1 The Hydrological Cycle
1.1 Introduction
Hydrology is one of the earth sciences. It studies the waters of the earth, their occurrence,
circulation and distribution, their chemical and physical properties, and their relation to
living things. Hydrology encompasses surface water hydrology, and groundwater
hydrology. Other related earth sciences include climatology, meteorology, geology,
geomorphology, sedimentology, geography, and oceanography.
Engineering hydrology is an applied science. It uses hydrologic principles in the solution
of engineering problems arising from human exploitation of the water resources of the
earth. In its broadest sense, engineering hydrology seeks to establish relations defining
the spatial, temporal, seasonal, annual, regional or geographical variability of water, with
the aim of ascertaining societal risks involved in sizing hydraulic structures and systems.
1.2 The Hydrologic Cycle
The hydrologic cycle describes the continuous recirculatory transport of the waters of the
earth, linking atmosphere, land, and oceans. The process is quite complex, containing
many subcycles. Fig 1 shows a pictorial representation of the hydrologic cycle.
In brief:
• Water evaporates from the ocean surface, driven by energy from the sun, and
joins the atmosphere, moving inland.
• Once inland, atmospheric conditions act to condense and precipitate water onto
the land surface, where, driven by gravitational forces, it returns to the oceans
through streams and rivers.
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Useful Terms:
Ocean: a store of salt water
Evaporation: Water transferring from ground
into water vapour in the air
Air: store of water vapour
Condensation: water vapour in the air turning
back into a liquid
Clouds: Fresh water droplets held in the air
Precipitation: water falling to the ground as rain,
hail, sleet or snow
Percolation: Water moving in the ground
Groundwater: water stored underground
Run-off : water running over the ground back to
the sea
Transpiration : water transferred by trees and
plants into the air as water vapour
The water-holding elements of the hydrologic cycle are
1- Atmosphere
2- Vegetation
3- Snowpack and icecaps
4- Landsurface
5- Soil
6- Streams, lakes, rivers
7- Aquifers
8- Oceans
Liquid-transport of the hydrologic cycle are
1- precipitation from the atmosphere onto land surface
2- throughfall from vegetation onto land surface
3- melt from snow and ice onto land surface
4- surface runoff from land surface to streams, lakes, and rivers, and from streams,
lakes, and rivers to ocean.
5- Infiltration from land surface to soil
6- Exfiltration from soil to land surface
7- Interflow from soil to streams, lakes, rivers, and vice versa
8- Percolation from soil to aquifers
9- Capillary rise from aquifers to soil
10- Groundwater flow from streams, lakes and rivers to aquifers and vice versa and
from aquifers to oceans and vice versa
Vapour-transport phases of the hydrologic cycle are
1- Evaporation from land surface, streams, lakes, rivers, and oceans to the
atmosphere.
2- Evapotranspiration from vegetation to the atmosphere
3- Sublimation from snowpack and icecaps to the atmosphere
4- Vapour diffusion from soil to land surface
1.3 Inventory of Earth’s Water
Table 1 lists estimates of the amount of water involved in the hydrological cycle and the
proportion (in %) of the total water on earth involved in each part of it.
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Table 1 Estimated Earth’s water inventory
Location
Fresh water lakes
Rivers
Soil moisture
Groundwater
Saline lakes and inland seas
Atmosphere
Polar ice-caps, glaciers and snow
Seas and oceans
Total
Volume (103 km3)
125
1.25
65
8250
105
13
29200
1320000
1360000
Percentage total water
0.62
0.008
0.001
2.1
97.25
100
Of the 0.6 per cent of total water that is available as fresh water, about half is below a
depth of 800m and so is not practically available on the surface. This means that the stock
of the Earth’s fresh water that is obtainable one way or another for man’s use is about
4x106 km3 and is mainly on the ground. Spread over the Earth’s land surface it would be
30 m deep.
1.4 The catchment and its hydrologic budget
A catchment is a portion of the earth’s surface that collects runoff and concentrates it at
its furthest downstream point, referred to as the catchment outlet. The runoff
concentrated by a catchment flows either into a larger catchment or into the ocean. The
place where a stream enters a larger stream or a body of water is referred to as the mouth.
The hydrologic budget refers to an accounting of the various transport phases of the
hydrologic cycle within a catchment, with the aim of ascertaining their relative
magnitudes. The following is a hydrologic budget equation that considers both surface
water and groundwater.
S = P − ( E + T + G + Q)
(1)
in which:
ΔS: Change in storage
P: precipitation
E: evaporation
T: evapotranspiration
G: groundwater flow out of the catchment
Q: surface runoff
In hydrologic practice, the terms of eq (1) are expressed in units of water depth, i.e. a
water volume uniformly distributed over the catchment area.
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A hydrologic budget equation that accounts only for the surface water is:
S = P − ( E + T + I + Q)
(2)
I: infiltration
Assuming ΔS=0, eq 2 reduces to:
Q = P−L
(3)
in which L: losses, or hydrologic abstractions.
1.5 Hydrology as applied in Engineering
Hydrology touches every human life in some manner. To some, it is simply a need for
drinking water, and to others, the need for water might be economic for just for
convenience. Among applications of hydrology are flood control, drought mitigation,
water supply, pollution control, urban development, industrial development, design of
hydraulic works (dams, culverts, spillways, bridge crossings, etc), agricultural
production, energy-resources development (thermal, nuclear, hydropower plants), land
conservation, environmental-impact assessment, land-use change, forest and wildlife
management, military operations, rural development, navigation, recreation, and
fisheries.
Engineering hydrology seeks to answer questions of the following type.
1. What is the maximum probable flood at a proposed dam site?
2. How does a catchment’s water yield vary from season to season and from year to
year?
3. What is the relationship between a catchment’s surface water and groundwater
resources?
4. When evaluating low flow characteristics, what flow level can be expected to exceed
90 percent of the time?
5. Given the natural variability of streamflows, what is the appropriate size of an
instream storage reservoir?
6. What hydrologic hardware (e.g. rainfall sensors) and software (computer models) are
needed for real-time flood forecasting?
7. …
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