1. Engineering Hydrology (CEE – 4420) TTU – Civil Engineering (Prepared by Abebe Gebregiorgis

2. 1. Basic Hydrology Concept
1.1. Introduction
Water is vital for all living organisms on Earth.
For centuries, people have been investigating where water comes from and where it goes, why some of it is salty and some is fresh, why sometimes there is not enough and sometimes too much. All questions and answers related to water have been grouped together into a discipline.
The name of the discipline is hydrology and is formed by two Greek words: "hydro" and "logos" meaning "water" and "science".

3. What is Hydrology?
It is a science of water.
It is the science that deals with the occurrence, circulation and distribution of water of the earth and earth’s atmosphere.
A good understanding of the hydrologic processes is important for the assessment of the water resources, their management and conservation on global and regional scales.

4. In general sense engineering hydrology deals with
Estimation of water resources
The study of processes such as precipitation, evapotranspiration, runoff and their interaction
The study of problems such as floods and droughts and strategies to combat them

5. 1.2 Hydrologic Cycle
Water exists on the earth in all its three states, viz. liquid, solid, gaseous and in various degrees of motion.

6. Hydrologic cycle….
Water, irrespective of different states, involves dynamic aspect in nature.
The dynamic nature of water, the existence of water in various state with different hydrological process result in a very important natural phenomenon called Hydrologic cycle.

7. Hydrologic Cycle Hydrologic cycle….
Evaporation of water from water bodies, such as oceans and lakes, formation and movement of clouds, rain and snowfall, stream flow and ground water movement are some examples of the dynamic aspects of water.
Hydrologic
Cycle

8. Hydrologic cycle….
Evaporation from water bodies
Water vapour moves upwards
Cloud formation
Condensation
Precipitate
Interception
Transpiration
Infiltration
Runoff–streamflow
Deep percolation
Ground water flow

9. Hydrologic cycle….
The hydrologic cycle has importance influence in a variety of fields agriculture, forestry, geography, economics, sociology, and political scene.
Engineering application of the knowledge are found in the design and operation of the projects dealing with water supply, hydropower, irrigation & drainage, flood control, navigation, coastal work, various hydraulic structure works, salinity control and recreational use of water.

10. 1.3 Water Budget Equation
Catchment area
The area of land draining in to a stream or a water course at a given location is called catchment area / drainage area / drainage basin / watershed.
A catchment area is separated from its neighbouring areas by a ridge called divide / watershed.

11. 1.3 Water Budget Equation Catchment area….
A watershed is a geographical unit in which the hydrological cycle and its components can be analysed. The equation is applied in the form of water-balance equation to a geographical region, in order to establish the basic hydrologic characteristics of the region. Usually a watershed is defined as the area that appears, on the basis of topography, to contribute all the water that passes through a given cross section of a stream.

12. Watershed and watershed divide
Watershed/ catchment
Watershed/ catchment
Watershed divide

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

14. Watershed characteristics

15. Mass inflow – Mass outflow = change in mass storage
Water Budget Equation
For a given catchment, in an interval of time ∆t, the continuity equation for water in its various phases can be given as:
Mass inflow – Mass outflow = change in mass storage
If the density of the inflow, outflow and storage volumes are the same:
Vi - Inflow volume in to the catchment, Vo - Outflow volume from the catchment and ∆S - change in the water volume

16. Water Budget Equation…
Therefore, the water budget of a catchment for a time interval ∆t is written as:
P – R – G – E – T = ∆S
P = Precipitation, R = Surface runoff, G = net ground water flow out of the catchment, E = Evaporation, T = Transpiration, and ∆S = change in storage
The above equation is called the water budget equation for a catchment
NOTE: All the terms in the equation have the dimension of volume and these terms can be expressed as depth over the catchment area.

17. Components of hydrologic cycle
Evapo transpiration
Precipitation
Stream flow
(Runoff)
Inter flow
Infiltration
Base flow
Groundwater flow

18. 1.3 World Water Budget
Total quantity of water in the world is estimated as 1386 M km3
M km3 of water is contained in oceans as saline water
The rest 48.5 M km3 is land water
13.8 M km3 is again saline
34.7 M km3 is fresh water
10.6 M km3 is both liquid and fresh
24.1 M km3 is a frozen ice and glaciers in the polar regions and mountain tops

20. Global annual water balanceSN
Item
Ocean
Land 1
Area (km2)
361.3
148.8 2
Precipitation (km3/year)
(mm/year)
458,000
1270
119,000
800 3
Evaporation (km3/year)
505,000
1400
72,000
484 4
Runoff to ocean
Rivers (km3/year)
Groundwater (km3/year)
44,700
2,200
Total Runoff (km3/year)
47,000
316

21. Water Balance of Continents

22. Water Balance …….
Drop of water …..
Matter…..

23. Water Balance of Oceans

24. 1.4 Application in Engineering
Hydrology finds its greatest application in the design and operation of water resources engineering projects
The capacity of storage structures such as reservoir
The magnitude of flood flows to enable safe disposal of the excess flow
The minimum flow and quantity of flow available at various seasons
The interaction of the flood wave and hydraulic structures, such as levees, reservoirs, barrages and bridges

25. THE END