Hydrologic LossesInfiltration, Evaporation and

Transpiration

Hydrology and Water Resources RSGIS

Institute of Space TechnologyOctober 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 Infiltrometer2. Double Ring Infiltrometer3. 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

3volume

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 exceedsthe infiltration rate

Total infiltration = Total precipitation – Surface runoff – Initial losses

• Total infiltration is expressed as depth of water

Approximate Equations: Calculating Infiltration

• 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 equationsF(t) = Ks t + ∆θ S ln [ 1 + F (t) / ∆θS ]

f (t) = Ks (1 + S∆ θ / F)F (t) = cumulative infiltration at time tt (t) = rate of infiltration at time tMoisture deficit = ∆θ = θs- θi

θi = initial water content θs= saturated water contentKs =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 tf(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

• Water Budget (mass balance)• Diffusion (mass transfer)• Energy Budget• Field Measurement (Pan Evaporation)

Water Budget Method

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)

• 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 soilDi = depth of ith layer of the soil (mm) within the root zonen = 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