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