Abstractions from Precipitation

Dr. Rallapalli Srinivas, Department of Civil Engineering

Engineering Hydrology, CE F321
Learning outcome
• Evaporation
• Evapotranspiration
• Infiltration
• Interception and depression losses
• Introduction to streamflow measurement
 Evaporation
• The process by which water is transferred from land and
water masses of the earth to the atmosphere
• There is a continuous exchange of water molecules between
an evaporating surface and its overlying atmosphere
• Evaporation is the net rate of vapour transfer, it depends on
(i) Vapor pressure at the water surface and air above
(ii) Air and water temperatures
(iii) Wind speed
(iv) Quality of water
(v) Size of water body
Estimation of evaporation
• Evaporimeters are water containing pans which are exposed
to the atmosphere and the loss of water by evaporation is
measured at regular intervals
CLASS A Evaporation Pan
Water depth is maintained between 18 cm and 20 cm.
Normally made of unpainted galvanized iron sheet
Evaporation is measured by measuring the depth of water with
a hook gauge
 Pan coefficient Cp
• Evaporation pans are not exact models of large reservoirs
and have drawbacks:
- Pans differ in heat storing capacity and heat transfer from the sides
and bottom.
- Heat transfer characteristics of pan material is different from that of
the reservoir
- Needs correction
Lake evaporation = Cp x Pan evaporation, where Cp is pan
coefficient
Evaporation stations
• Arid zones: One station for every 30,000 km2
• Humid temperature climates: One for every 50,000
km2
• Cold regions- One for every 100,000 km2
India has 220 pan-evaporimeter stations
 Empirical evaporation equations
• Dalton-type equation

• Meyer’s formula

• Rohwer’s formula
Numerical
Analytical method of evaporation estimation

• Water budget method

• Energy balance
• Mass-transfer method
Water budget method
Hydrologic continuity equation

All quantities are either in mm or m3

Reservoir evaporation
Volume of water lost through evaporation from a
reservoir in a month is calculated as

Evaporation from a water surface is a continuous process. In India,

evaporation loss from a water body is about 160 cm in a year, quantity of
water lost is considerable.
Indian reservoirs and water lost
Methods to reduce evaporation losses
1. Reduction in Surface Area
2. Mechanical covers
3. Chemical films
Evapotranspiration
Transpiration: Process by which water leaves the
body of a living plant and reaches the atmosphere as
water vapor. Water is taken up by the plant system
and escapes through leaves.
It is affected by atmospheric vapor pressure,
temperature, wind. Light intensity and characteristics
of plant such as root and leaf systems
Major difference between evaporation and
transpiration: Transpiration is essentially confined to
day-light hours and evaporation continues all
throughout the day and night
Evapotranspiration
• Evaporation + Transpiration
• In hydrology and irrigation practice,
both transpiration and evaporation
processes can be considered
advantageously under one head as
evapotranspiration
• Consumptive use: Consumptive use
is the sum of water need of crop
and volume of transpirated in  a
specific time
• Knowledge of consumptive use
helps determine irrigation
requirement at the farm which
should, obviously, be the difference
between the consumptive use and
the effective precipitation.
Evapotranspiration

• Depends on availability of water

• Potential evapotranspiration (PET): If sufficient
moisture is always available to completely meet
the needs of vegetation fully covering the area
• Actual evapotranspiration: Real
evapotranspiration occurring in a specific
situation
• Field capacity (FC): Maximum quantity of
water the soil can retain against the force of
gravity
• Permanent wilting point (PWP): Moisture
content of a soil at which the moisture is no
longer available in sufficient quantities to
sustain the plants.
 Evapotranspiration
• FC and PWP depends on soil characteristics
• FC-PWP = Water available for plant growth
• If water supply to the plant is adequate, soil moisture will be at field
capacity, AET = PET
• If water supply is less than PET, soil dries out and the ratio of AET/PET
< 1.
 Field capacity and Saturation
• To understand field capacity, it is important to first
understand how water moves through soil.
• When soil is initially saturated with water, excess water
begins to drain away due to the force of gravity. This process
is called drainage.
• As water drains from the soil, it leaves behind a layer of
moist soil with a relatively stable water content. This is the
soil at field capacity.
• Saturation is the soil water content when all pores are filled
with water. The water content in the soil at saturation is
equal to the percent porosity. Field capacity is the soil water
content after the soil has been saturated and allowed to drain
freely for about 24 to 48 hours.
 Field capacity and Saturation
• Field capacity is the water content of soil after excess water has
drained away, but the soil is still moist. Soils at field capacity
contain the maximum amount of water that can be held against the
force of gravity, and this water is available for plant roots to uptake.
• Saturation, on the other hand, occurs when the soil is completely
saturated with water and no more can be held. At this point, water
starts to drain away, and soil aeration is reduced, which can have
negative effects on plant growth.
• Free drainage occurs because of the force of gravity pulling on the
water. When water stops draining, we know that the remaining water
is held in the soil with a force greater than that of gravity. Permanent
wilting point is the soil water content when plants have extracted all
the water they can. At the permanent wilting point, a plant will wilt
and not recover.
 How both are different?
• Not all of the pores in the soil are filled with water when
the soil has reached its field capacity.
• In soil, water is held in the pores between soil particles.
When the soil is at field capacity, some of these pores are
filled with water, while others contain air. The exact
distribution of water and air in the soil depends on the soil
type, texture, structure, organic matter content, and bulk
density, among other factors.
• It is important to note that field capacity is not a constant
value, as it can vary depending on the soil and
environmental conditions. For example, soil that is heavily
compacted may have a lower field capacity than soil that is
well-aerated.
What happens in transmission zone
• In the soil transmission zone, the soil moisture content
is greater than field capacity, but less than saturation,
meaning that there is a sufficient amount of water
available for plant roots to uptake, but the soil is not
saturated and the soil structure is not negatively
impacted. This is an important factor for healthy plant
growth and overall soil health.

• In general, it is important to maintain soil moisture

content at or near field capacity for optimal plant
growth, as soil that is too dry or too wet can have
negative impacts on plant health and growth.
Important terms
• Gravitational water refers to the amount of water
held by the soil between saturation and field
capacity. Water holding capacity refers to the
amount of water held between field capacity and
wilting point. Plant available water is that portion of
the water holding capacity that can be absorbed by a
plant.
 Sponge example
• A sponge is just like the soil
• Place it under water in a dishpan, and allow it to soak up as much
water as possible. At this point, the sponge is at saturation.
• Carefully support the sponge with both hands and lift it out of the
water. When the sponge stops draining, it is at field capacity, and
the water that has freely drained out is gravitational water.
• Now, squeeze the sponge until no more water comes out. The
sponge is now at permanent wilting point, and the water that was
squeezed out of the sponge is the water holding capacity. About
half of this water can be considered as plant available water.
• You may notice that you can still feel water in the sponge. This is
the unavailable water.
Measurement of evapotranspiration
• Lysimeters

https://www.youtube.com/watch?
v=mMPxPt7fxPQ
Evapotranspiration equations
• Using climate data
• Penman’s equation
Penman’s equation
Penman’s equation
Penman’s equation
Penman’s equation
Numerical
Reference crop evapotranspiration
Blaney-Criddle Formulae
Blaney-Criddle Formulae
Numerical
Thornthwaite Formula
Initial losses
(i) Interception process
(ii) Depression storage

Initial loss represents the quantity of storage that

must be satisfied before overland runoff begins.
Interception
• Part of rainfall caught by vegetation and
subsequently evaporated
Depression Storage
• When the precipitation of a storm reaches the ground, it
must first fill up all depressions before it can flow over
the surface, the volume of water trapped in these
depressions is called depression storage
• Eventually lost to runoff through processes of
infiltration and evaporation
• Depends on
1. Type of soil
2. Condition of surface reflecting the amount and nature of
depression
3. Slope of catchment
4. Soil moisture
Infiltration
• Flow of water int the ground through the soil
surface
• The distribution of soil moisture within the soil
profile during the infiltration is given
Infiltration
model

•Infiltration capacity:
Maximum rate at which
ground can absorb water
•Field capacity: The
volume of water that
ground can hold
Infiltration capacity
• Maximum rate at which ground can absorb water
(fp)
• Units are cm/h

• fp is dependent on large number of factors

1. Characteristics of soil
2. Surface of entry
3. Fluid characteristics
Measurement of Infiltration
• Flooding type infiltrometers
1. Simple infiltrometer
2. Double ring infiltrometer
3. Modified Philip-Dunne (MPD) infiltrometer
• Rainfall simulator
• Hydrograph analysis
Modelling Infiltration Capacity
• Horton’s Equation
Modelling Infiltration Capacity
• Green-Ampt Equation
Estimation of Parameters of infiltration models
Numerical
Homework problem
• Values of infiltration capacities at various times
obtained from an infiltration test are given below.
Determine the parameters of Green-Ampt equation.
 Classification of infiltration capacities
• Steady state infiltration capacity is divided into four
infiltration classes as given below:

• In hydrologic calculations involving floods, it is found

convenient to use a constant value of infiltration rate for
the duration of the storm, the defined average infiltration
rate is called infiltration index.
 Φ- Index
• Average rainfall above which the rainfall volume is equal to runoff
volume
• Derived from rainfall hyetograph
• Initial loss is also considered infiltration
• Φ value is found by treating it as a constant infiltration capacity
• Constant infiltration rate that yields ERH with total depth = depth of
direct rainfall
• If rainfall intensity is less than Φ then the infiltration rate is equal to the
rainfall intensity, otherwise difference between rainfall and infiltration
represents surface runoff volume
• Rainfall excess: Amount of rainfall in excess to Φ index. Or the rainfall
that contributes to surface runoff.
• Effective rainfall: Rainfall excess + Lateral flow
• Obtained using hyetograph
Φ- Index

, where is time when rainfall intensity is > index

, no runoff
W-index
Numerical
• The rainfall in the three successive 8h periods is 1.6,
5.4 and 4.1 cm. If the initial loss is 0.6 cm and
surface runoff resulting from the storm is 4.7cm.
Find the Φ- Index and w-index.
Homework
Stream flow measurement
• Area velocity method
• Acoustic doppler current profiler

• https://www.youtube.com/watch?v=VgsMzlWfboQ
• https://www.youtube.com/watch?v=qTfRfi5HQQk
• https://www.youtube.com/watch?v=xRVagxn6Hho
THANK YOU
Q&A