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The hydrologic budget for a catchment in a given duration can be written as
•This water budget can be used to estimate AET, if other terms are known or measured or estimated.
Measurement of ET
For a given type of vegetation, ET can be measured
– Using Lysimeters
– From Field Experimental Plots
– From Soil Moisture Depletion Studies etc
Lysimeters
• A lysimeter is a special water tight tank containing soil and set in a cropped field (so buried that the level of soil is the same both inside and outside the container). The same type of plants as in the surrounding field are grown in a lysimeter. The soil in the lysimeter along with the vegetation in it is hydrologically isolated from the surrounding soil. Lysimeters shall be designed so as to accurately reproduce the soil and soil moisture conditions, type and size of vegetation etc of the surrounding area.
• Lysimeter studies involve growing crops in large containers and measuring the water losses and gains. ET can be estimated by determining the amount of water required to maintain constant soil moisture conditions within the tank.
• Types of Lysimeters
– Weighing Type
– Non-weighing Type
• Limitations of Lysimeters
– Reproduction of physical conditions in field (say, temperature, water table position, soil texture, density etc)
Field Experimental Plots
The different elements of the water budget (other than ET) in a known interval of time are measured in special experimental plots established in the field. ET is then estimated as
ET = Precipitation + Irrigation Input – Runoff – Increase in Soil Moisture Storage – Groundwater Loss
• Since groundwater loss due to deep percolation is difficult to measure, it is minimised by maintaining the soil moisture condition in the plot at field capacity.
• This method provides fairly reliable results.
Evapotranspiration Equations
• A number of methods are available to estimate the potential evapotranspiration (PET) using climatological data
• These formulae range from those backed by theoretical concepts to purely empirical methods
• Penman’s equation is based on sound theoretical reasoning and is obtained from a combination of the energy balance and mass transfer approach
• A modified form of the original Penman equation is discussed here
Penman’s Equation
![clip_image002[4]](https://test.theconstructor.org/wp-content/uploads/2010/10/clip_image00241.jpg "clip_image002[4]")
![clip_image002[6]](https://test.theconstructor.org/wp-content/uploads/2010/10/clip_image00261.jpg "clip_image002[6]")
| Surface | Range of r values |
| Close ground corps | 0.15 to 0.25 |
| Bare lands | 0.05 to 0.45 |
| Water surface | 0.04 |
| Snow | 0.45 to 0.95 |
![clip_image002[8]](https://test.theconstructor.org/wp-content/uploads/2010/10/clip_image00281.jpg "clip_image002[8]")
• To compute PET, data pertaining to 
, mean air temperature, and nature of the surface(ie. the value of albedo) are needed
• These are obtained from actual measurements or using the available meteorological data of the region
• Penman’s equation can be used to compute evaporation from an open water surface by putting r=0.05 in the above equation
Empirical Formulae
• A large number of empirical formulae are available to estimate PET, using climatological data
• These formulae are not universally applicable to all climatic zones
• To be used with caution
• Blaney-Criddle and Thornthwaite Formulae
Blaney-Criddle Formula
• A purely empirical formula developed based on data from arid Western US
• Assumes that PET is related to the hours of sunshine and temperature (these are measures of solar radiation in an area)
• PET (in cm) in a crop growing season
Thornthwaite Formula
• Developed from data of Eastern US
• Uses only mean monthly temperature along with an adjustment for day length
![clip_image002[10]](https://test.theconstructor.org/wp-content/uploads/2010/10/clip_image002101.jpg "clip_image002[10]")