1) A watershed is an area of land that drains rainwater to a common outlet such as a river, lake, or ocean. Watersheds can be classified based on size, land use, slope, and other characteristics. 2) Watersheds are commonly classified as small (<250 km2), medium (250-2500 km2), or large (>2500 km2) based on size. Land use is another important classification criterion. 3) Runoff generation in watersheds occurs in two phases: overland flow across the land and through channel networks. Small watersheds have a dominant land phase while large watersheds have well-developed channel networks.

1. Geomorphology of Watershed

2.  Watershed is an isolated area with a well demarcated boundary line,
draining the rainwater to a single outlet.
 A watershed having the measured hydrological parameters such as rainfall,
runoff and others is called gauged watershed
 Other terms synonymous with drainage basin are watershed, catchment,
basin, river basin, runoff area, and stream basin.
 Watershed, catchment and basin are most commonly used terms by
hydrologists.

3. Classification of Watersheds
Watersheds can be classified based on size, mean slope, length, land
use, etc.
Two hydrologically meaningful criteria are size and land use.
Runoff generation on these watersheds can be considered in
two phases: i) land phase and ii) channel phase.
Classification of Watersheds by Size
1. Small size: < 250 km2
2. Medium size: between 250 km2- 2500 km2
3. Large: >2500 km2

4. Classification of Watersheds
Watersheds can be classified based on size, mean slope, length, land
use, etc.
Two hydrologically meaningful criteria are size and land use.
Runoff generation on these watersheds can be considered in
two phases: i) land phase and ii) channel phase.
Classification of Watersheds by Size
1. Small size: < 250 km2
2. Medium size: between 250 km2- 2500 km2
3. Large: >250 km2

5. Classification of Watersheds
Large Watersheds
1) They have well-developed channel networks and channel phase, and, thus,
channel storage is dominant.
2) They are less sensitive to high-intensity rainfalls of short duration.
Small Watersheds
1) They have dominant land phase and overland flow, have relatively less
conspicuous channel phase.
2) They are highly sensitive to high-intensity, short-duration rainfalls.
Two watersheds of the same size may behave very differently if they do not
have similar land and channel phases.
 Small watersheds are usually least heterogeneous and large watersheds are
most heterogeneous. In other words, spatial variability of watershed
characteristics increases with size.

6. Classification of Watersheds
Lee (1980) has reported that flow rates per unit area, Q (m3/s-ha, or depth/time),
generally follow the relationship
Where
k is empirical constant, and x< 1 for peak flows (typically x =
0.8); x> 1 for low flows (typically x = 1.2), and x 1 for average
discharge

7. Classification of Watersheds
Watersheds are also classified into different categories based on area
that the watershed contains

8. Classification of Watersheds by Land Use
Land use defines exploitation of watershed. Accordingly,
watersheds can be classified as
1) Urban watersheds: urban hydrology
2) Agricultural watersheds: agricultural hydrology
3) Forest watersheds: forest hydrology
4) Mountainous watersheds: mountain hydrology
5) Desert watersheds: desert hydrology
6) Coastal watersheds: coastal hydrology
7) Wetland/marsh watersheds: wetland hydrology

9. Watershed Morphology-
 It includes overall
surface characteristics
of watershed including
the stream network
comprising the
 Stream ordering
 Stream length
 Stream slope
 Areal aspect
 relief aspects etc.
 Related to watershed
morphology there are various
laws defining the features of
watershed; they are the
 Law of steam order
 Law of stream length
 Law of stream slope
 Law of stream area etc.

10. Watershed Morphology-
Stream order : An approach for categorizing the
streams into different orders as per their sequence
of their origin. This also provides a basis for
dividing the entire area of watershed for grouping,
stream wise.
Rule-1: The finger tips like streams are taken first
for ordering. These are provided stream order –Ist.
Rule-2: When two same order streams join
together the resulting steam will be of next higher
order. Say for example, if Stream A and B of Ist
order join together the order of resulting stream C
will be 2nd order.
Rule-3: If a lower order stream joins to a higher
order stream the order of resulting stream will be
the same, i.e., remain the higher order as it is.

11. Watershed Morphology-
Law of Stream Number It states that the number of stream of each order
forms an inverse geometric sequence with the stream order number,
expressed as,
Where, Nu = Number of steam of order u.
K = trunk order of stream segment in the watershed.
Rb = Bifurcation ratio, defined as the ratio of number of stream segments
of a given order u to the number of stream segment of next higher order,
expressed as under,

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16. Law of Stream Length
The law of stream length states that the
mean length of stream segment of
successive order basin approximates a
direct geometric sequence represented by
the following expression
In which
𝐿̅ 𝑢 is the mean length of stream of order u;
𝐿̅ 1 is the mean length of stream of order 1;
Rl is the length ratio and
K is the trunk order of the stream
Length ratio