Hydrologic Losses Infiltration, Evaporation and Transpiration Hydrology and Water Resources RSGIS Institute of Space Technology October 16, 2015
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 Zone of Aeration: Region in the ground in which pore spaces are filled with air.
Zone of Saturation: Region in the ground in which pore spaces are filled with water. Water table: The interface between the zone of saturation and the zone of aeration.
Infiltration: The downward movement of water through pore spaces of permeable rock or soil. Capillarity: The upward movement of water through very small pore spaces.
Infiltration Infiltration depends on Soil surface condition Soil moisture condition Vegetation providing channelization through roots Compaction, cracks, urbanization, etc.
Storm conditions Rainfall intensity and duration, etc. Underground condition Soil texture and transmission characteristics Storage available underground Others???
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 Infiltrometers Single Ring Double Ring
http://en.wikipedia.org/wiki/Infiltrometer Typical Infiltration Curve Sprinkler Infiltrometer Relating as much as possible to the actual basin conditions
Superimposition of Rainfall Hyetograph and Infiltration Curve Infiltration Indices The average infiltration rate is called the Infiltration Index 2 types of indices
index W index 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 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 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 Green-Ampt Model SCS Method
Horton's Model others Green-Ampt Method (1911) Based on Darcys law and unsteady continuity equation Then integrating the differential equations F(t) = Ks t + S ln [ 1 + F (t) / S ] S ln [ 1 + F (t) / S ln [ 1 + F (t) / S ]S ]
F (t) = cumulative infiltration at time t t (t) = rate of infiltration at time t Moisture deficit = S ln [ 1 + F (t) / S ] = s- i i = initial water content s= saturated water content Ks =Saturated hydraulic conductivity S = capillary suction And rate of infiltration
f (t) = Ks (1 + S S ln [ 1 + F (t) / S ] / F) Limitations Green-Ampt model applies only when rainfall intensity > infiltration rate For unsteady rainfall, G-A model applies as long as the rainfall variation is not large
The model would not yield correct results if there is excessive periods of low or zero rainfall. 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 = Hortons 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 . Estimating Evaporation
Water Budget (mass balance) Diffusion (mass transfer) Energy Budget
Field Measurement (Pan Evaporation) Water Budget Method
inflows outflows= Storage Measure inflows Determine all outflows other than Evaporation and change in storage Calculate Evaporation E = S + I + P O GW Where: S = 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 diameter, 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 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] Groundwater loss is difficult to measure and can be minimized by keeping the moisture condition of the plot at field capacity.
Evapotranspiration Equations Penmans Equation Based on energy balance and mass transfer approach
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