earth
and
which
are
merely
errors
in
the
data.
The
tools
in
the
Hydrology
toolset
of
the
Ar
c
GIS
Spatial
Analyst
extension
are
useful
in
preparing
a
depressionless
elevation
surface.
Sinks
can
be
located
using
the
Sink
tool.
This
tool
requires
a
direction
raster
that
is
created
by
the
Flow
Direction
tool.
The
result
is
a
raster
that
identifies
any
existing
sinks
in
the
data.
Depending
on
the
results,
you
can
fill
the
sinks,
or
you
can
use
the
output
to
help
determine
the
fill
limit.
Sinks
can
be
filled
using
the
Fill
tool.
To
use
the
output
from
Sink
to
determine
the
fill
limi
t
,
refer
to
"Finding
sink
depth"
in
this
topic
(
below
).
Spatial
Analysis
69
(Q
u
a
nt
i
tative
Mo
r
p
h
o
m
e
t
r
ic
An
a
l
y
s
i
s
)
Dr.
Aurass
Muhi
Taha
-
r-_
-
-
-
r-
r-
P
ro
f
i
l
e
1.
1
i
e
w
o
f
a
s
i
nk
The
Fill
tool
uses
a
variety
of
Spatial
Analyst
tool
s
,
including
several
of
the
hydrologic
analysis
tools
discussed
earlier,
to
create
a
depressionless
DEM.
This
tool
requires
an
input
surface,
a
fill
limit,
and
an
output
raster.
When
a
sink
is
filled,
it
is
filled
to
its
pour
point,
the
minimum
elevation
along
its
watershed
boundary.
The
identification
and
removal
of
sinks
when
creating
a
depressionless
DEM
is
an
iterative
process.
When
a
sink
is
filled,
the
boundaries
of
the
filled
area
may
create
new
sinks, which
then
need
to
be
filled.
For
a
large
DEM
or
one
with
many
sinks,
this
can
take
minutes
to
hours.
Finding
sink
depth:
It
is
useful
to
know
the
depth
of
a
sink
or
group
of
sinks.
This
information
can
be
used
to
determine
an
appropriate
z
limit
for
the
Fill
tool,
to
understand
the
type
of
errors
present
in
the
dat
a
,
and
to
determine
if
the
sinks
are
legitimate
morphological
features.
The
following
steps
outline
the
general
process
to
find
sink
depth:
1.
Create
a
raster
of
sinks
with
values
that
identify
their
depth
by
running
the
Sink
tool
to
locate
sinks
in
the
raster.
2.
Use
the
Watershed
tool
to
create
a
raster
of
the
contributing
area
for
each
sink
using
the
flow
direction
from
the
elevation
raster
and
the
output
from
the
Sink
tool
as
input
for
pour
points.
3.
With
Map
Algebra
in
Python,
use
the
Zonal
Statistics
tool
with
the
Minimum
option
to
create
a
raster
of
the
minimum
elevation
in
the
watershed
of
each
sink:
sink_min
=
Z
onalStatistics
(
sink_areas,
"Value",
ele
v
_ras,
"Minimum"
)
(
Quantilawe
Moroh>metric
Analysi
<
)
Dr.
Aurass
Mu:hi
Taha
The
sink
areas
input
is
the
output
from
the
Watershed
tool.
4.
Create
a
raster
containing
the
lowest
eleval:i
on
a
!
ong
the
boundary
of
each
watershed
with
the
Zonal
Fill
tool
(
this
corresponds
to
the
elevation
at
which
flow
would
leave
the
basin
after
filling
to
the
rim)
:
sink_max
=
ZonalFill
(
sink_area.s,
elev_ra.s)
5.
Use
the
Minus
tool
to
subtract
the
minimum
value
from
the
maximum
value
to
find
the
depth
again:
sink_depth
=
Minus(
sink_max,
sink_min)
The
above
sequence
can
also
be
completed
using
the
Sink,
Watershed,
Zonal
Statistics,
Zonal
Fill,
and
Minus
geoprocessing
tools.
How
·
watel'S
hed
work
s
:
A
watershed
is
the
upslope
area
that
contributes
flow-generally
water-to
a
common
outlet
as
concentrated
drainage.
It
can
be
part
of
a
larger
watershed
and
can
also
contain
smaller
watersheds,
called
subbasins.
The
boundaries
between
watersheds
are
termed
drainage
divides.
The
outlet,
or
pour
point,
is
the
point
on
the
surface
at
which
water
flows
out
of
an
area
It
is
the
I
owest
point
along
the
boundary
of
a
watershed.
D
W
a
t
e
r
s
h
ed
bo
u
n
d
a
ry
D
--....
--....
•
W
r
e
r
s
h
e
d
c
o
m
p
o
n
e
n
r
s
S
u
b
b
a
s
i
n
D
r
a
i
n
a
g
e
d
i
v
i
d
e
s
S
t
r
e
a
m
n
e
t
w
o
r
k
O
u
t
l
e
ts
(
po
u
r
po
i
n
t
s
)
70
Quantitative morphometric analysis in geomorphology focuses on measuring and mathematically analyzing the earth's surface configuration and landform dimensions. It involves using GIS techniques to assess terrain parameters and drainage systems, providing valuable insights into watershed characteristics and hydrological processes. The hydrology tools aid in modeling water flow and understanding drainage systems for various applications such as regional planning and forestry.
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(Quantitative Morphometric Analysis) Spatial Analysis Dr. Aurass Muhi Taha Quantitative Morphometric Analysis: Morphometry is the measurement and mathematical analysis of the configuration of the earth's surface, shape and dimension of its landforms. A major emphasis in geomorphology over the past several decades has been on the development of quantitative physiographic methods to describe the evolution and behavior of surface drainage networks. The morphometric characteristics at the watershed scale may contain important information regarding its formation and development because all hydrologic and geomorphic processes occur within the watershed. Morphometric analysis of a watershed provides a quantitative description of the drainage system, which is an important aspect of the characterization of watersheds. GIS techniques are now a day used for assessing various terrain and morphometric parameters of the drainage basins and watersheds , as they provide a flexible environment and a powerful tool for the manipulation and analysis of spatial information. The measurement and mathematical analysis of the configuration ofthe earth's surface and of the shape and dimensions of its landform provides the basis of the investigation of maps for a geomorphological survey. This approach has recently been termed as Morphometry. The area, altitude, volume, slope, profile and texture of landforms comprise principal parameters of investigation. Hydrology tools: The Hydrology tools are used to model the flow of water across a surface. Information about the shape of the earth's surface is useful for many fields, such as regional planning, agriculture , and forestry. These fields require an understanding of how water flows across an area and how changes in that area may affect that flow. When modeling the flow of water, you may want to know where the water came from and where it is going. The following topics explain how to use the hydrologic analysis functions to help model the movement of water across a surface, the concepts and key terms regarding drainage systems and surface processes, how the tools can be 63
(Quantitative Morphometric Analysis) Spatial Analysis Dr. Aurass Muhi Taha used to extract hydrologic information from a digital elevation model (DEM), and sample hydrologic analysis applications. The Hydrology tools can be applied individually or used in sequence to create a stream network or delineate watersheds. Tool Description I I II Basin Creates araster delineating all drainage basins. Fill Fills sinks in a sutface raster to remove small imperfections in the data. Flow Accumulation Creates a raster of accumulated flow into each cell. A weight factor can optionally be applied. Flow Direction Creates araster of flow direction from each cell to its steepest downslope neighbor. ICalculates theupstream or downstream distance, orweighted distance, along the flow path for each cell. Creates a raster identifying all sinks or areas of internal drainage. Flow Length I Sink SnapPour Point Snaps pour points to the cell of highest flow accumulation within a specified distance. IAssigns quevalues to sections of a raster linear network between mtersectwns. Stream Link I Stream Order Assigns a numeric order to segments of a raster representing branches of a linear network. Stream to Feature Converts a raster representing a linear network to features representing the linear network. IDetermines the contributing area above a set of cells in a raster. Watershed I Understanding Drainage Systems: The area upon which waterfalls and the network through which it travels to an outlet are referred to as a drainage system. The flow of water through a drainage system is only a subset of what is commonly referred to as the hydrologic cycle, which also includes precipitation, evapotranspiration, and groundwater flow. The hydrology tools focus on the movement of water across a surface. 64
Spatial Analysis A drainage basin is an area that drains water and other substances to a common (Quantitative Morphometric Analvsis) Dr. Aurass Muhi Taha outlet. Other common terms for a drainage basin are watershed, basin, catchment, or contributing area. This area is normally defined as the total area flowing to a given outlet, or pour point. A pour point is the point at which water flows out of an area. This is usually the lowest point along the boundary of the drainage basin. The boundary between two basins isreferred to as a drainage divide or watershed boundary. Watershed {Basin,Catchment, Contributing Area) Watershed boundaries (Drainagedivid ) Pour Points (OUtlets) fAmponenrs of drainage lnsin The network through which water travels to the outlet can be visualized as a tree, with the base of the tree being the outlet. The branches of the tree are stream channels. The intersection of two stream channels is referred to as a node or junction. The sections of a stream channel connecting two successive junctions or ajunction and the outlet are referred to as stream links. Exploring Digital Elevation Iodels: The most common digital data of the shape of the earth's surface is cell-based digital elevation models (DEMs). This data is used as input to quantify the characteristics of the land surface. A DEM is a raster representation of a continuous surface, usually referencing the surface of the earth. The accuracy of this data is determined primarily by the resolution (the distance between sample points). Other factors affecting accuracy are data type (integer or floating point) and the actual sampling of the surface when 65 creating the original DEM.
Spatial Analysis Dr. Aurass Muhi Taha Errors in DEMs are usually classified as either sinks or peaks. A sink is an area surrounded by higher elevation values and is also referred to as a depression or pit. This is an area of internal drainage. Some of these may be natural, particularly in glacial or karst areas (Mark 1988), although many sinks are imperfections in the DEM. Likewise, a spike, or peak, is an area surrounded by cells of lower value. These are more commonly natural features and are less detrimental to the calculation of flow direction. Errors such as these, especially sinks, should be removed before attempting to derive any surface information. Sinks, being areas of internal drainage, prevent downslope flow routing of water. The number of sinks in a given DEM is normally higher for coarser-resolution DEMs. Another common cause of sinks results from storing the elevation data as an integer number. This can be particularly troublesome in areas of low vertical relief. It is not uncommon to find 1 percent of the cells in a 30- meter-resolution DEM to be sinks. This can increase as much as 5 percent for a 3-arc secondDEM. DEMs may also contain noticeable striping artifacts, a result of systematic sampling errors when creating the DEM. Again, this is most noticeable on integer data in flat areas. The hydrologic analysis tools are designed to model the convergence of flow across a natural terrain surface. There is an assumption that the surface contains sufficient vertical relief that a flow path can be determined. The tools operate on thc66
(Quantitative Morphometric Analysis) Spatial Analysis Dr. Aurass Muhi Taha assumption that for any single cell, water can flow in from many adjacent cells but out through only one cell. Deriving Runoff Characteristics: When delineating watersheds or defining stream networks, you proceed through a series of steps. Some steps are required, while others are optional depending on the characteristics of the input data. Flow across a surface will always be in the steepest downslope direction. Once the direction of flow out of each cell is known, it is possible to determine which and how many cells flow into any given cell. This information can be used to define watershed boundaries and stream networks. The following flowchart shows the process of extracting hydrologic information, such as watershed boundaries and stream networks, from a digital elevation model (DEM). Flow Accumulation Stream Order IStream To Feature Apply threshold Stream Unk Flowlength J Yes .---------- Fill .---------''--------'------,1 Hydrologia l modeling flowcharr Regardless of your goal, start with an elevation model. The elevation model is used to determine which cells flow into other cells (the flow direction). However, if there are errors in the elevation model or if you are modeling karst geology, there may be some cell locations that are lower than the surrounding cells. If this is the case, all water traveling into the cell will not travel out. These depressions are called sinks. The hydrologic analysis tools allow you to identify the sinks andgive you tools to fill them.67
(Quantitative Morphometric Analysis) Spatial Analysis Dr. Aurass Muhi Taha The result is a depressionless elevation model. You can then determine the flow direction on this depressionless elevation model. If you are delineating watersheds, you need to identify pour points (locations for which you want to know the contributing watershed). Usually these locations are mouths of streams or other hydrologic points of interest, such as a gauging station. Using the hydrologic analysis tools, you can specify the pour points, or you can use the stream network as the pour points. This creates watersheds for each stream segment between stream junctions. To create the stream network, you must first calculate the flow accumulation for each cell location. If you are defining stream networks, you not only need to know the direction water flows from cell to cell but also how much water flows through a cell, or how many cells flow into another cell. When enough water flows through a cell, the location is considered to have a stream passing through it. Creating a depressionless DEM: A digital elevation model (DEM) free of sinks-a depressionless DEM-is the desired input to the flow direction process. The presence of sinks may result in an erroneous flow-direction raster. In some cases, there may be legitimate sinks in the data. It is important to understand the morphology of the area well enough to know what features may truly be sinks on the surface ofthe earth and which are merely errors in the data. The tools in the Hydrology toolset of the ArcGIS Spatial Analyst extension are useful in preparing a depressionless elevation surface. Sinks can be located using the Sink tool. This tool requires a direction raster that is created by the Flow Direction tool. The result is a raster that identifies any existing sinks in the data. Depending on the results, you can fill the sinks, or you can use the output to help determine the fill limit. Sinks can be filled using the Fill tool. To use the output from Sink to determine the fill limit, refer to "Finding sink depth" in this topic (below). 68
(Quantitative Morphometric Analysis) Spatial Analysis Dr. Aurass Muhi Taha -r - _ - -- r- r - Profile 1 . 1 iew of asink The Fill tool uses a variety of Spatial Analyst tools, including several of the hydrologic analysis tools discussed earlier, to create a depressionless DEM. This tool requires an input surface, a fill limit, and an output raster. When a sink is filled, it is filled to its pour point, the minimum elevation along its watershed boundary. The identification and removal of sinks when creating a depressionless DEM is an iterative process. When a sink is filled, the boundaries of the filled area may create new sinks, which then need to be filled. For a large DEM or one with many sinks, this can take minutes to hours. Finding sink depth: It is useful to know the depth of a sink or group of sinks. This information can be used to determine an appropriate z limit for the Fill tool, to understand the type of errors present in the data, and to determine if the sinks are legitimate morphological features. The following steps outline the general process to find sink depth: 1.Create a raster of sinks with values that identify their depth by running the Sink tool to locate sinks in the raster. 2.Use the Watershed tool to create a raster of the contributing area for each sink using the flow direction from the elevation raster and the output from the Sink tool as input for pour points. 3.With Map Algebra inPython, use the Zonal Statistics tool with the Minimum option to create a raster of the minimum elevation in the watershed of each sink: 69 sink_min = ZonalStatistics(sink_areas, "Value",elev_ras, "Minimum")
Dr. AurassMu:hiTaha (Quantilawe Moroh>metric Analysi<) The sink areas input is the output from the Watershed tool. 4.Create a raster containing the lowest eleval:i on a!ong the boundary of each watershed with the Zonal Fill tool (this corresponds to the elevation at which flow would leave the basin after filling to the rim): sink_max = ZonalFill(sink_area.s, elev_ra.s) 5.Use the Minus tool to subtract the minimum value from the maximum value to find the depth again: sink_depth = Minus(sink_max, sink_min) The above sequence can also be completed using the Sink, Watershed, Zonal Statistics, Zonal Fill, and Minus geoprocessing tools. How watel'Shed works: A watershed is the upslope area that contributes flow-generally water-to a common outlet as concentrated drainage. It can be part of a larger watershed and can also contain smaller watersheds, called subbasins. The boundaries between watersheds are termed drainage divides. The outlet, or pour point, is the point on the surface at which water flows out of an area It is the I owest point along the boundary of a watershed. D boundary D--.... --.... Watershed Subbasin Drainage divides Stream network Outlets (pour points) W rershed componenrs 70
