Introduction Hydrology

Engineering Engineering HydrologyHydrology(CEE – 4420)(CEE – 4420)

TTU – Civil EngineeringTTU – Civil Engineering(Prepared by Abebe Gebregiorgis (Prepared by Abebe Gebregiorgis

1. Basic Hydrology 1. Basic Hydrology ConceptConcept

Water is vital for all living organisms on Earth. Water is vital for all living organisms on Earth. For centuries, people have been investigating For centuries, people have been investigating

where water comes from and where it goes, where water comes from and where it goes, why some of it is salty and some is fresh, why why some of it is salty and some is fresh, why sometimes there is not enough and sometimes sometimes there is not enough and sometimes too much. All questions and answers related to too much. All questions and answers related to water have been grouped together into a water have been grouped together into a discipline. discipline.

The name of the discipline is hydrology and is The name of the discipline is hydrology and is formed by two Greek words: "hydro" and formed by two Greek words: "hydro" and "logos" meaning "water" and "science". "logos" meaning "water" and "science".

1.1. Introduction1.1. Introduction

What is Hydrology?What is Hydrology? – It is a science of water.It is a science of water.– It is the science that deals with the It is the science that deals with the

occurrence, circulation and distribution of occurrence, circulation and distribution of water of the earth and earth’s atmosphere.water of the earth and earth’s atmosphere.

A good understanding of the A good understanding of the hydrologic processes is important for hydrologic processes is important for the assessment of the water resources, the assessment of the water resources, their management and conservation on their management and conservation on global and regional scales. global and regional scales.

In general sense engineering In general sense engineering hydrology deals with hydrology deals with – Estimation of water resourcesEstimation of water resources– The study of processes such as The study of processes such as

precipitation, evapotranspiration, precipitation, evapotranspiration, runoff and their interactionrunoff and their interaction

– The study of problems such as The study of problems such as floods and droughts and strategies floods and droughts and strategies to combat themto combat them

1.2 Hydrologic Cycle 1.2 Hydrologic Cycle

Water exists on the earth in all its Water exists on the earth in all its three states, viz. liquid, solid, three states, viz. liquid, solid, gaseous and in various degrees of gaseous and in various degrees of motion.motion.

Hydrologic cycle….Hydrologic cycle….

Water, irrespective of different Water, irrespective of different states, involves dynamic aspect in states, involves dynamic aspect in nature.nature.

The dynamic nature of water, the The dynamic nature of water, the existence of water in various state existence of water in various state with different hydrological process with different hydrological process result in a very important natural result in a very important natural phenomenon called phenomenon called

Hydrologic cycleHydrologic cycle. .

Hydrologic cycle….Hydrologic cycle…. Evaporation of water from water bodies, such Evaporation of water from water bodies, such

as oceans and lakes, formation and movement as oceans and lakes, formation and movement of clouds, rain and snowfall, stream flow and of clouds, rain and snowfall, stream flow and ground water movement are some examples ground water movement are some examples of the dynamic aspects of water. of the dynamic aspects of water.

Evaporation from Evaporation from water bodieswater bodies

Water vapour Water vapour moves upwardsmoves upwards

Cloud formationCloud formation CondensationCondensation PrecipitatePrecipitate InterceptionInterception TranspirationTranspiration InfiltrationInfiltration Runoff–Runoff–

streamflowstreamflow Deep percolationDeep percolation Ground water flowGround water flow



The hydrologic cycle has importance The hydrologic cycle has importance influence in a variety of fields agriculture, influence in a variety of fields agriculture, forestry, geography, economics, forestry, geography, economics, sociology, and political scene. sociology, and political scene.

Engineering application of the knowledge Engineering application of the knowledge are found in the design and operation of are found in the design and operation of the projects dealing with water supply, the projects dealing with water supply, hydropower, irrigation & drainage, flood hydropower, irrigation & drainage, flood control, navigation, coastal work, various control, navigation, coastal work, various hydraulic structure works, salinity control hydraulic structure works, salinity control and recreational use of water. and recreational use of water.

The area of land draining in to a The area of land draining in to a stream or a water course at a stream or a water course at a given location is called given location is called catchment catchment area area / / drainage area drainage area // drainage drainage basin basin / / watershed.watershed.

A catchment area is separated A catchment area is separated from its neighbouring areas by a from its neighbouring areas by a ridge called ridge called divide divide / / watershedwatershed. .

1.3 Water Budget 1.3 Water Budget Equation Equation

Catchment areaCatchment area

A watershed is a geographical unit in A watershed is a geographical unit in which the hydrological cycle and its which the hydrological cycle and its components can be analysed. The components can be analysed. The equation is applied in the form of water-equation is applied in the form of water-balance equation to a geographical region, balance equation to a geographical region, in order to establish the basic hydrologic in order to establish the basic hydrologic characteristics of the region. Usually a characteristics of the region. Usually a watershed is defined as the area that watershed is defined as the area that appears, on the basis of topography, to appears, on the basis of topography, to contribute all the water that passes contribute all the water that passes through a given cross section of a stream. through a given cross section of a stream.

1.3 Water Budget 1.3 Water Budget Equation Equation

Catchment area….Catchment area….

Watershed and watershed divideWatershed and watershed divide

Watershed/ catchment

Watershed/ catchment

Watershed divide

If a permeable soil covers an impermeable If a permeable soil covers an impermeable substrate, the topographical division of substrate, the topographical division of watershed will not always correspond to the line watershed will not always correspond to the line that is effectively delimiting the groundwater.that is effectively delimiting the groundwater.

Catchment area….Catchment area….

Watershed characteristicsWatershed characteristics

Water Budget Equation Water Budget Equation

For a given catchment, in an interval of For a given catchment, in an interval of time time ∆t, the continuity equation for ∆t, the continuity equation for water in its various phases can be water in its various phases can be given as:given as:Mass inflow – Mass outflow = change in mass Mass inflow – Mass outflow = change in mass

storagestorage If the density of the inflow, outflow and If the density of the inflow, outflow and

storage volumes are the same:storage volumes are the same:

VVi i - - Inflow volume in to the catchment, VInflow volume in to the catchment, Vo o - Outflow - Outflow

volume from the catchment and volume from the catchment and ∆S -∆S - change in the change in the water volume water volume

Soi

Therefore, the water budget of a Therefore, the water budget of a catchment for a time interval catchment for a time interval ∆t is ∆t is written as:written as:

P – R – G – E – T = ∆SP – R – G – E – T = ∆SP = Precipitation, R = Surface runoff, G = net ground water flow P = Precipitation, R = Surface runoff, G = net ground water flow

out of the catchment, E = Evaporation, T = Transpiration, and out of the catchment, E = Evaporation, T = Transpiration, and ∆S = change in storage ∆S = change in storage

The above equation is called The above equation is called the water the water budget equation for a catchment budget equation for a catchment

NOTE:NOTE: All the terms in the equation have the All the terms in the equation have the dimension of dimension of volume and these terms can be volume and these terms can be expressed as depth expressed as depth over the catchment area. over the catchment area.

Water Budget Equation… Water Budget Equation…

Components of hydrologic Components of hydrologic cyclecycle

Precipitation

Infiltration

Evapo transpiration

Inter flow

Groundwater flow

Base flow

Stream flow

(Runoff)

Total quantity of water in the Total quantity of water in the world is estimated as world is estimated as 1386 M km1386 M km33

– 1337.5 M km1337.5 M km33 of water is contained of water is contained in oceans as saline waterin oceans as saline water

– The rest The rest 48.5 M km48.5 M km33 is land water is land water 13.8 M km13.8 M km33 is again saline is again saline 34.7 M km34.7 M km33 is fresh water is fresh water

– 10.6 M km10.6 M km33 is both liquid and fresh is both liquid and fresh– 24.1 M km24.1 M km33 is a frozen ice and glaciers in is a frozen ice and glaciers in

the polar regions and mountain topsthe polar regions and mountain tops

1.3 World Water Budget 1.3 World Water Budget

Global annual water Global annual water balancebalance

SSNN

ItemItem OceanOcean LandLand

11 Area (kmArea (km22)) 361.3361.3 148.8148.8

22 Precipitation Precipitation (km(km33/year)/year)

(mm/year)(mm/year)

458,000458,000

12701270119,000119,000

800800

33 Evaporation (kmEvaporation (km33/year)/year)

(mm/year)(mm/year)505,000505,000

1400140072,00072,000

484484

44 Runoff to oceanRunoff to oceanRivers (kmRivers (km33/year)/year)

Groundwater (kmGroundwater (km33/year)/year)

44,70044,700

2,2002,200

Total Runoff (kmTotal Runoff (km33/year)/year)

(mm/year)(mm/year)47,00047,000

316316

Water Balance of Water Balance of Continents Continents

Area (M km^2)

30.3

8.7 9.8

20.717.8

45

0

10

20

30

40

50

Africa Asia Australia Europe N.America S.America

Precipitation (mm/ yr)

686 736 734 670726

1648

0

500

1000

1500

2000


Water Balance ……. Water Balance ……. Precipitation (mm/ yr)

686 736 734 670726

1648

0

500

1000

1500

2000


Evaporation (mm/ yr)

547 510415 383

1065

433

0

200

400

600

800

1000

1200


Total Runoff (mm/ yr)

139226

319 287293

583

0

100

200

300

400

500

600

700


Drop of water Drop of water ….. ….. Matter…..Matter…..

Water Balance of OceansWater Balance of Oceans

10712

75167

780

240

1010

1210

1040

120

1380

1140

0

200

400

600

800

1000

1200

1400

1600

Atlantic Arctic Indian Pacific

Area M km^2

Precp (mm/yr)

Evap. (mm/yr)

Water flow in Ocean

200 230

70 60

350

-300

130

-60

-400

-200

0

200

400

Atlantic Arctic Indian Pacific

Continental Inflow (mm/yr)

water exch. with ocean(mm/yr)

1.4 Application in 1.4 Application in Engineering Engineering

Hydrology finds its greatest Hydrology finds its greatest application in the design and application in the design and operation of water resources operation of water resources engineering projectsengineering projects– The capacity of storage structures such The capacity of storage structures such

as reservoir as reservoir – The magnitude of flood flows to enable The magnitude of flood flows to enable

safe disposal of the excess flowsafe disposal of the excess flow– The minimum flow and quantity of flow The minimum flow and quantity of flow

available at various seasonsavailable at various seasons– The interaction of the flood wave and The interaction of the flood wave and

hydraulic structures, such as levees, hydraulic structures, such as levees, reservoirs, barrages and bridges reservoirs, barrages and bridges

THE ENDTHE END

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Introduction Hydrology