Transcript Slide 1

CE 3205
Water and Environmental
Engineering
Watershed and Introduction to
Precipitation
Mdm. Norhidayah Rasin
Hydrologic Cycle
Water moves
throughout
the Earth by
different
pathways and
at different
rates
Cloud Formation
Rain Clouds
Evaporation
Precipitation
Soil
Storage
Runoff
Transpiration
Stream
Vegetation
Infiltration
Percolation
Ocean
Groundwater Flow
Hydrologic Cycle
The Watershed or Basin
• Area of land that drains water, sediments and
dissolved materials along a stream channel to a
single outlet and is separated from other
watersheds by a drainage divide.
• Rainfall that falls in a watershed will generate
runoff to that watershed outlet.
• Topographic elevation is used to define a
watershed boundary
Outlet
Watershed
• Watershed – Area of land
draining into a stream at a
given location
• Streamflow – Gravity
movement of water in
channels
– Surface and subsurface flow
– Affected by climate, land
cover, soil type, etc.
Watershed – Hydrologic System
Watershed/Drainage Basin Terms
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Catchment
Catchment area
Catchment basin
Drainage area
River basin
Water basin
Watershed
Drainage Patterns/Networks
Dendritic
Rectangular
Parallel
Radial
Trellis
Centripetal
Deranged
Watershed Characteristics
Divide

Size
 Slope
 Shape
Reservoir
Natural
stream
Urban
 Soil type
 Storage capacity
 Land use / cover
Concrete
channel
Basin size
• Delineate watershed according to the height of
land that separates water draining to the point of
interest from water that drains to adjacent basins
• Watershed area (km2, ha)
– smaller watersheds tend to have a more
peaked hydrograph, more intermittent water
supply
– larger watersheds have flatter hydrographs
because larger channel network can store
more water
Watershed Land Slope
• The slope of the sides of a watershed govern
how fast water will drain to the channel
• steep slopes - peaked hydrograph
• gentle slopes - flat hydrograph
• slope is vertical over horizontal distance, derived
•
from topographic maps
An objective repeatable formula for land slope:
(L)(CI)
S
A
where L is the total length of contours,
CI is the contour interval and A is the
watershed area.
Strahler’s Order of Streams
• A headwater stream with no
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tributaries is a first order
stream
When two first order streams
join they form a second order
stream
Two second order streams
form a third order stream etc.
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2
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2
2
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2
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3
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Bifurcation Ratio (RB)
The ratio of the number of stream segments of a
given order, Nn, to the number of segments of the
next highest order, Nn+1, is called the bifurcation
ratio, RB:
**Bifurcation-splitting of a main body into two parts
Bifurcation Ratio - Example
Stream
order
No.of stream
order
Bifurcation
ratio
1
17
17/6
2
6
6/2
3
2
2/1
4
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Watershed Delineation
Catchment Surface
DEM Data
GIS
SKC Bridge
Upper Bernam
Basin
Outlet
Upper Bernam River Basin
Typical Drainage Basin – Langat River
Basin
SubWatershed – Bagan and Sat Rivers
Sg. Bagan
Sg. Sat
Watershed Delineation
Draw Sub-watersheds
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Stream Networks
Precipitation
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Introduction
• All forms of water that reach the earth from the
atmosphere is called Precipitation.
• The usual forms are rainfall, snowfall, frost, hail,
dew. Of all these, the first two contribute
significant amounts of water.
• Rainfall being the predominant form of
precipitation causing stream flow, especially the
flood flow in majority of rivers. Thus, in this
context, rainfall is used synonymously with
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precipitation.
Introduction….
• In nature water is present in three
aggregation states:
– solid: snow and ice;
– liquid: pure water and solutions;
– gaseous: vapors under different grades of
pressure and saturation
• The water exists in the atmosphere in these
three aggregation states.
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Introduction….
• Types of precipitation
– Rain, snow, hail, drizzle, glaze, sleet
• Rain:
– Is precipitation in the form of water drops of size
larger than 0.5 mm to 6mm
– The rainfall is classified in to
• Light rain – if intensity is trace to 2.5 mm/h
• Moderate – if intensity is 2.5 mm/hr to 7.5 mm/hr
• Heavy rain – above 7.5 mm/hr
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Introduction….
• Snow:
– Snow is formed from ice crystal masses, which
usually combine to form flakes
• Hail (violent thunderstorm)
– precipitation in the form of small balls or lumps
usually consisting of concentric layers of clear
ice and compact snow.
– Hail varies from 0.5 to 5 cm in diameter and can
be damaging crops and small buildings.
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Temporal and Spatial Variation of Rainfall
• Rainfall varies greatly both in time
and space
– With respect to time – temporal variation
– With space – Spatial variation
• The temporal variation may be defined as
hourly, daily, monthly, seasonal variations
and annual variation (long-term variation of
precipitation)
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Temporal Variation of rainfall at a particular site
Rainfall Intensity, cm/hr
Total Rainfall amount = 6.17 cm
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12
10
8
6
4
2
0
0
20
40
60
80
100
120
140
Time, min
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Long term Precipitation variation at Arba Minch
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Annual rainfall, mm
40
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30
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20
15
10
5
0
1986
1988
1990
1992
1994
1996
Years
1998
2000
2002
2004
2006
Annual Precipitation
average precipitation
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Measurement of Rainfall
• Rainfall and other forms of precipitation are
measured in terms of depth, the values being
expressed in millimeters.
• One millimeter of precipitation represents the
quantity of water needed to cover the land with a
1mm layer of water, taking into account that
nothing is lost through drainage, evaporation or
absorption.
• Instrument used to collect and measure the
precipitation is called rain gauge.
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Rainfall measurement…
1. Non recording gauge
Precipitation gauge
1 - pole
2 - collector
3 - support- galvanized metal
sheet
4 – funnel
5 - steel ring
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2. Recording gauge / graphic rain gauge
• The instrument records the graphical variation of
the fallen precipitation, the total fallen quantity in a
certain time interval and the intensity of the rainfall
(mm/hour).
• It allows continuous measurement of the rainfall.
The graphic rain gauge
1-receiver
2-floater
3-siphon
4-recording needle
5-drum with diagram
6-clock mechanism
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3. Tele-rain gauge with tilting baskets
• The tele-rain gauge is used to transmit measurements of
precipitation through electric or radio signals.
• The sensor device consists of a system with two tilting
baskets, which fill alternatively with water from the collecting
funnel, establishing the electric contact.
• The number of tilting is proportional to the quantity of
precipitation, hp
The tele-rain-gauge
1 - collecting funnel
2 - tilting baskets
3 - electric signal
4 - evacuation
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4. Radar measurement of rainfall
• The meteorological radar is the powerful
instrument for measuring the area extent,
location and movement of rainstorm.
• The amount of rainfall overlarge area can be
determined through the radar with a good degree
of accuracy
• The radar emits a regular succession of pulse of
electromagnetic radiation in a narrow beam so
that when the raindrops intercept a radar beam,
its intensity can easily be known.
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Rain gauge Network
• Since the catching area of the rain gauge is very
small as compared to the areal extent of the
storm, to get representative picture of a storm
over a catchment the number of rain gauges
should be as large as possible, i.e. the catchment
area per gauge should be small.
• There are several factors to be considered to
restrict the number of gauge:
– Like economic considerations to a large extent
– Topographic & accessibility to some extent.
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Raingauge Network…..
• World Meteorological Organization (WMO) recommendation:
– In flat regions of temperate, Mediterranean and tropical
zones
• Ideal  1 station for 600 – 900 km2
• Acceptable 1 station for 900 – 3000 km2
– In mountainous regions of temperate , Mediterranean and
tropical zones
• Ideal  1 station for 100 – 250 km2
• Acceptable  1 station for 250 – 1000 km2
– In arid and polar zone
• 1 station for 1500 – 10,000 km2
• 10 % of the rain gauges should be self recording to
know the intensity of the rainfall
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END
THANK YOU
Next topics to be continued..
-Preparation data
-Estimation of missing data
-Test for consistency record
-Mean Precipitation over an area
•arithmetic mean
•the method of the Thiessen polygons
•the isohyets method
-Runoff estimation
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