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CE 3205 Water and Environmental
Engineering
Watershed and Introduction to Precipitation
Mdm. Norhidayah Rasin
Hydrologic Cycle
Rain Clouds
Precipitation
Cloud Formation
Evaporation
VegetationStream
Soil
Ocean
Transpiration
Infiltration
Percolation
Groundwater Flow
RunoffStorage
Water moves throughout the Earth by different pathways and at different rates
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
• Catchment
• Catchment area
• Catchment basin
• Drainage area
• River basin
• Water basin
• Watershed
Drainage Patterns/Networks
Dendritic Parallel Trellis
Rectangular Radial Centripetal
Deranged
Watershed Characteristics
Size
Slope
Shape
Soil type
Storage capacity
Land use / cover
Reservoir
Divide
Natural stream
Urban
Concrete channel
Basin sizeBasin size
• Delineate watershed according to the height of Delineate watershed according to the height of land that separates water draining to the point of land that separates water draining to the point of interest from water that drains to adjacent basinsinterest from water that drains to adjacent basins
• Watershed area (kmWatershed area (km22, ha), ha)
– smaller watersheds tend to have a more peaked smaller watersheds tend to have a more peaked hydrograph, more intermittent water supplyhydrograph, more intermittent water supply
– larger watersheds have flatter hydrographs larger watersheds have flatter hydrographs because larger channel network can store more because larger channel network can store more waterwater
Watershed Land SlopeWatershed Land Slope• The slope of the sides of a watershed govern The slope of the sides of a watershed govern
how fast water will drain to the channelhow fast water will drain to the channel• steep slopes - peaked hydrographsteep slopes - peaked hydrograph
• gentle slopes - flat hydrographgentle slopes - flat hydrograph
• slope is vertical over horizontal distance, derived slope is vertical over horizontal distance, derived from topographic mapsfrom topographic maps
• An objective repeatable formula for land slope:An objective repeatable formula for land slope:
A(L)(CI)
S where L is the total length of contours, CI is the contour interval and A is the watershed area.
Strahler’s Order of StreamsStrahler’s Order of Streams
• A headwater stream with no A headwater stream with no tributaries is a first order tributaries is a first order streamstream
• When two first order streams When two first order streams join they form a second order join they form a second order streamstream
• Two second order streams Two second order streams form a third order stream etc.form a third order stream etc.
1
12
21
1 1
2 1
123
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Bifurcation Ratio (RBifurcation Ratio (RBB) )
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 1 -
Watershed Delineation
Upper Bernam Basin
Upper Bernam River Basin
Catchment Surface
SKC Bridge
GIS
Outlet
DEM Data
Typical Drainage Basin – Langat River Basin
SubWatershed – Bagan and Sat Rivers
Sg. Bagan
Sg. Sat
Watershed Delineation
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Draw Sub-watersheds
Stream Networks
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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 precipitation.
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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
Total Rainfall amount = 6.17 cm
0
2
4
6
8
10
12
14
0 20 40 60 80 100 120 140
Time, min
Rain
fall
Inte
nsi
ty, cm
/hr
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Long term Precipitation variation at Arba Minch
0
5
10
15
20
25
30
35
40
45
1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006
Years
Ann
ual r
ainf
all,
mm
Annual Precipitationaverage 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…
Precipitation gauge1 - pole2 - collector3 - support- galvanized metal sheet 4 – funnel5 - steel ring
1. Non recording gauge
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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 needle5-drum with diagram6-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 baskets3 - electric signal4 - 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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ENDTHANK 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