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20. Measurement of Evapotranspiration

Evapotranspiration encompasses the combined processes of evaporation and transpiration, playing a vital role in the hydrologic cycle and water resource management. Accurate measurement methods, both direct and indirect, include lysimeters, water balance, and remote sensing techniques, which provide insights into water balance, irrigation planning, and drought assessment. Various factors such as climate and plant characteristics influence evapotranspiration rates, which can be quantified using mathematical models and empirical methods.

Sections

Measurement of Evapotranspiration

Evapotranspiration is the process combining evaporation and transpiration, crucial for water resource management.

20 Section Overview

Start current section content and materials

20.1 Evapotranspiration – Basic Concepts

Evapotranspiration combines evaporation and transpiration processes, critical for water management and hydrology.

20.2 Factors Affecting Evapotranspiration

Multiple climatic, soil, and plant-related factors influence the process of evapotranspiration.

20.3 Methods of Measurement of Evapotranspiration

This section outlines both direct and indirect methods for measuring evapotranspiration, essential for effective water resource management.

20.3.1 Direct Methods

Direct methods measure evapotranspiration through actual field observations, providing precise data on water loss.

20.3.1.a Lysimeter Method

The lysimeter method involves using a specialized tank to measure evapotranspiration by monitoring changes in weight due to water loss.

20.3.1.b Field Experimental Plots

Field experimental plots utilize controlled water applications and observe resulting plant growth and soil moisture changes to calculate evapotranspiration.

20.3.2 Indirect Methods

Indirect methods utilize meteorological and empirical data to estimate evapotranspiration (ET).

20.3.2.a Water Balance Method

The Water Balance Method estimates evapotranspiration (ET) using a continuity equation that accounts for precipitation, runoff, deep percolation, and change in soil water storage.

20.3.2.b Energy Balance Method

The Energy Balance Method estimates evapotranspiration (ET) based on the conservation of energy principle, taking into account net radiation, soil heat flux, sensible heat flux, and latent heat flux.

20.3.2.c Aerodynamic Method

The Aerodynamic Method estimates evapotranspiration using wind speed and vapor pressure differences.

20.3.2.d Combination Method (Penman Equation)

The Combination Method, illustrated by the Penman Equation, integrates both energy balance and aerodynamic principles to estimate evapotranspiration effectively.

20.4 Empirical Methods

Empirical methods utilize well-established equations to estimate evapotranspiration when direct measurement data is absent.

20.4.1 Blaney-Criddle Method

The Blaney-Criddle Method is an empirical formula used to estimate evapotranspiration (ET) based on temperature and daylight hours.

20.4.2 Thornthwaite Method

The Thornthwaite Method estimates potential evapotranspiration (PET) based on mean monthly temperature and day length.

20.4.3 Hargreaves Method

The Hargreaves Method is a simplified approach for estimating evapotranspiration using temperature data, and is regarded as being more accurate than the Thornthwaite Method.

20.5 Measurement of Pan Evaporation

Pan evaporation measurements are crucial for estimating reference evapotranspiration using evaporation pans.

20.6 Remote Sensing and ET Estimation

Remote sensing offers advanced tools to estimate evapotranspiration (ET) using satellite-based sensors.

20.7 Use of Lysimeters in India

Lysimeters in India are critical tools used in agriculture to measure evapotranspiration, helping to develop crop coefficients for various agro-climatic zones.

20.8 Crop Coefficients and Reference ET (ET₀)

This section discusses the computation of reference evapotranspiration (ET₀) using crop coefficients and the significance of the FAO Penman-Monteith method.

Learning Objectives

  • Evapotranspiration is a key component of the hydrologic cycle.

  • Measurement of evapotranspiration is essential for effective water resource management.

  • Both direct and indirect methods can be employed to estimate evapotranspiration accurately.

Key Concepts

Evapotranspiration

The total water loss from both evaporation and transpiration processes.

Potential Evapotranspiration (PET)

The amount of evapotranspiration that occurs with an unlimited water supply.

Lysimeter

A device used to measure the amount of water lost through evapotranspiration.

Water Balance Method

A method of estimating evapotranspiration based on the continuity equation using precipitation, runoff, deep percolation, and soil water storage changes.

Crop Coefficient (Kc)

A factor that accounts for the specific crop's water use compared to reference evapotranspiration.

Practice Exercises

Total Questions

2

Estimated Time

4 min

Passing Score

70%

Instructions

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  • You can use hints if you need help
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