Hydrology Lecture Note-I

Hydrologic Cycle

Prepared by
Rifat Talha Khan
Lecturer
Department of Water Resources Engineering
Chittagong University of Engineering & Technology
Introduction
• Hydrology means the science of water.
• It is the science that deals with the occurrence, circulation and
distribution of water of the earth’s atmosphere.
• As a branch of science, it is concerned with the water in streams and
lakes, rainfall and snowfall, snow and ice on the land and occurring
below the earth’s surface in the pores of the soils and rocks
Introduction
• In general, engineering hydrology deals with:
i. Estimation of water resources
ii. The study of processes such as precipitation, runoff, evapotranspiration
and their interaction,
iii. The study of problems such as floods and droughts and strategies to
combat them.
Hydrologic Cycle
Hydrologic Cycle
1. Water evaporates from the oceans and lakes of the earth.
2. The evaporated water vapor forms into clouds.
3. The clouds move through the atmosphere in global weather patterns.
4. The water vapor condenses and precipitates in the form of rain, snow,
or hail.
5. The rain lands on the ground and flows to the streams.
6. The streams flow to rivers and eventually into the oceans and lakes.
Hydrologic Cycle
7. streams and rivers are fed by rainfall in three ways.
8. Some of the rain is lost immediately to evaporation and
evapotranspiration (the loss of water vapor from plants to the
atmosphere),
9. some flows by gravity over the surface of the ground and eventually into
the stream, and the remainder infiltrates into the ground.
10. Of the infiltrated water, some flows underground or in the subsurface by
gravity to the stream, and some percolates to the stored underground
water body called groundwater.
11. Although the overland flow is relatively quick, reaching the stream in
minutes or, at most, hours, the subsurface flow and groundwater flow
are much slower, lasting many days.
12. Therefore, after an initial quick surge of overland flow when it rains,
streams receive a constant feed of subsurface water throughout the days
between rainfalls.
Hydrologic Cycle
• The constant low-level flow in streams due to the subsurface feed is
called base flow, and the quick surge due to overland flow is called
direct runoff.
• It is direct runoff that we are concerned with in the design of
hydraulic structures because direct runoff represents the greatest
volume of water that the structure must handle.
Catchment Area
• The area of land draining
into a stream or a water
course at a given location
is known as catchment
area, drainage area or
drainage basin or
Watershed (in USA).
• A catchment area is
separated from its
neighbouring areas by a
ridge called divide in USA
and watershed in the UK.
Delineation of Catchment Area
Brahmaputra River Catchment Area
Water Budget Equation
• For a given problem area, say a catchment, in an interval of time Δt, the continuity
equation for water in its various phases is written as:
Mass inflow - mass outflow = change in mass storage
If density of inflow, outflow and storage volumes are the same,
Vi-Vo=ΔS
Where,
Vi= inflow volume of water into the problem area during the time period,
Vo= Outflow volume of water from the problem are during the time period,
ΔS = change in storage of the water volume over and under the given area during the given period
Water Budget Equation
• An expression for the water budget of a catchment for a time interval
Δt is written as:
P – R – G – E – T = ΔS
P = Precipitation
R = Surface Runoff
G = Net ground water flow out of the catchment
E = Evaporation
T = Transpiration
ΔS = change in storage
 Renewable and Non-Renewable Water
• Water exists in various forms in the hydrosphere (liquid and frozen surface
waters, groundwater held in soil and rock, and atmospheric water vapour) (The
Editors of Encyclopaedia Britannica, 2018).
• These various forms of water are completely replenished in quantity as well as in
quality during the hydrologic cycle but at varying frequencies.
• For complete replenishment, it takes 2500 years for oceanic waters, 10,000 years
for permafrost and ice, 1500 years for mountainous glaciers and deep
groundwater aquifers, 17 years for lakes and 16 days for rivers.
• The portion of the water resources of a region which takes years or decades for
complete replenishment (such as large lakes, deep groundwater aquifers,
glaciers, etc.) is considered as static storage component whereas that portion of
water resources replenished annually through the water cycle (such as river
discharge and upper groundwater aquifer) is known as renewable water resource
(Shiklomanov, 1998).
Aquifer

a geological formation of
permeable rock, gravel, or sand
containing or conducting ground
water, esp. one that supplies
the water for wells, springs, etc.
Applications of Hydrology in Civil Engineering
• Hydrology is used to find out maximum probable flood at proposed sites
e.g. Dams.
• The variation of water production from catchments can be calculated and
described by hydrology.
• Engineering hydrology enables us to find out the relationship between a
catchments’s surface water and groundwater resources
• The expected flood flows over a spillway, at a highway Culvert, or in an
urban storm drainage system can be known by this very subject.
• It helps us to know the required reservoir capacity to assure adequate
water for irrigation or municipal water supply in droughts condition.
• It tells us what hydrologic hardware (e.g. rain gauges, stream gauges etc)
and software (computer models) are needed for real-time flood forecasting
• Used in connection with design and operations of hydraulic structure
 GBM Basin
• The Ganges-Brahmaputra-Meghna (GBM) river basin is a transboundary river basin with
a total area of just over 1.7 million km2, distributed between India (64 percent), China
(18 percent), Nepal (9 percent), Bangladesh (7 percent) and Bhutan (3 percent) (Table 1).
• Nepal is located entirely in the Ganges river basin and Bhutan is located entirely in the
Brahmaputra river basin.
• The GBM river system is considered to be one transboundary river basin, even though
the three rivers of this system have distinct characteristics and flow through very
different regions for most of their lengths.
• They join only just a few hundred kilometres upstream of the mouth in the Bay of
Bengal. Not only is each of these three individual rivers big, each of them also has
tributaries that are important by themselves in social, economic and political terms, as
well as for water availability and use. Many of these tributaries are also of a
transboundary nature (Biswas, after 2006). The GBM river system is the third largest
freshwater outlet to the world’s oceans, being exceeded only by the Amazon and the
Congo river systems (Chowdhury and Ward, 2004).
Hydrological Regions of Bangladesh
 Hydrological Regions of Bangladesh
• According to National Water Management Plan 2001, Bangladesh has
been divided into 8 distinct hydrological regions considering the
principal rivers and the natural geographical features as boundaries.
These 8 regions are: Northwest (NW), North Central (NC), Northeast
(NE), Southeast (SE), South Central (SC), Southwest (SW), Eastern Hills
(EH), plus the active floodplains and charlands of the Main Rivers and
Estuaries (RE) (WARPO, 2001).
 Hydrology of Bangladesh
• Three large rivers systems e.g. Ganges, Brahmaputra and Meghna, in the
world covering a combined total catchments area of about 1.7 million sq.
km. extending over Bhutan, China, India and Nepal, flow through this
country. Out of these huge catchments only 7% lies in Bangladesh. Rivers
are classified into three broad categories depending on the flow range and
are as follows:
i) Major Rivers: 300 to 120,000 cumec e.g Ganges, Brahmaputra, Padma,
Meghna
ii) Semi major Rivers: 100 to 15000 cumec e.g. Old Brahmaputra, Dhaleswari,
Gorai, Arial Khan, Surma, Kushiyara, Teesta etc.
iii) Minor River: 1 to 1000 cumec e.g. Sitalakhya, Buriganganga, Khowai,
Manu, Gumti, Dharla, Dudkumar, Karnafuli, Halda, Sangu etc.
Hydrographs of Some of the Major Rivers
 Climate of Bangladesh
Temperature and seasonality:
The general characteristics of the seasons of Bangladesh have a subtropical monsoon climate
characterized by wide seasonal variations in rainfall, moderately warm temperatures, and high
humidity.
In Bangladesh there are four prominent seasons, namely, winter, pre-monsoon, monsoon and
post-monsoon. In general, maximum summer temperatures range between 30°C and 40°C.
April is the warmest month in most parts of the country. January is the coldest month, when
the average temperature for most of the country is about 10°C.
Heavy rainfall is characteristic of Bangladesh. With the exception of the relatively dry
western region of Rajshahi, where the annual rainfall is about 1600 mm, most parts of the
country receive at least 2000 mm of rainfall per year. Because of its location just south of the
foothills of the Himalayas, where monsoon winds turn west and northwest, the regions in
northeastern Bangladesh receives the greatest average precipitation, sometimes over 4000 mm
per year. About 80 percent of Bangladesh's rain falls during the monsoon season.
 Climate of Bangladesh
• Winter (December to February):
Winter is relatively cooler and drier, with the average temperature
ranging from a minimum of 7.2 to 12.8°C to a maximum of 23.9 to
31.1°C. In winter there is not usually much fluctuation in temperature
which ranges from minimum of 7°C to 13°C to maximum of 24°C to
31°C. The minimum temperature occasionally falls below 5°C in the
north though frost is extremely rare. There is a south to north thermal
gradient in winter mean temperature: generally the southern districts
are 5°C warmer than the northern districts.
 Climate of Bangladesh
• Pre-monsoon (March to May):
Pre-monsoon is hot with an average maximum of 36.7°C,
predominantly in the west for up to 10 days, very high rate of
evaporation, and erratic but occasional heavy rainfall from March to
June. In some places the temperature occasionally rises up to 40.6°C or
more. The peak of the maximum temperatures are observed in April,
the beginning of pre-monsoon season. In pre-monsoon season the
mean temperature gradient is oriented in southwest to northeast
direction with the warmer zone in the southwest and the cooler zone in
the northeast.
 Climate of Bangladesh
• Monsoon (June to early-October):
Monsoon is both hot and humid, brings heavy torrential rainfall
throughout the season. About four-fifths of the mean annual rainfall
occurs during monsoon. The mean monsoon temperatures are higher
in the western districts compared to that for the eastern districts.
Warm conditions generally prevail throughout the season, although
cooler days are also observed during and following heavy downpours.
Climate of Bangladesh: Variation of Annual Mean Rainfall
 Technical Terms
• Hydrograph
• Discharge
• Direct Runoff
• Streamflow
• Infiltration
• Percolation
• Soil Moisture
• Baseflow