7 and 8 Measurement of Transpiration and Evapotranspiration

Measurement of Transpiration

This document outlines the process of transpiration in plants and methods for its measurement, as well as related concepts like evapotranspiration.

Transpiration:

• Definition: Transpiration is the process by which plants release water vapor into the atmosphere from their leaves, stems, and trunks. It's a significant water loss mechanism, with up to 99% of absorbed water being transpired.  
• Mechanism:
  • Water is absorbed by roots and transported to leaves.  
  • During photosynthesis, stomata (pores) on leaves open to allow CO2 intake.  
  • Water vapor escapes through these stomata.
• Transpiration Ratio (T.R.):
  • T.R. = (Weight of water transpired) / (Weight of dry matter produced)
  • Typical T.R. values for crops range from 300 to 800.
• Factors Affecting Transpiration:
  • Temperature: Higher temperatures increase transpiration.  
  • Wind velocity: Increased wind speed enhances water vapor removal, thus increasing transpiration.  
  • Atmospheric pressure: Lower pressure (higher altitude) increases transpiration.  
  • Humidity: Higher humidity decreases transpiration.  
  • Solar radiation: Increased radiation increases transpiration.  
  • Soil moisture: Decreased soil moisture reduces transpiration.  
  • Physiological factors: Stomata density, leaf structure, and protective coatings influence transpiration.  

Measurement of Transpiration:

• Phytometer Method:
• A phytometer is a sealed, watertight container filled with soil and a plant.
• The phytometer is weighed initially and after a period of growth.
• The amount of water added during growth is recorded.
• Transpiration loss (T) is calculated as: T = (W1 + W) - W2, where:
  • W1 = Initial weight
  • W = Total water added
  • W2 = Final weight
• This is a laboratory measurement, and field adjustments are necessary.

Source: https://the-jena-experiment.de/index.php/trait-evolution/

Evapotranspiration:

• Definition: Evapotranspiration is the combined water loss through transpiration and evaporation from the soil surface.  
• Potential Evapotranspiration (PET): Evapotranspiration when water supply is unlimited.
• Actual Evapotranspiration (AET): Evapotranspiration under real conditions.
• Factors Affecting Evapotranspiration:
  • Meteorological factors (temperature, wind, humidity, solar radiation).  
  • Vegetation density.
  • Soil moisture.
  • Plant growth stage.
  • Adjoining land conditions.
  • Leaf surface color.

Measurement of Evapotranspiration:

• Direct Measurement Methods:
• Tank and Lysimeter Method:
  • Tanks: Measure water supplied to maintain plant growth.
  • Lysimeters: Measure water supplied minus drainage.
• Field Experimental Plots: Measure water applied with adjustments for runoff.
• Soil Moisture Studies: Track changes in soil moisture content.
• Integration Method: Sum evapotranspiration from different land types.
• Inflow and Outflow Studies: Calculate water balance for large areas.

Source: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lysimeters

• Use of Empirical Formulae:
  • Blaney-Criddle Equation: Uses temperature and daylight hours.
  • Thornthwaite Equation: Uses temperature and heat index.  
  • Penman's Method: Combines energy balance and wind transfer.