Watershed

1. CHAPTER 2
ANALYSIS OF WATERSHED
CHARACTERISTICS
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2. CHAPTER 2: ANALYSIS OF WATERSHED
CHARACTERISTICS
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 Characteristics of a watershed are broadly
divided into:
i. Biophysical characteristics
ii. Socioeconomic characteristics
2.1 Bio-physical Characteristics of a Watershed
These include:
 Watershed hydrological cycle
 Geology
 Soil characteristics
 Topography
 Geomorphology
 Climate
 land use/land cover

3. A. Watershed Hydrological Cycle
• The watershed hydrological cycle is an open
system which has a range of inputs, storages,
transfers/flows, and outputs.
• Energy from the sun and precipitation
(including rain and snow) enter the system and
water leaves it.
• The figure below illustrates the inputs, storages,
transfers, and outputs of the watershed
hydrological cycle.

5. a. Inputs – water coming into the system
• Precipitation – all forms of moisture that reach
the Earth’s surface e.g. rain, snow, sleet and hail.
b. Storage – water stored in the system
• Interception – this is when precipitation lands on
buildings, vegetation and concrete before it
reaches the soil.
• Interception storage is only temporary as it is
often quickly evaporated.
• Vegetation storage – this is water taken up by
vegetation. It is all the moisture in vegetation at
any one time.

6. • Surface storage – the total volume of water held
on the Earth’s surface in lakes, ponds and
puddles.
• Groundwater storage – the storage of water
underground in permeable rock strata.
• Channel storage -the water held in a river or
stream channel.
c. Flows and Processes – water moving from one
place to another
• Base flow – water that reaches the channel by fast
through flow and from permeable rock below the
water table forms base flow.
• Channel flow – the movement of water within
the river channel. This is also called a river’s
discharge.

7. • Groundwater flow – the deeper movement of
water through underlying permeable rock strata
below the water table.
• Limestone is highly permeable with lots of joints
and can lead to faster groundwater flow.
• Infiltration – the downward movement of water
into the soil surface.
• Interflow – water flowing downhill through
permeable rock above the water table.
• Percolation – the gravity flow of water within
soil.
• Stem flow – water running down a plant stem or
tree trunk.

8. • Surface Runoff – the movement of water over the
surface of the land, usually when the ground is
saturated or frozen or when precipitation is too
intense for infiltration to occur.
• Through flow- the movement of water down
slope within the soil layer.
• Through flow is fast through pipes (cracks in the
soil or animal burrows).
d. Outputs – water leaving the system
• Evaporation – the transformation of water
droplets into water vapor by heating.

9. • Evapo-transpiration – the loss of water from a
drainage basin into the atmosphere from the
leaves of plants + loss from evaporation.
• Transpiration – evaporation from plant leaves.
• River discharge – the amount of water that
passes a given point, in a given amount of time.

11. B. Geology
Geology refers to the bedrock underlying
an area or the type of rock or mineral
formations of the watershed.
These bedrock formations developed as a
result of geologic processes that have
operated for many years.
The geology of the watershed must be
known in order to estimate the watershed’s
hydrological reaction.
The geology of the watershed substrate
influences both the runoff and the
groundwater flow.
The main geologic characteristic is the
permeability of the soil substrate.
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12. A watershed that has an impermeable
substrate presents a faster and more violent
increase of the runoff in comparison to a
watershed with a permeable substrate.
A watershed with a permeable substrate
will provide a base run-off during dry
periods that will last longer.
Weak geology of the watershed combined
with rainfall and human activities lead to
various forms of landslide in the watershed.
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13. Other characteristics related to geology:
 Historical geology of the watershed
 Local and regional structural formations
(e.g., faults, basins, etc.)
 Types of rock groups present in the
watershed
 Stratigraphy of the rock types
 A geologic cross-section
 Near-surface geology, including
descriptions of the major soil units;
glacial and/ or depositional history
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14. C. Soil
Soil consists of material weathered in place
from the underlying parent material (i.e.,
bedrock) and mixed with organic material
near the surface.
The knowledge of soils, their physical and
chemical properties are important in
watershed management planning.
Because,
It helps in understanding the soil fertility
and productivity of a watershed.
Soil particles and their sizes are
important factors for soil erosion.
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15. The detachability and transportability of soil in
the erosion process is based on kind and size of
soil particles.
For example, the clay particles are
difficult to detach than sand but easier to
transport.
Infiltration capacity of soil play
important role in soil erosion.
When rainfall intensity exceeds the
infiltration capacity of soil, runoff or
overland flow occurs, which causes erosion.
If the infiltration capacity of soil is higher
than the intensity of rainfall, then the runoff
or overland flow will be lower and less
erosion occurs.
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16. • Soils are classified according to their infiltration
rate after prolonged wetting with all vegetation
removed. They are divided into four hydrologic
groups:
• Group A soils have low runoff potential and
high infiltration rates even when thoroughly
wetted.
• They consist chiefly of deep, well to excessively
drained sands or gravels.
• These soils have a high rate of water transmission
(greater than 0.30 in/hr).

17. • Group B soils have moderate infiltration rates
when thoroughly wetted and consist chiefly of
moderately deep to deep, moderately well to well
drained soils with moderately fine to moderately
coarse textures.
• These soils have a moderate rate of water
transmission (0.15 - 0.30 in/hr).
• Group C soils have low infiltration rates when
thoroughly wetted and consist chiefly of soils
with a layer that impedes downward movement
of water and soils with moderately fine to fine
texture.
• These soils have a low rate of water transmission
(0.05 - 0.15 in/hr).

18. • Group D soils have very high runoff potential.
• They have very low infiltration rates when
thoroughly wetted and consist chiefly of clay
soils with a high swelling potential, soils with a
permanent high water table, soils with a clay
pan or clay layer at or near the surface, and
shallow soils over nearly impervious material.
• These soils have a very low rate of water
transmission (0.0 - 0.05 in/hr).

19. D. Topography
Topography is a product of the
underlying geologic formations and the
geologic history of an area, as well as
human activities that alter the natural
landscape.
The topography or terrain of an area has
a significant influence on:
Infiltration rate
Runoff rate
Erosion rate
Sedimentation
Vegetation type
Flood storage and conveyance
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20. Natural storage of water in depressions on
the ground surface during rainfall reduces
surface runoff volume and velocity.
On relatively flat terrain, precipitation stored
in surface depressions has the opportunity to
infiltrate into the soil.
Depending on soil characteristics and
vegetative cover, the rainwater may be taken
up by plants and transpired back to the
atmosphere, flow subsurface down the slope,
and/or percolate to the groundwater.
However, as the surface slope increases, the
water storage decreases.
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21. In small watersheds on steep hilly slopes,
most surface depressions are filled to
capacity very quickly, reducing
opportunities for infiltration and increasing
overland runoff.
Increasing surface slope not only decreases
surface storage and infiltration, but also
increases the velocity of overland runoff
generated.
Increased runoff velocity means that
erosion rates increase as soil particles are
more easily detached and transported down
the slope.
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22. The topography of the watershed also
determines the character of the stream
valleys and streams themselves.
For example,
The steeper headwater areas of a watershed
tend to have steeper stream channels
confined by adjacent hilly slopes.
As a result, headwater streams generally
have more energy available to erode and
transport stream bank and streambed
materials.
They also have relatively few areas to store
flows that overtop the stream banks.
As a consequence, floodwaters are
conveyed quickly to downstream reaches.
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23.  Lowland areas (downstream of the watershed)
tend to be flatter, with broader valleys.
 unaltered streams flowing across broad, flat
valleys tend to have less energy available for
erosion and transport of materials.
 they usually have significant areas available
for storing and slowing the downstream
conveyance of floodwaters.
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24. E. Geomorphology
 Geomorphology refers to the physical features
of the surface of the earth and their relation to
its geological structures.
 It includes size, length, shape, slope, etc. of the
watershed.
 Size of a watershed reflects the volume of water
that can be generated from a rainfall.
 Length of the watershed is the distance traveled
by the surface drainage and sometimes more
appropriately labeled as hydrologic length.
 Shape of a watershed reflects the way that
runoff will be collected at the outlet.
 Slope of a watershed affects the force of runoff.
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25. i. Size of a Watershed
• A large watershed takes longer time for draining
the runoff to the outlet than smaller watershed
and vise-versa.
a. Drainage area-watershed area
• The drainage area/watershed area is the single
most important factor affecting the magnitude of
peak flows.
• Watershed area is used to indicate the potential
for rainfall to provide a volume of runoff,
whereas length of watershed indicates the time
taken by runoff through watershed.

26. • Accordingly, large watershed area indicates high
volume of runoff and long watershed indicates low
volume of runoff.
• In general, a large watershed area implies a large peak
flow; however, human activities like urbanization can
modify this behavior.
Factors responsive to watershed size:
• Overland flow is more in small watersheds as
there is less network of drainage systems while in
large watersheds channel flow is dominant
• Sheet and rill erosion is dominant in small
watersheds while in large watersheds gully
erosion could be more significant
• Channel/Stream storage is more significant in
larger watersheds

27. • Development of erosion: sheet erosion, rill
erosion, gully/channel erosion, stream flow
b. Channel/stream Length
• The effective length of a channel depends on flow
magnitude.
• Large flows overtop the banks and fill the
floodplain whose length is usually shorter than
that of the meandering streambed.
• A long drainage channel usually indicates a long
runoff removal time.
• Therefore, longer channels cause a response to
rainfall slower than for shorter channels.

28. c. Shape of a Watershed
• The shape of the watershed has an effect on the
rate of runoff.
• The rate of runoff will be lower for a long narrow
watershed than for a fan-shaped watershed.
• Watersheds have an infinite variety of shapes,
and the shape supposedly reflects the way that
runoff will “bunch up” (gathered) at the outlet.
• A circular watershed would result in runoff from
various parts of the watershed reaching the outlet
at the same time.
• Long and narrow watersheds are likely to have
longer time of concentration, resulting in lower
runoff rates than broad and compact watersheds
of the same size.

29. Example:
i. Fan-shape- shape looks like part of a circle
[tends to produce higher runoff very early]
ii. Fern shape- shape looks like feathers of birds
[tends to produce less runoff]
• In general terms, in more compact watershed, the
runoff hydrograph is expected to be sharper with
a greater peak and shorter duration.
• For a watershed that is partly long and narrow
and partly compact, the runoff hydrograph is
expected to be a complex composite of the above
mentioned hydrographs

30. ii. Slope of a Watershed
• The principal effect of land slope is on the rate of
runoff.
• Runoff will flow faster on a steeper slope.
• This results in higher peaks at downstream
locations.
• The effect of land slope on the volume is usually
minor.
• Slope determines the flood magnitude and
speed: naturally, the steeper the slope of a field,
the greater the speed of runoff.
• Soil erosion by water also increases as the slope
length increases due to the greater accumulation
of runoff.

31. Slope of the line (in mathematics)

33. Calculating slope from a contour map

36. • The slope is obtained by dividing the rise over run.
• Multiply this ratio by 100 to express slope as a
percentage.
• The slope angle expressed in degrees is found by
taking the arctangent of the ratio between rise and
run.
• Here we want to find the average slope of the face of
this mountain (the section from point A to point B).
• The vertical distance or rise is the elevation difference
between point A and point B.
• Checking the topo map below Point A is at 2500m.
• Contour interval is 20m (five contour lines per 100m
elevation difference).
• Therefore, elevation of point B is 2780m.
Rise = 2780m – 2500m = 280m.

37. • The run or the horizontal distance between two points
is found by using the map's scale bar.
• Using a ruler we can measure the scale bar of a Google
Map at bottom left corner.
• 17mm or 1.7cm on the map is equal to 100m in the real
world.
• Again using a ruler, the next step is measuring the
horizontal distance between point A and point B on
the map: 42mm or 4.2cm.
• Calculating the real world distance: Run = 4.2cm *
(100m / 1.7cm) = 247m.
• (Note that the numbers corresponding to
measurements on the image may be different on your
computer monitor due to resolution difference or
when the image is printed. The end result however
should be the same).

38. • Gradient (decimal) = Rise / Run = 280m / 247m
= 1.1336
Here, for every 1 unit (e.g. meter, foot, etc.) of
horizontal travel, there is 1.1336 units of altitude
gain or vertical increase.
• Alternatively for every 0.882 unit horizontal
travel, there is one unit of vertical gain.
• Therefore, as a ratio, the gradient would be
expressed as (1 in 0.882).
Gradient (percentage) = 1.1336 * 100 = 113.4%
• Slope angle is the angle α in the diagram.
• By definition of tangent in trigonometry:
tan α = Rise / Run

39. Slope classification on the basis of % values
Code Class Description
A Little or none Little or none slope: 0-3%
gradient
B Gentle Gentle slopes: 4-9% gradient
C Moderate Moderate slopes: 10-15%
gradient
D Steep Steep slopes: 16-30% gradient
E Extremely steep Extremely steep slopes: 31-60%
gradient
F Excessively steep Excessively steep slopes: > 60%

40. Slope angle classification
• Slope angles are classified on the basis of
geomorphological parameters.
• Although the continuous variables of slope angle
are arbitrary, but it delineates the micro units of
landform.
• For the hydrological purpose, the slope angles are
best determiner of sediment load transported by
a stream.
• Taking all these into consideration, the slope
angles are divided into five categories

41. Slope gradient classification on the basis of angle classes
Angle class Description
< 12° Gentle slope
12-22° Moderate
23-31° Moderately steep slope
32-45° Steep slope
> 45° Very steep slope

42. • Some slope features are important in field studies
like geomorphology, avalanches and backcountry
travel decision making.
• Examples include convex and concave slopes.
• Convex slopes roll from less steep (gentler) to
steeper terrain.
• Depending on the contour interval and the size of
the feature, convexities on terrain may be detected
by wider contour spacing on top and closer
contour lines on the bottom of the roll.
• Concave slopes go from steeper to gentler terrain
with movement down slope.
• There are closer contour spacings at the top and
wider spacings at the bottom indicating steeper
and gentler slopes, respectively.

45. • In watershed rehabilitation and management, it
is important to determine how much proportion
of the watershed lies within each class of the
slope gradient.

46. iii. Roughness
• Roughness affects the velocity of overland flow
and stream flow.
• A rough channel will cause smaller peaks than a
smooth channel.
• For a given discharge, stage levels (water surface
elevations) in a stream are higher for rough
channels.
iv. Stream Order
• Stream order is a measure of the degree of
branching of streams within a Watershed.
• First order streams are defined as those channels
that have no tributaries.

47. • In this case, the flow is depended entirely on
surface overland flow to them.
• The junction of two first order stream forms a
second order channel.
• Please note that when a low order stream
segment joins the higher order stream segment,
then the order of the stream remained as it is
• Second order channel receives flow from the two
first order channels that form it and from
overland flow from the ground surface and
might receive flow from another first order
channel that flow directly in to it.

48. • Third order channel is formed by the junction of
two second order channels.
• It receives flow not only from the two second
order channels that form it, but also direct
overland flow and possibly from first order
channels that flow directly into it and possibly
from other second order stream that might join
it.
• In general, an nth order stream is a tributary
formed by two or more streams of order (n-1)
and streams of lower order.
• Numerical ordering begins with the tributaries
at the streams headwaters being assigned the
value one.

49. Figure 1: Stream/channel and basin Order

50. Stream/channel and basin Order
• The bifurcation ratio (Rb) is defined as the ratio of
the number of streams of any order to the number
of streams of the next higher order.
• The bifurcation ratio is calculated as:
Rb = Ni/Ni+1
• Values of Rb typically range from the theoretical
minimum of 2 to around 6.
• The bifurcation ratio of a whole watershed is the
average of the bifurcation ratios of each stream
order
• The lower the value of bifurcation ratio, the flatter
or rolling the drainage basin/watershed is.
• It is also suggestive that the area is underlain by a
homogenous rock
• The higher the value of bifurcation ratio, the
steeper and more dissected the drainage basin.

51. Figure 2: Example of Bifurcation Ration

52. v. Drainage Density
• The drainage density is a measure of the total
length of well defined channels/streams that
drain the watershed (sometimes measured as the
blue lines representing the streams on a
topographic map).
 Drainage density affects the response of the
watershed to rainfall.
 High densities usually allow fast runoff removal.
 Mathematically, drainage density is defined as the
sum of the lengths of all the streams (in km or
miles) divided by the total watershed area (km2 or
mile2).

53. Where,
–Dd is drainage density (km km-2)
–L is length of stream segment (km)
–A is basin area/watershed area
(km2) or (mi²)
• This ratio can be determined from
topographic maps.
• A high drainage density reflects
highly dissected basins, and relatively
rapid response to a rainfall input,
while low drainage density reflects a
poorly drained basin with low
hydraulic response.

54.  Therefore, greater peaks and hydrographs with
shorter durations are expected for watersheds
with higher drainage densities.
 The effect of drainage density on runoff volume
is associated with the time during which the
runoff remains in the watershed.
 Low densities allow for long residence times;
therefore, abstraction mechanisms have more
time to remove water.

55. • Stream density – also known as stream
frequency over the basin, and it is expressed as
the ratio of the total number of streams to the
area of the basin.
• Stream density (Sd) = No. of streams/Basin
Area.
• It is also possible to calculate Sd of first order
streams over the watershed, as
• Sd1 = No. of 1st order streams/basin Area

56. vi. Drainage Patterns
This refers to the arrangement of streams in a
drainage, which often reflects structural and/ or
lithological control of underlying rocks.
–Drainage patterns tell much about the
substance of which the land surface is made
• The drainage pattern of an area is the outcome of
– the geological processes,
– nature and structure of rocks,
– topography,
– amount of flow
– periodicity of the flow

57. Some examples of drainage pattern: dendritic,
parallel, rectangular, radial, centripetal, …
a. Dendritic drainage pattern
–Develops in area where the type of rocks remain
the same all over the basin and where no
geological processes, like folding or faulting
have created structures that would control the
development of river system
–Weak rock structure usually form dendritic
drainage pattern
–Dendritic drainage pattern is characterized by
the fact that tributaries flow in the same
direction as the main stream, joining at an acute
angle

58. b. Radial drainage pattern
- it is made up of a pattern of stream flowing
outward, down the slopes of a dome or cone-
shaped upland
c. Rectangular drainage pattern
- The rectangular pattern is found in regions that
have undergone faulting.
- Movements of the surface due to faulting offset
the direction of the stream.
- As a result, the tributary streams make sharp
bends and enter the main stream at high angle
d. Trellis drainage pattern
- It develops in area where softer and harder rocks
alternate with one another or where folding and
faulting results in the formation of structures that
control the development of river system

59. Figure 3: Dendritic Drainage pattern

60. Figure 4: Radial drainage pattern

61. Figure 5: Rectangular drainage pattern

62. Figure 6: Trellis drainage patterns

63. F. Climate
Climate refers to the prevailing weather
conditions in an area for long period of
time, which affects the flow, pattern, and
shape of streams.
Climate influences:
Amount and type of precipitation
Timing of runoff
Evaporation rate
Vegetation type
Erosion rate
Groundwater recharge rate
Climate influences the amount and seasonal
distribution of precipitation and thereby determines
other processes. 63

64. Rainfall and temperature play crucial
roles in watershed condition.
There is direct relationship between the
amount of rainfall and erosion in a
watershed.
Rainfall intensity influences both the rate
and volume of runoff and then the scale
of erosion.
Temperature affects climatic type, which
governs the types of crop grown and the
amount of ground cover that exists in
watershed.
Temperature is important in producing
desired level of ground cover to protect
soil from erosion and landslides in the
watershed.
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65. Climate also affects stream flow and
sediment by influencing the type and
density of vegetation in a watershed.
In addition, climate has a significant effect
on the chemical characteristics of streams.
The chemical composition of streams
derives from atmospheric, soil and rock
sources.
Chemical and physical weathering of rock
and soil contribute the greatest proportion
of dissolved and suspended material to
natural stream systems.
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66. G. Land use/ land cover
Land cover refers to the types of
vegetation found in an area.
A related factor is land use, which refers
to the types of activities which people
conduct on a given land area.
Land use and land cover are major
factors controlling the volume and rate of
runoff from a watershed, soil erosion and
sediment loadings, the stability of
valley/hilly slopes, stream channel
morphology, and overall water quality.
The location and intensity of a particular
land use activity will determine its effect
on the watershed.
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67. Interception on leaves, stems, and surface
litter of vegetation allows water to be
retained during smaller storms,
evaporating back into the atmosphere
without ever reaching the ground.
It also reduces the impact of raindrops by
preventing the detachment of soil particles.
Vegetation increases infiltration by
preserving loose soil structure and
scattering the flow of water.
Vegetation also interrupts overland flow,
slowing the velocity, physically binding the
soil and inhibiting erosion.
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68. 2.2 Socioeconomic Characteristics of a Watershed
This includes the social, cultural and economic
condition of the watershed community.
a. Socio-cultural factors
The information on the people's social and
cultural norms and activities should be collected
and analyzed for watershed management.
A planner must carefully collect and study socio-
cultural information before making any
recommendations for drastic change.
Population growth in a watershed results
degradation of watershed and its environment.
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69. Population growth results in deficit of
food, fodder, wood and land.
This will result in cultivation of marginal
land, over grazing, over cutting and
removal of trees.
The consequence of such activities result
in land degradation like soil erosion/
watershed/ environmental degradation.
Cultural information of watershed is
equally important for the development of
watershed management.
Because, watershed management programs
can bring cultural transformation in the
society. 69

70. To consider local culture in planning is to
minimize possible resistance in future
implementation.
Farmers are relatively conservative.
Any improvement which is compatible with the
local culture and with a gradual path will have
better potential for success.
b. Economic factors
Among other factors, economic factors also
play crucial role in soil conservation and
watershed management.
Soil conservation and watershed management
programs need investment.
Poor farmers cannot invest in soil and water
conservation because of low level of economy.
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