Hydrologic Losses
Infiltration, Evaporation and
Transpiration
Hydrology and Water Resources
RSGIS
Institute of Space Technology
October 11 & 23, 2013
Infiltration, Percolation, Seepage
• Infiltration
– Water moving into soil by crossing the soil surface
• Percolation
– Water moving through soil (movement within the
soil profile)
• Seepage
– Water moving out of soil
Infiltration
• Infiltration depends on
– Soil type
– Soil moisture condition
– Vegetation providing channelization through roots
– Others???
Estimation of Infiltration
• Infiltration Capacity
– The maximum rate at which water may infiltrate into a
ground is called the infiltration capacity
– Differs soil to soil
– also different for the same soil in its moist and dry states
Measurement of Infiltration
1. Ring Infiltrometer
2. Double Ring Infiltrometer
3. Sprinkler Infiltrometer
Ring Infiltrometer
• Constant water level (head) in the ring is maintained
by adding water to compensate the infiltration losses
• Infiltration losses can be measured by keeping track
of added water during time interval
• Problem of water leaving out of edges (use of double
ring infiltrometer)
Double Ring Infiltrometer
• Water tables in both rings are maintained
• Only track the water added in the inner ring
• More representative of actual vertical
infiltration rates
Another view By Das & Saikia
Typical Infiltration Curve
Sprinkler Infiltrometer
• Relating as much as possible to the actual
basin conditions
Superimposition of Rainfall Hyetograph
and Infiltration Curve
October 23, 2013
Infiltration Indices
• The average infiltration rate is called the
Infiltration Index
• 2 types of indices
– φ – index
– W – index
φ – index
• Rate of infiltration above which the rainfall volume
equals runoff volume
• Depends on soil type, vegetative cover, initial
moisture conditions, storm duration, rainfall
intensity, and time of the year
• If rainfall intensity is less than φ then the infiltration
rate is equal to the rainfall intensity
• The amount of rainfall in excess of the index is called
rainfall excess or effective rainfall
φ – index
Example
volume
3
Conti….
• Assume a value of phi index and calculate
runoff from graph. If runoff from graph is
equal to calculated runoff then phi value is
correct. Otherwise assume another value of
phi index and redo calculation till runoff from
graph equals runoff calculated.
W – index
• Average infiltration rate during the time when the
rainfall intensity exceeds the infiltration rate
W = Total Infiltration/ time during which the rainfall intensity exceeds
the infiltration rate
Total infiltration = Total precipitation – Surface runoff – Initial losses
• Total infiltration is expressed as depth of water
Approximate Equations: Calculating Infiltration
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Green-Ampt Model
SCS Method
Horton's Model
others
Green-Ampt Method (1911)
• Based on Darcy’s law and unsteady continuity
equation
• Then integrating the differential equations
F(t) = Ks t + ∆θ S ln [ 1 + F (t) / ∆θS ]
f (t) = Ks (1 + S∆ θ / F)
F (t) = cumulative infiltration at time t
t (t) = rate of infiltration at time t
Moisture deficit = ∆θ = θs- θi
θi = initial water content
θs= saturated water content
Ks =Saturated hydraulic conductivity
S = capillary suction
Limitations
• Green-Ampt model applies only when rainfall
intensity > infiltration capacity
• For unsteady rainfall, G-A model applies as
long as the rainfall variation is not large
• others
SCS Method
Horton's Model (1940)
• Considering exponential behavior of infiltration
• Following equation developed for finding rate
curve of infiltration capacity f(t) at any time t
f(t) = fc + (f0 + fc)e-kt
• f0 = infiltration rate just at the beginning of rainfall (mm/hr)
• fc = steady state infiltration capacity (mm/hr)
• k = Horton’s infiltration constant dependent on vegetal cover
and type of soil (hr-1)
Horton's Infiltration Curve
Typical Values of Parameters Used in
Horton's Equation
Evapotranspiration
• Evaporation:
– Change of liquid to vapor from soil or water surface
• Transpiration:
– Change of liquid to vapor from plant
• Evapotranspiration= Evaporation + Transpiration
Evaporation
• Depends on:
– Solar radiation
– Temperature of water and air
– Difference in vapor pressure between water and
the overlying air
– Wind speed
– others
Estimating Evaporation
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Water Budget (mass balance)
Diffusion (mass transfer)
Energy Budget
Field Measurement (Pan Evaporation)
Water Budget Method
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Σ inflows – Σ outflows= Δ Storage
Measure inflows
Determine all outflows other than Evaporation
and change in storage
Calculate Evaporation
E = – ΔS + I + P – O – GW
Where:
– ΔS = change in storage
O = surface Outflow
– I = surface Inflow
P = Precipitation
– GW= subsurface seepage to groundwater
Open Pan Evaporation
• Simple and inexpensive
• Cylindrical vessel of 1.2 to 1.8 m dia, 0.3 m high with
open top made of galvanized iron
• Depth of evaporation during any time interval is
measured as the drop in water level in the pan
(corrected for precipitation)
• Water level is maintained in the pan
• Observation taken on daily basis
• Pan reading may be different from actual reading
because of the differences in temperatures and vapor
pressures between pan water and lake water
• Actual evaporation = pan coefficient x pan evaporation
Factors Affecting Transpiration
• Plant Factor
– Efficiency of root system to absorb moisture
– Leaf area and structure
• Soil Factor
– Amount of moisture in soil
• Climate Factor
– Solar radiation
– Atmospheric pressure, temperature and wind
– Etc.
Evapotranspiration (ET)
• ET rates depends on
– Humidity (ET increases with decrease in Humidity)
– Temperatures
– Sunshine
– Wind
– Moisture available
– Vegetation type and coverage
Potential Evapotranpiration (PET)
• For given atmospheric conditions, it is the
maximum ET rate possible
• The real evapotranspiration occurring in a
specific situation is called Actual
Evapotranspiration (AET)
• AET < PET
• Field Capacity: maximum quantity of water
that soil can retain against the force of gravity
• Permanent Wilting Point: moisture content of
soil at which the moisture is no longer
available in sufficient quantity to sustain the
plants
• Both depends on soil characteristics
• The difference between these two moisture
contents is called ‘available water’- the
moisture available for plant growth
Measurement of Evapotranspiration (Field
Methods)
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Water Budget Method
Soil Moisture Depletion studies
Lysimeters
Field Plots
Using remotely sensed data from satellites to
evaluate ET over vast areas (SEBAL Model –
Energy Balance)
Soil Moisture Depletion Studies
• Soil moisture depletion studies are made
• Large number of samples are taken from various depths in the
roots zone through out the growth period of a crop
M1i and M2i are soil moisture % in the ith layer at the time of 1st
and 2nd sampling respectively
Gi = apparent specific gravity of ith layer of the soil
Di = depth of ith layer of the soil (mm) within the root zone
n = number of soil layers considered in the entire root zone
Lysimeters
• Special watertight tank containing block of soil
• Set into the field of growing plants
• Plants growing inside the Lysimeter are same
as growing in the surrounding
• Evapotranspiration is estimated in term of
amount of water required to maintain
constant moisture conditions in the tank
Field Plots
• In special plots, all elements of water budget
are measured and Evapotranspiration is
calculated
ET = [Precipitation+ Irrigation Input – runoff –
increase in soil storage groundwater loss]
Evapotranspiration Equations
• Penman’s Equation
– Based on energy balance and mass transfer
approach