Module 1_Hydraulic Cycle, Rainfall & Runoff Process 2016

8/18/2019 Module 1_Hydraulic Cycle, Rainfall & Runoff Process 2016

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Module

The Hydrologic Cycle

and

Rainfall Runoff Processes

Ir Dr Kelvin Kuok

Faculty of Engineering, Computing & Science

Room E613

[email protected]

CVE 30001 – Urban Water Resource

2016


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Hydrology

Applied Hydrology Hydrologic cycle

Hydrology Component

Effect of urbanisation

All about water Inventory of water

Global Distribution

Renewal

Rainfall Runoff Process

Rainfall Intensity

Rainfall intensity calculation methods

Outline


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What is Engineering Hydrology?

Hydrology is the science of water.

It is the study of the

occurrence

character

movement

of water within and between the physical and biological

components of the environment.


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Applied Hydrology

The practical application of hydrology

Provide guidance for planning and management of water resource.

Found in such tasks as

Design and operation of hydraulic structures

Water supply

Wastewater treatment and disposal

Irrigation

Drainage

Hydropower generation

Flood control etc


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Hydraulics Structure


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Dam (water supply & hydroelectric)


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Irrigation


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Hydrological Cycle

An orderly scheme to systematically examine and analyze the

movement of water through the landscape

The hydrologic cycle is the central focus of hydrology.

The cycle has no beginning or end.

Its processes occur continuously


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Hydrologic cycle


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Evaporation-moisture passes into atmosphere as vapor.

Surface runoff – runoff travels over the soil surface to the

nearest stream channel.

Percolation – passage of water under hydrostatic pressure

through intersectics of a soil and rock.

Infiltration- passage of water through surface of soil.


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Hydrologic Cycle - Animation


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Evaporation

The conversion of water from a liquid into a gas

Water is transferred from the surface to the atmosphere through

evaporation, the process by which water changes from a liquid to a gas.


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Precipitation

Precipitation is the primarymechanism for transporting waterfrom the atmosphere to the surface

of the earth. There are several formsof precipitation, the most commonof which is rain. Other forms of

precipitation include; hail, snow,sleet, and freezing rain. A well

developed extra-tropical cyclone could be responsible for thegeneration of any or all of theseforms of precipitation.

Transfer of water from the atmosphere back to earth

http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/rain_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/hail_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/snow_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/sleet_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/freeze_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/cyclone.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/cyclone.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/freeze_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/sleet_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/snow_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/hail_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/rain_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1


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Hail – frozen form of precipitation, associated with

thunderstorm.

Sleet – wintry precipitation, snows partially melted on its

way to ground.

Tropical cyclone: a storm system characterized by a large

low pressure centre and numerous thunderstorms that

produced strong winds and heavy winds.


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Transpiration

Transfer of water from plants to the atmosphere

Transpiration is theevaporation of water intothe atmosphere from theleaves and stems of plants.

Transpiration accounts forapproximately 10% of allevaporating water.

http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/evaporation.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/evaporation.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1


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Groundwater

Water that has penetrated the earth's surface

Groundwater is all the water that haspenetrated the earth's surface and isfound in one of two soil layers. The onenearest the surface is the "zone ofaeration", where gaps between soil are

filled with both air and water. Belowthis layer is the "zone of saturation",where the gaps are filled with water.

The water table is the boundarybetween these two layers. As the

amount of groundwater water

increases or decreases, the water tablerises or falls accordingly. When the

entire area below the ground issaturated, flooding occurs because allsubsequent precipitation is forced toremain on the surface.

http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/1997_flood.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/1997_flood.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1


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Runoff

Transfer of landwater to the oceans

Runoff is the movement oflandwater to the oceans, chiefly inthe form of rivers, lakes, andstreams. Runoff consists of

precipitation that neither evaporates,transpires nor penetrates thesurface to become groundwater.Even the smallest streams areconnected to larger rivers that carry

billions of gallons of water intooceans worldwide.

http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/precip_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/evaporation.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/transpiration.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/groundwater.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/groundwater.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/transpiration.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/evaporation.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1http://ww2010.atmos.uiuc.edu/(Gh)/wwhlpr/precip_hyd.rxml?hret=/guides/mtr/hyd/smry.rxml&prv=1


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Hydrology Component

Precipitation Evaporation

Interception

Transpiration

Overland Flow Surface Runoff Runoff to Streams

and Ocean

Infiltration Subsurface Flow

Groundwater

RechargeGroundwater Flow

A t m o s p h e r i c W a t e r

S u b s u r f a c e W a t e r

S u r f a c e W a t e r

+

+

+

+

+

+

+


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Infiltration

Evaporation

Runoff

Rainfall

HydrologicData

Rainfall-Runoff Model

-Double Mass Curve Analysis-Arial Rainfall

-Runoff

-Flow Duration Curve

-Hydrograph/UnitHydrograph

-Infiltration Analysis

Evapotranspiration

-Evaporation Analysis

Hydrologic Component


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System Concept

Precipitation, I(t)

Streamflow, Q(t)

System BoundaryWatershed Surface

Watershed Divide


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Process(Basin)

Input

(Rainfall)

Output

(Runoff)

Simple Hydrologic System Model

Unsteady Flow Equation ; I-Q=dS/dt

I = Input (volume/time)O= Output (volume/time)

dS/dt = Time rate of change of storage

Basic Equation of Hydrologic Cycle


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All about Water

Approximately 97% of all the water on the Earth is in the oceans.

The other 3% is held as freshwater in glaciers and icecaps,groundwater, lakes, soil, the atmosphere, and within life.

The oceans supply most of the evaporated water found in theatmosphere.

Only 91% of it is returned to the ocean basins by way of precipitation

The remaining 9% is transported to areas over landmasses whereclimatological factors induce the formation of precipitation. Theresulting imbalance between rates of evaporation and precipitation

over land and ocean is corrected by runoff and groundwater flow tothe oceans.

Water is continually cycled between its various reservoirs.


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Inventory of water at the earth’s surface

Reservoir Volume (cubic kmx 1,000,000) Percent of Total

Oceans 1370 97.25

Ice Caps and Glaciers 29 2.05

Groundwater 9.5 0.68

Lakes 0.125 0.01

Soil Moisture 0.065 0.005

Atmosphere 0.013 0.001

Streams and Rivers 0.0017 0.0001

Biosphere 0.0006 0.00004

Source: Pidwirny, M. (2006)


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Global Distribution of Water

60% of river water in Amazon and Congo

about 8% of Canada is lake (largest area of lake in the

world)

global precipitable water vapour = 25 mm; global average

annual ppt. = 1000 mm, therefore, atmospheric water is

completely recycled 40X per year or every 9 days


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Renewal of Water

On average water is renewed in rivers once every 16 days

Water in the atmosphere is completely replaced once every 8

days

Slower rates of replacement occur in large lakes, glaciers, ocean

bodies and groundwaterSome of these resources (especially groundwater) are being used

by humans at rates that far exceed their renewal times. This type

of resource use is making this type of water effectively

nonrenewable


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Typical residence times of water found in various reservoirs

Table 8b-2:.Reservoir Average Residence Time

Glaciers 20 to 100 years

Seasonal Snow Cover 2 to 6 months

Soil Moisture 1 to 2 months

Groundwater: Shallow 100 to 200 years

Groundwater: Deep 10,000 years

Lakes 50 to 100 years

Rivers 2 to 6 months

Source: Pidwirny, M. (2006)


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Effect of urbanisation to Hydrologic Cycle

Change in Land Use

Eg: Tree removals, Building ofhouses, Filling in of farm ponds,

Building of roads, Diverting

streams to supply water for

people, Discharging of sewage

into streams, Building of industrialbuildings etc

Effect on water system

More storm runoff and erosion

More sediment is washed into streams

Flooding can occur as water-drainage

patterns are changed

Harms the water quality of the streams

Increased sewage in streams causes

pollution -- it can kill fish and make

water unusable for other purposes

downstream

More pavement means less water will

soak into the ground, hence the

underground water table will have less

water to recharge it. Thus will lower the

water table


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Fixing the Problem

Change in Land Use

Improvements in the storm

drainage system

Wells are drilled to recharge

underground aquifers

Reuse wastewater

Ecological-designed recharge

ponds disperse some storm

drainage to artificially recharge

shallow aquifers

Effect on Water System

New storm-drainage systems

reduce flooding during storms.

Water is actually injected into

recharge wells to put water back

into underground aquifers Reusing wastewater means less

pollution, more water

conservation, and additional water

for recharging aquifers


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Average Annual Precipitation


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Relationship between rainfall, infilt ration and runoff

Water reaching the ground surface infiltrates into the soil until it

reaches a stage where the rate of rainfall (intensity) exceeds theinfiltration capacity of the soil.

Thereafter, surface puddles, ditches, and other depressions are filled(depression storage), after which runoff is generated.

Source: L ins ley et al. 1958


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The infiltration capacity of the soil depends on

texture and structure and soil moisture content. The

initial capacity (of a dry soil) is high but, as the storm

continues, it decreases until it reaches a steady value

termed as final infiltration rate

The process of runoff generation continues as long as the

rainfall intensity exceeds the actual infiltration capacity of

the soil but it stops as soon as the rate of rainfall dropsbelow the actual rate of infiltration.


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Physical characteristics affecting runoff

Soil type

The infiltration capacity is dependent on the porosity of a soilwhich determines the water storage capacity and affects theresistance of water to flow into deeper layers.

The highest infiltration capacities are observed in loose, sandy

soils while heavy clay or loamy soils have considerable smallerinfiltration capacities.

The infiltration capacity depends on the moisture contentprevailing in a soil at the onset of a rainstorm.

The initial high capacity decreases with time (provided the raindoes not stop) until it reaches a constant value as the soil profilebecomes saturated


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Physical characteristics - Soil Type

Clay Sand


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Physical characteristics - Vegetation

The amount of rain lost to interception storage on the foliagedepends on the kind of vegetation and its growth stage.

A dense vegetation cover shields the soil from the raindropimpact and reduces the crusting effect as described earlier.

The root system as well as organic matter in the soil increase thesoil porosity thus allowing more water to infiltrate.

Vegetation also retards the surface flow particularly on gentleslopes, giving the water more time to infiltrate and to evaporate.

An area densely covered with vegetation, yields less runoff thanbare ground.


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Physical characteristics -Vegetation

Exposed Soil

Road/pavement Forest


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Physical characteristics - Slope & Catchment Size

Steep slope plots yield more runoff than those with gentle slopes.

Quantity of runoff decreased with increasing slope length.

Mainly due to lower flow velocities and subsequently a longer

time of concentration This means that the water is exposed for a

longer duration to infiltration and evaporation before it reaches

the measuring point.

The runoff efficiency (volume of runoff per unit of area) increases

with the decreasing size of the catchment i.e. the larger the size

of the catchment the larger the time of concentration and the

smaller the runoff efficiency.


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Meteorological factors affecting runoff

• Type of precipitation (rain, snow, sleet, etc.)

• Rainfall intensity• Rainfall amount

• Rainfall duration

• Distribution of rainfall over the watersheds

• Direction of storm movement

• Antecedent precipitation and resulting soil moisture

• Other meteorological and climatic conditions that affect

evapotranspiration, such as temperature, wind, relativehumidity, and season.


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Rainfall Intensity

The most common historical rainfall data are daily total

amounts at selected locations

obtained from a "standard raingauge" (tube) which gives the

depth of rainfall accumulated between observations.

Traditional Rain Gauge Optical Rain Gauge Wireless Rain Gauge

http://images.google.com/imgres?imgurl=http://www.chilbolton.rl.ac.uk/weather/images/Optical%2520Rain%2520Gauge.jpg&imgrefurl=http://www.chilbolton.rl.ac.uk/weather/raingaugedetails.htm&h=534&w=416&sz=45&hl=en&start=48&sig2=w4N_YLVAmVCWL-33qFtmbQ&um=1&tbnid=viHLglp8yPPmLM:&tbnh=132&tbnw=103&ei=SXCRSPGJO4Ko6wO-vuWdBw&prev=/images%3Fq%3Drain%2Bgauge%26start%3D40%26ndsp%3D20%26um%3D1%26hl%3Den%26rlz%3D1T4SKPB_enMY274MY274%26sa%3DNhttp://www.compleatnaturalist.com/images/Prof%20Rain%20Gauge.jpg


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Rainfall Intensity Calculation Methods

Arithmetic Mean Method

Thiessen Method

Isohyetal Method


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Arithmetic Mean

Simplest method

Averaging rainfall depth across a number of gauges

Uniformly distributed

Does not vary greatly


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Isohyetal method

Locate all rainfall stations on

a base map and record the

rainfall amount.Draw isohyets (lines of equal

rainfall) by proportioning the

distances between adjacent

gauges according todifferences in catch.

The most basic method of representing the spatial distribution.

This is generally the most accurate method but is also the mostlaborious.


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Isohyetal method Cont.

• Then calculate the mean precipitation for the area

corresponding to each isohyet (Contour).

• Calculate the fraction of catchment area under each isohyet,

multiply by the mean precipitation for that area and sum to

get the catchment average.


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Thiessen method

Locate all rainfall stations on a

base map and record the rainfall

amount.

Connect each station bystraight lines with the several

nearest stations to form a series

of triangles.

Erect perpendicular bisectors

on each of these lines andextend them to the intersect with

other bisectors, thus forming a

series of irregular polygons

This involves determining the area of influence for each station,

rather than assuming a straight-line variation. It is easier thanthe isohyetal method but less accurate


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Thiessen method cont.

Measure the fraction of the catchment area in each polygon (called

the Thiessen constant), multiply by the rainfall catch at the stationwithin the polygon and sum to get the catchment average.


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Exercise

Area of Basin = 32.7 km2

SationNumber Station Precipi tation(mm)

1 20

2 26

3 25

4 32

5 35

6 38

7 40

8 509 52

10 48


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Exercise Hints when finding Area

The scale of basin map is 1:50,000

Make the scale of the grid paper 1:50,000

Meaning every cm on the grid paper represents 50,000cm on the

ground or 0.5km

Count up all the large blocks first, then the small blocks (find outhow many blocks in each big block)

Multiply by the number of km2 that each large block represents

Area should be not too far from the given area (which is 32.7km2)

Solution to exercise


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So ut o to e e c se

Sation

Number Station Precip (mm) [A]

Area of

polygon

(km2)

Fractional

Area [B]

Area weighted

precipitation

(A*B)

1 20 0.67 2.06 0.41

2 26 1.60 4.89 1.27

3 25 2.97 9.10 2.28

4 32 1.95 5.97 1.91

5 35 1.82 5.56 1.95

6 38 3.39 10.38 3.95

7 40 5.70 17.46 6.99

8 50 6.17 18.91 9.46

9 52 3.38 10.35 5.38

10 48 4.99 15.31 7.35

Total 32.63 100.00 40.93

AveragePreci itation = 40.93 mm


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Question?

Module 1_Hydraulic Cycle, Rainfall & Runoff Process 2016

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