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19. Evapotranspiration

Evapotranspiration (ET) is a crucial component of the hydrologic cycle, integrating both evaporation and transpiration processes, influencing water demand and resource management. Understanding ET is essential for effective agricultural practices, particularly in regions with limited water resources, as it depends on various climatic, crop, and soil factors. The chapter explores methods for estimating ET, measurement techniques, and the impacts of climate change on ET dynamics.

Sections

Evapotranspiration

Evapotranspiration (ET) combines evaporation and transpiration processes, representing water loss to the atmosphere, crucial for water resource management.

19 Section Overview

Start current section content and materials

19.1 Evapotranspiration – Definition and Concept

Evapotranspiration (ET) comprises the combined processes of evaporation and transpiration, representing the total water loss from vegetated surfaces to the atmosphere.

19.2 Components of Evapotranspiration

Evapotranspiration consists of two main components: evaporation and transpiration, both of which are influenced by climatic and biophysical factors.

19.2.1 Evaporation (E)

Evaporation is a key component of evapotranspiration that refers to the physical loss of water from soil, plants, and water bodies to the atmosphere.

19.2.2 Transpiration (T)

Transpiration is a key process in the evapotranspiration cycle where plants release water vapor into the atmosphere, influenced by various botanical and environmental factors.

19.3 Factors Affecting Evapotranspiration

Evapotranspiration is influenced by various climatic, crop, soil, and management factors that collectively determine water loss from the atmosphere.

19.4 Potential and Actual Evapotranspiration

This section defines Potential Evapotranspiration (PET) and Actual Evapotranspiration (AET), highlighting the difference that conditions have on water loss in the atmosphere.

19.5 Methods of Estimating Evapotranspiration

Various methods are employed to estimate evapotranspiration (ET), classified into empirical, energy balance, and combination methods.

19.5.1 Empirical Methods

Empirical methods are used to estimate evapotranspiration (ET) through the analysis of climatological data and regression equations.

19.5.1.1 Blaney-Criddle Method

The Blaney-Criddle Method is an empirical approach to estimate potential evapotranspiration based on temperature and daylight hours.

19.5.1.2 Hargreaves Method

The Hargreaves Method is a straightforward temperature-based approach for estimating evapotranspiration (ET) using maximum and minimum temperature data along with extraterrestrial radiation.

19.5.1.3 Pan Evaporation Method

The Pan Evaporation Method estimates evapotranspiration based on measurements from a Class A evaporation pan, utilizing a pan coefficient to adjust values for various environmental conditions.

19.5.2 Energy Balance Methods

Energy Balance Methods for estimating evapotranspiration involve the conservation of energy at the land surface.

19.5.2.1 Energy Budget Equation

The Energy Budget Equation details the relationship between net radiation and heat fluxes at the land surface, critical for estimating evapotranspiration (ET).

19.5.3 Combination Methods

Combination methods for estimating evapotranspiration integrate various approaches to enhance accuracy.

19.5.3.1 Penman Method

The Penman Method combines energy balance and aerodynamic approaches to estimate evapotranspiration, providing a standard and accurate method for reference ET estimation.

19.5.3.2 FAO Penman-Monteith Method

The FAO Penman-Monteith Method provides a standardized approach for estimating reference evapotranspiration (ET) based on climatic data and vegetation characteristics.

19.6 Measurement Techniques for ET

This section discusses various measurement techniques for evapotranspiration (ET), including both direct and indirect methods.

19.6.1 Lysimeter

A lysimeter is a crucial device used for accurately measuring evapotranspiration (ET) by observing mass changes in a soil-plant column.

19.6.2 Atmometer (Evaporimeter)

The atmometer, or evaporimeter, is an instrument used to measure the rate of evaporation as a proxy for evapotranspiration.

19.6.3 Eddy Covariance System

The Eddy Covariance System is an advanced method for measuring evapotranspiration using high-frequency wind and humidity data.

19.6.4 Bowen Ratio Energy Balance Method

The Bowen Ratio Energy Balance Method is a technique used to measure evapotranspiration by analyzing temperature and humidity gradients.

19.6.5 Remote Sensing Techniques

Remote sensing techniques utilize satellite data to estimate evapotranspiration (ET) over extensive areas using various vegetation indices.

19.7 Crop Coefficient (Kc) Concept

The Crop Coefficient (Kc) is a vital parameter used to calculate the evapotranspiration (ET) of various crops based on their growth stage.

19.8 Applications of Evapotranspiration Data

Evapotranspiration data is crucial for various applications related to water management, agriculture, and climate studies.

19.9 Evapotranspiration under Changing Climate

This section discusses how climate change impacts evapotranspiration (ET) through changes in temperature, precipitation, and wind patterns.

Learning Objectives

  • Evapotranspiration is the total water loss from a vegetated surface due to evaporation and transpiration.

  • Factors influencing evapotranspiration include climatic conditions, crop characteristics, soil properties, and management practices.

  • Potential ET represents maximum losses under optimal conditions, while actual ET reflects the effects of varying moisture availability.

Key Concepts

Evapotranspiration

The total water loss from soil and vegetation due to the processes of evaporation and transpiration.

Potential Evapotranspiration (PET)

The maximum possible evaporation from a given area assuming optimal moisture conditions.

Actual Evapotranspiration (AET)

The actual measurement of evaporation that occurs under the prevailing conditions, usually less than PET.

Crop Coefficient (Kc)

A factor used to estimate ET of different crops based on crop type and growth stage.

Measurement Techniques

Methods to quantify ET, including lysimeters, atmometers, eddy covariance systems, and remote sensing.

Practice Exercises

Total Questions

2

Estimated Time

4 min

Passing Score

70%

Instructions

  • Read each question carefully
  • You can use hints if you need help
  • Complete all questions before submitting
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