Hydrology
TOPIC 1
BASIC OF HYDROLOGY
The Concept of Hydrology
Discusses the hydrologic cycle, it’s processes, water balance, precipitation
types, estimation of precipitation, and analysis of precipitation data. Also methods of
measurement of stream flow, stage discharge relation, unit hydrograph theory,
Transposition of Hydrograph, Synthesis of hydrograph from basin characteristics,
stream flow routing, flood frequency analysis and attenuation of flood flows.
Emphasis is given towards the calculation of rain fall data and urban drainage concept
in developing new areas.
What Is Hydrology?
a. The study of water on, under, and over the Earth’s surface, and from its
origins to all its destinations on the earth is called hydrology.
b. The scientific study of water, seeking to explain the water balance equation
in terms of time and space, and assessing the impact of physical and
chemical processes and their role in ecosystems.
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Hydrology
Uses of Engineering Hydrology
Engineering Hydrology Helps in the following ways:

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
catchment’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

Used in prediction of flood over a spillway, at highway culvert or in urban
storm drainage

Used to assess the reservoir capacity required to assure adequate water for
irrigation or municipal water supply during drought

Hydrology is an indispensable tool in planning and building hydraulic
structures.

Hydrology is used for city water supply design which is based on
catchments area, amount of rainfall, dry period, storage capacity, runoff
evaporation and transpiration.
Branches of Hydrology
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Hydrology
Hydrological Cycle
Figure 1.2: Hydrologic Cycle
Refer to clause 2.2.1, Urban Stormwater Management Manual for Malaysia
(MASMA) vol. 1: Introduction to the Manual, the hydrologic cycle is the continuous,
unsteady circulation of water from the atmosphere to and under the land surface and
back to the atmosphere by various processes. It is dynamic in that the quantity and
quality of water at a particular location may vary greatly with time. Temporal variations
may occur in the atmosphere, on land surface, in surface waters, and in the
groundwater of an area. Within the hydrologic cycle, water may appear in all three of
its states; solid, liquid, and gas. Figure 1.2 shows the hydrologic cycle in schematic
form. The important processes are described below with emphasis on factors that
influence each process and its significance in the planning, design, and operation of
stormwater management systems (Walesh, 1989).
How the water cycle works
1. Solar energy heats up the oceans water surface, lake, river etc.
2. The water evaporates and rises into the air.
3. The vapor condenses into clouds and turns into rain.
4. Rain falls back to the surface. Some of rain infiltrates in soil.
5. Surface runoff makes its way into rivers and streams.
6. Rivers flow back into the ocean due to the force of gravity.
7. The cycle starts all over again.
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Hydrology
The Process in Hydrological Cycle
a. Evaporation
b. Condensation
c. Precipitation
d. Surface runoff,
e. interception
f. Transpiration
g. Infiltration
h. Sub-surface runoff
i. Sublimation
Evaporation
Evaporation is the process by which water is converted from its liquid form to
its vapor form and thus transferred from land and water masses to the atmosphere.
The rate of evaporation depends upon:

Wind speed: the higher the wind speed, the more evaporation

Temperature: the higher the temperature, the more evaporation

Humidity: the lower the humidity, the more evaporation
Condensation
The change of water from its gaseous form (water vapor) into liquid (water).
Condensation generally occurs in the atmosphere when warm air raises, cools and
looses its capacity to hold water vapor. As a result, excess water vapor condenses to
form cloud droplets.
Precipitation
Precipitation can occur primarily as rain. Annual amounts of precipitation are
unpredictable and variable, ranging from approximately 1500 mm to 4000 mm in
various locations in Malaysia. In essence, precipitation is the most important process
in the hydrologic cycle because it is the 'driving force' providing water that must be
accommodated in the urban environment.
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Hydrology
Surface runoff
Sometimes referred to as overland flow, is the process whereby water moves
from the ground surface to a waterway or water body. Urbanisation usually
dramatically increase surface runoff volume and rates.
Interception
Interception is the amount of precipitation that wets and adheres to
aboveground objects (primarily vegetation) until it is evaporated back into the
atmosphere. The annual amount of interception in a particular area is affected by
factors such as the amount and type of precipitation, the extent and type of
vegetation, and winds. Interception is not likely to be an important process in urban
stormwater management programs.
Transpiration
Transpiration is the process by which moisture is carried through plants from
roots to small pores on the underside of leaves, where it changes to vapor and is
released to the atmosphere. Transpiration is essentially evaporation of water from
plant leaves. Transpiration also includes a process called guttation, which is the loss
of water in liquid form from the uninjured leaf or stem of the plant, principally through
water stomata.
Environmental factors that affect the rate of transpiration
1. Light
Plants transpire more rapidly in the light than in the dark. This is largely
because light stimulates the opening of the stomata (mechanism). Light also
speeds up transpiration by warming the leaf.
2. Temperature
Plants transpire more rapidly at higher temperatures because water evaporates
more rapidly as the temperature rises. At 30°C, a leaf may transpire three
times as fast as it does at 20°C.
3. Humidity
The rate of diffusion of any substance increases as the difference in
concentration of the substances in the two regions increases.When the
surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.
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Hydrology
4. Wind
When there is no breeze, the air surrounding a leaf becomes increasingly
humid thus reducing the rate of transpiration. When a breeze is present, the
humid air is carried away and replaced by drier air.
5. Soil water
A plant cannot continue to transpire rapidly if its water loss is not made up by
replacement from the soil. When absorption of water by the roots fails to keep
up with the rate of transpiration, loss of turgor occurs, and the stomata close.
This immediately reduces the rate of transpiration (as well as of
photosynthesis). If the loss of turgor extends to the rest of the leaf and stem,
the plant wilts.
Infiltration
Infiltration is defined as the passage of water through the air-soil interface.
Infiltration rates are affected by factors such as time since the rainfall event started,
soil porosity and permeability, antecedent soil moisture conditions, and presence of
vegetation. Infiltration is a very important process in urban stormwater management
and, therefore, essentially all hydrologic methods explicitly account for infiltration.
Urbanisation usually decreases infiltration with a resulting increase in runoff volume
and discharge.
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Hydrology
Sub-surface runoff.
Interflow, sometimes referred to as subsurface stormflow, is the process
whereby water moves laterally beneath the land surface, but above the groundwater
table. Interflow occurs until water enters a waterway or water body, or is
evapotranspired. Interflow is affected by the same factors as those for surface runoff.
Interflow is rarely explicitly analyses; it is usually considered part of the surface runoff.
Surface runoff, interflow, and precipitation falling directly on water bodies are
sometimes lumped together and called direct runoff.
The Effect of Soils Use Toward Hydrological Cycle.
When development occurs, the resultant alterations to the land can lead to
dramatics changes to the hydrology or the way water is transported and stored,
Impervious man-made surfaces (asphalt, concrete, rooftops) and compacted earth
associated with development create a barrier to percolation of rainfall into the soil,
increasing surface runoff and decreasing ground water infiltration.
Effects of Urbanization on Stormwater (Ministry of Environment, 2006)
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Hydrology
If over 10% of a watershed
is convered by impervious
surfaces ,stream quality
may be moderately
impacted
Watersheds with over 25%
impervious surfaces have
severely impacted streams
Figure 1.3 : Relationships between impervious cover and surface runoff
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Hydrology
This disruption of the natural water cycle leads to a number of changes, including:
a) Increased volume and velocity of runoff
b) Increased frequency and severity of flooding
c) Peak (storm) flows many times greater than in natural basins
d) Loss of natural runoff storage capacity in vegetation, wetlands and soil
e) Reduced groundwater recharge
f) Decreased base flow (the ground water contribution to stream flow). This
can result in stream becoming intermittent or dry and also affects water
temperature.
The Hydrology Continuity Equation
Inputs can include:

Precipitation - rain;

Groundwater influx from an adjacent aquifer or a transboundary (trans-river
basin) aquifer;

Snow melt; and

Inter-basin transfer -(water transferred into the basin from an adjacent river
basin).
Extractions include:

Evaporation;

Transpiration;

Extraction for consumptive use from streams and rivers - water for industrial or
domestic use and irrigation;

Extraction for consumptive use from groundwater aquifers; and

Inter-basin transfer (water transferred out of the basin to adjacent river basin).
A simple approach to a water balance equation could be considered as (Wanielista et
al. 1997):
P + R + B - F - E -T = ΔS
Abbreviations:
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Hydrology
P = Precipitation
R = Runoff or excess rainfall
B = Subsurface flow
F = Infiltration
E = Evapotranspiration
T = Transpiration
S = Change in storage in the saturated zone - soil or groundwater
Inflow – Outflow
=
Change in Storage
I–O
=
ds/dt
I–O
=
∆S
P – DRO – E – T-G
=
∆S
=
∆S
@
P – ( R + ET + G)
Example;
1)
Kelantan's river catchment's expected to accept rain as much as 350 mm from
the beginning October 2003 to December 2003. Evaporation and infiltration
respectively was estimated at 35 mm and 25 mm in that time period. The
catchment’s area was 90 km2. There is a reservoir in these catchments.
Estimate runoff volume in m3 if level of reservoir unchanged.
Solution :
Given;
P = 350 mm ,
E = 35mm,
I = 25mm ,
A = 90km2
Hydrology equation balance:
Inflow – Outflow
=
Change in Storage
=
Change in Storage
P – ( E + I + DRO )
=
0
350 – ( 35 + 25 + DRO)
=
0
=
290 mm @ 0.29mm
ds/dt
( I – O)
DRO
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Hydrology
Volume of direct runoff, DRO =
0.29m x
( 90km2 x (1000m )2
(1km)2
=
26.1 x 106 m3.
Exercise:
1)
In 6 month period, Sungai Lui catchment’s were estimated will get rain as much
as 350 mm. Evaporation were estimated as much as 100 mm and infiltration to
subsurface were estimated at the 40 mm. Estimated the volume of runoff in
cubic meters (m3) that will be storage in reservoir if area for catchment was 85
km2.
2)
Hydrology record for a catchment's as wide as 500 km2 show excess rainfall
annual and average the surface runoff annual respectively was 90 cm and 33
cm. One reservoir as wide 1700 km2 had planned the construction in the outlet
part of catchment area. The annual evaporation average to that reservoir was
expected as much as 150 mm. Determine storage values that occur in that
reservoir.
3)
In a year, a wide rain catchment area is 103 km2 accept rain as much as 1000
mm/ year. Annual discharge of the river is 19 m3/s. Estimated
evapotranspiration to the catchment area.
4)
In period three months, Ketereh district are expected to receive rain as much
as 245 mm. evapotranspiration were estimate as 80 mm and diffusion to sub
surface as much as 20 mm. Wide of basin was 36 km2. Estimate :
– Excess rain depth
– calculate direct runoff volume
– If direct runoff may be stored in a reservoir, determine population
of people which can accept water supply for now if per-capita
daily utilizability was 200 liters.
5)
A storage pool has as much as water total saving 20 x 103 m3, in times that
been taken. Where discharge reading inflow and outflow is 10 m 3/ s and 15 m3/
s. After an hour later flow reading in and out change to 15 m 3/ s and 16 m3 / s.
Calculate water reserve change and water total saving that new after 1 hours.
6)
A catchment area as wide as 2.5 km2 accepts rainfall intensity 100 mm/ hour
for 6 hours. Run volume of water that noted in this period is 720,000 m 3. Get
rate of water loss from rain 6 hours.
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7)
8)
Hydrology
One reservoir 400 hectare expanse, produce evaporation as much as 50 cm in
24 hours. Expansion due to heavy rain into reservoir was in value 65 m 3/ s.
Determine hectare-meter deep water's volume that seeping reservoir policy on
that day if unchanged water level.
Catchment area in Kuala Krai has area 1720 km2. Annual average rainfall data
is 3200 mm. There are two rivers which flowed to that catchment area, namely
Sungai Kuala Nal and Sungai Krai. Discharge from Sungai Kuala Nal is 23m 3/s
while data from Sungai Krai not obtained. Record that made to show loss result
condensation process and bypass is 12% from average annual rainfall.
Calculate discharge value for Sungai Krai.
References
1. http://www.aboutcivil.com/uses-of-engineering-hydrology.html
2. http://msmam.com/msmam/chapter2/Ch2-EnvironmentalProcesses.html
3. http://techalive.mtu.edu/meec/module01/EvaporationandTranspiration.htm
4. http://www.poweredbymothernature.com/what-is-hydropower
5. http://www.kunenerak.com/en/river/hydrology/principles+of+hydrology/water+bala
nce.aspx
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