Hydrology notes

Some Hydrological concepts for university students.

1. Potamology: Originated from Greek word potamos (meaning-river) and logos (meaning-study).
According to Oxford Dictionary, potamology is the study of rivers.
According to Penck (1897), the science of rivers, which may be called potamology, must be treated
under five different heads-
1. The physics of running water
2. The volume of water and its fluctuations
3. The action of water on its bed
4. The distribution of running water on the earth
5. Rivers as a scene of organic life.

Limnology: Originated from Greek word limne (meaning-lake) and logos (meaning-study).
According to Oxford Dictionary, limnology is the study of the biological, chemical, and physical
features of lakes and other bodies of fresh water.
Limnology may study- water flow, oxygen in the water, food web dynamics, animal movement
patterns, minerals and other water chemicals, pollution, ecosystem structure, the economics of
water, light influences, nutrient cycles, plants that live on, in, or near inland waters, sediments,
bacteria, human influences, ecosystems, animal communities etc.

Cryology: Originated from Greek word cryo (meaning-ice) and logos (meaning-study). According
to Oxford Dictionary, the study of ice, snow, and the parts of the natural environment below
freezing point. According to Seligman (1947), cryology is the scientific study of ice. Seligman also
mentioned the scope of cryology. Those are-
1. Standardization of maps of snow-cover and ice-cover for the world.
2. Uniform classification of different types of snow and snow-cover, and uniform
nomenclature for the same.
3. A system of classification for the international bibliography of snow and ice.
4. Standardization of methods of snow-surveying and forecasting run-off from snow.

Watershed: According to Oxford Dictionary of Geography, watershed is the boundary between
two river systems. It marks the divide between drainage basins and usually runs along the highest
points of the interfluves.

2. Geohydrology: Geohydrology is the study of geology with an emphasis on hydrological aspects.
In other word, it is a branch of hydrology that explains geological phenomena from hydrological
aspects. According to Grand Valley State University, Michigan, Geohydrology is the study of
groundwater and its physical and chemical interactions with the physical environment.

Venturimeter: A venturimeter is a device used for measuring the rate of flow of a fluid flowing
through a pipe.
Construction:
a) Short converging part: It is a tapered portion whose radius decreases.
b) Throat: It is middle portion of the venturi. Here the velocity of the fluid increases and
pressure decreases. It possesses the least cross section area.
c) Diverging part: In this portion the fluid diverges.
Principals: The principle of venturimeter is that when a fluid flows through the
venturimeter, it accelerates in the convergent section and decelerates in the divergent
section, resulting in a drop in the static pressure followed by a pressure recovery in the
flow direction. By measuring the difference in the pressures at an axial station upstream of
the convergent section and at the throat, the volumetric flow rate can be estimated.

Measurement:
??????
????????????��????????????=??????
�
??????
1??????
2 √(2??????ℎ)
√(??????
1
2
−??????
2
2
)

where,
Qactual = Actual discharge in m3 /s
Cv=Discharge co-efficient of a
venturimeter
A1=area at the inlet side in m2
A =area at the throat side in m2
h =Differential pressure head of liquid in m
g =Acceleration due to gravity (9.81 m/s2)

3. Capillary rise: A rise in a liquid above the level of zero pressure due to a net upward force
produced by the attraction of the water molecules to a solid surface, e.g. glass, soil (for those cases
where the adhesion of the liquid to the solid is greater than the cohesion of the liquid to itself).

Figure: Capillary action (rise/fall) of water and mercury

4. RUNOFF
Runoff: The movement over ground of rain water.
Components: surface runoff, subsurface runoff, groundwater.
Factors:
1. Climatic: type of rainfall, intensity of rainfall, duration of rainfall, distribution of rainfall,
antecedent precipitation, direction of storm movement, other: temperature, wind, humidity,
air pressure etc.
2. Physiographic: land use, soil type, basin area, basin shape, elevation, slope, orientation,
drainage network type, artificial drainage, indirect drainage.



STREAM FLOW MEASUREMENT STEPS
Step-1: Choosing method
Direct method: area velocity method, dilution method, electromagnetic method,
ultrasonic method.
Indirect method: slope area method, hydraulic structure.
Other: bucket stopwatch method, float method.
Step-2: Setting up instruments.
Step-3: Undertake experiments
Step-4: Reading data
Step-5: Calculation
Bucket-Stopwatch: discharge=discharge volume/time
Float method: area x velocity x 0.8


GROUND WATER
Groundwater: Ground water is the water found in the surface of the ground containing minerals
and is not connected with underground stream channels (Oxf. D. Geo.).
Groundwater quality: Groundwater quality refers to the state of water that is located beneath
Earth's surface. Normally it has following components-
Primary (1-1000 mg/L) cations: sodium, calcium, magnesium
Primary anions: bicarbonate, sulfate, silica, chloride
Secondary (0.01-10 mg/L) cations: potassium, iron, strontium
Secondary anions: carbonate, nitrate, boron, fluoride.
Trace elements (less than that): others (Br, As, Al etc.)
Deterioration of the above quality considered as ground water pollution.
Causes of ground water pollution: Naturally-occurring (geogenic), On-site sanitation systems,
Sewage (treated and untreated), Fertilizers and pesticides, Commercial and industrial leaks,
Hydraulic fracturing, Landfill leachate, other.
Safe yield of ground water: amount of ground water, which can be drawn annually without
producing an undesired result.

5. Factors:
a) Water supply (should not yield more than long time mean water)
b) Water quality (should not mix up with external contents)
c) Water right (law should be established before yielding)
d) Economics (yield cost should not be extreme)

POTENTIAL AND ACTUAL EVAPOTRANSPIRATION
Actual evapotranspiration: Actual evapotranspiration or AE is the quantity of water that is
actually removed from a surface due to the processes of evaporation and transpiration.
Potential evapotranspiration: Potential evapotranspiration or PE is a measure of the ability of
the atmosphere to remove water from the surface through the processes of evaporation and
transpiration assuming no control on water supply. Wind and Sun Radiation are two energy two
sources for PE.
Relation between AE and PE: Scientists consider these two types of evapotranspiration for the
practical purpose of water resource management. Around the world humans are involved in the
production of a variety of plant crops. Many of these crops grow in environments that are naturally
short of water. As a result, irrigation is used to supplement the crop's water needs. Managers of
these crops can determine how much supplemental water is needed to achieve maximum
productivity by estimating potential and actual evapotranspiration. Estimates of these values are
then used in the following equation:

Crop water need = potential evapotranspiration - actual evapotranspiration

INTERCEPTION:
Interception: The proportion of precipitation that does not reach the ground and considered as
precipitation loss.
Factors: Season, density of precipitation, age of the plants.
Interception Controlling Phenomena:
1. Vegetation characteristics
a) Group form of plants
b) Plant communities structure
2. Meteorological factor
a) Precipitation density
b) Precipitation intensity
c) Wind speed
d) Precipitation type
e) Precipitation frequency
Significance:
1. Reduces basin quantity.
2. Lowers the infiltration
3. Washes away solid particle from leaves.

6. SOIL MOISTURE
Soil moisture: Water stored in or at the land surface and available for evapotranspiration (IPCC,
2012). It is measured with Tensiometers.
Importance: Soil moisture is a key variable in controlling the exchange of water and heat energy
between the land surface and the atmosphere through evaporation and plant transpiration. As a
result, soil moisture plays an important role in the development of weather patterns and the
production of precipitation.
Soil Moisture Movement: Movement of soil moisture content towards the inner portion of the
surface and to take place infiltration. It has 3 types-
i) Saturated Flow
ii) Unsaturated Flow
iii) Water Vapor Movement

o Saturated flow: This occurs when the soil pores are completely filled with water.
Saturated flow is water flow caused by gravity’s pull. It begins with infiltration,
which is water movement into soil when rain or irrigation water is on the soil
surface.
 Factors: 1. Texture 2.Structure 3.Amount of organic matter 4.Depth of soil
to hard pan 5.Amount of water in the soil 6.temperature and 7. Pressure
o Unsaturated flow: It is flow of water held with water potentials lower than-1/3 bar.
Water will move toward the region of lower potential (towards the greater “pulling”
force). In a uniform soil this means that water moves from wetter to drier areas.
The water movement may be in any direction.
 Factors: Unsaturated flow is also affected in a similar way to that of
saturated flow. Amount of moisture in the soil affects the unsaturated flow.
The higher the percentage of water in the moist soil, the greater is the
suction gradient and the more rapid is the delivery.
o Water Vapor Movement: The movement of water vapor from soils takes place in
two ways: (a) Internal movement—the change from the liquid to the vapor state
takes place within the soil, that is, in the soil pores and (b) External movement—
the phenomenon occurs at the land surface and the resulting vapor is lost to the
atmosphere by diffusion and convection.
 Factor: The movement of water vapor through the diffusion mechanism
taken place from one area to other soil area depending on the vapor pressure
gradient (moving force).


CLIMATE CHANGE AND WATER RESOURCE MANAGEMENT
IN BANGLADESH
Climate Change Vulnerability:
1. Bangladesh located on a Delta
2. High Population density

7. 3. Most of Bangladesh < 5m above sea level
4. One of the poorest countries in the world
5. Poor building construction
6. Most people working in fishing and rice farming
7. Monsoon rains
8. Cyclones
9. Flooding of Ganges and Brahmaputra Rivers
10. Drought in North of country
11. Glacier melt in Himalayas

Climate Change effect in Bangladesh: Flash flood, cyclone, salinity intrusion, sea level rise,
biodiversity loss, extreme temperature etc.
Responses: Bangladesh Climate Change Strategy and Action Plan 2009, Bangladesh Climate
Change Strategy (BCCS) etc.
Water resource management and climate change:
 For irrigation as courses and precipitation behaviour change.
 Saline water intrusion observation and remedies.
 Water storage system and artificially enhancing recharge technology.
 Water resource saving strategy and planning for saving from salinization.
 Preparation for droughts occurred due to climate change.
 Crop diversification and irrigation planning considering climate change.

Infiltration for seasonal and spatial variation:
Seasonal variation: High infiltration in summer, low in rainy season.
Causes: In summer:
 High porosity of land allows runoff to infiltrate.
 Temperature is high, consequently the infiltration is high as well since
temperature influences viscosity.
 Surface is dry. Surface can absorb a lot of water.
 High value of soil moisture, soil cover and soil temperature causes high f
(infiltration capacity).
Result: high infiltration.
Causes: In rainy season:
 Low porosity of land.
 High compaction of wet surface particles.
 Temperature is not as high as summer.
 Surface is wet enough and impermeable.
 Low value of soil moisture, soil cover and soil temperature.
Result: Low infiltration.
Spatial Variation: It depends on-
 Land use patterns (e.g. cropping practice, surface covering)
 Major changes in land use (e.g. sudden deforestation)

8.  Land types (Varies in plain land, coastal area, slopes, mountain, ice covered area
etc.)
 Geographical position (high infiltration in equatorial area, low in northern and
southern region etc.).
 Other Spatial phenomena associated with infiltration (existence of water table,
drainage pattern influencing runoff etc.
Special case in storm: Storm creates high infiltration due to high precipitation and affecting
surface condition. According to a study Cerda (1999), a 40 minute storm in a slope increases the
infiltration rate from 72.5% to 74.3%. This is the reason of high infiltration in monsoon climate in
subcontinent since it is associated with a lot of climatic phenomena such as stormy weather and
high rainfall intensity.

9. Q: For the storm commenced at 7 hours. Cumulative rainfall at 15 min interval beginning at 7.00
hours are: 0, 9.5, 17, 27, 40.5, 49, 63, 84, 95, 102, 110, 112, and 112. Plot duration intensity
graph.

Answer:

Time Rainfall 15
m
30
m
45m 60m Re
me
dial
90m R
e
m
e
di
al
120
m
Rem
edial
180m
7 0
7.15 9.5 9.5
7.3 17 7.5 17
7.45 27 10 17.
5
27
8 40.5 13.
5
23.
5
31 40.5
8.15 49 8.5 22 32 39.5 49
8.3 63 14 22.
5
36 46 63
8.45 84 21 35 43.5 57 74.5 8
4

9 95 11 32 46 54.5 78 95
9.15 102 7 18 39 53 75 92.5 102
9.3 110 8 15 26 47 69.5 93 110
9.45 112 2 10 17 28 63 85 112
10 112 0 2 10 17 49 71.5 112
Highest Value (p) 21 35 46 57 78 95 112
Min to hour
conversion (t)
0.2
5
0.5 0.75 1 1.5 2 3
Intensity (p/t) 84 70 61.33 57 52 47.5 37.33

10. 0
10
20
30
40
50
60
70
80
90
15 30 45 60 90 120 180
Intesity (mm/hr)
Time in minute
Intensity-duration graph