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21. Evapotranspiration Equations: Penman and Blaney & Criddle Methods

Evapotranspiration is a crucial process in the hydrological cycle, combining evaporation and transpiration, significantly influencing water resource management. The Penman and Blaney & Criddle methods are key techniques for estimating evapotranspiration, with distinct applications based on data availability and climate conditions. Understanding these methods is essential for effective irrigation scheduling, water resource planning, and hydrological modeling.

Sections

Evapotranspiration Equations: Penman and Blaney & Criddle Methods

This section covers the essential concepts of evapotranspiration (ET) and introduces the Penman and Blaney & Criddle methods for estimating ET.

21 Section Overview

Start current section content and materials

21.1 Concept of Evapotranspiration

Evapotranspiration (ET) is the combined process of evaporation and transpiration, essential for understanding water loss in hydrological systems.

21.2 Penman Method

The Penman Method estimates potential evapotranspiration (PET) using a combination of energy balance and aerodynamic principles.

21.2.1 Introduction to Penman Equation

The Penman Equation estimates potential evapotranspiration (PET) using energy balance and aerodynamic principles, making it essential for accurate water resource management.

21.2.2 Components of Penman Equation

This section details the key components of the Penman Equation used for estimating potential evapotranspiration (PET), specifically net radiation, aerodynamic terms, and temperature/vapor pressure parameters.

21.2.2.a Net Radiation (Rn)

Net Radiation (Rn) is the balance of incoming and outgoing radiation at a crop surface and is crucial for estimating potential evapotranspiration.

21.2.2.b Aerodynamic Term

The aerodynamic term plays a crucial role in the Penman equation for estimating potential evapotranspiration, accounting for wind speed and vapor pressure deficits.

21.2.2.c Temperature and Vapor Pressure Parameters

This section discusses the computation of saturation vapor pressure and actual vapor pressure in relation to temperature and humidity.

21.2.3 Advantages and Limitations of Penman Method

The Penman Method for estimating potential evapotranspiration (PET) has significant advantages, including its accuracy under varied conditions, yet it also presents limitations such as the requirement for detailed meteorological data.

21.3 Blaney and Criddle Method

The Blaney and Criddle method is an empirical approach for estimating crop evapotranspiration based on temperature and daylight hours.

21.3.1 Introduction

The Blaney and Criddle method estimates seasonal crop evapotranspiration using temperature and daylight hours, making it suitable for regions with limited climatic data.

21.3.2 Blaney and Criddle Equation

The Blaney and Criddle Equation estimates crop evapotranspiration based on temperature and daylight hours.

21.3.3 Calculation of the p Factor

The p factor is crucial for estimating crop evapotranspiration in the Blaney and Criddle method, calculated based on daylight hours and monthly considerations.

21.3.4 Crop Coefficient (k)

The crop coefficient (k) is a dimensionless value that varies based on crop type, growth stage, and climatic zone, essential for calculating evapotranspiration.

21.3.5 Limitations of Blaney and Criddle Method

The Blaney and Criddle method has significant limitations, including its inability to consider humidity, wind speed, or radiation, making it less accurate in regions with high climatic variability.

21.4 Comparison of Penman and Blaney & Criddle Methods

This section provides a comparison between the Penman and Blaney & Criddle methods for estimating evapotranspiration, highlighting their differences in approach, data requirements, and applicability.

21.5 Applications in Civil Engineering

This section discusses the key applications of evapotranspiration methods in civil engineering, focusing on their importance in irrigation scheduling, water resource planning, urban hydrology, climate impact studies, and drought analysis.

Learning Objectives

  • Evapotranspiration is the total loss of water from soil through evaporation and transpiration.

  • The Penman Method combines energy balance and aerodynamic principles for estimating potential evapotranspiration (PET).

  • The Blaney & Criddle Method provides a simpler empirical approach suitable for regions with limited climatic data.

Key Concepts

Evapotranspiration (ET)

The combined process of evaporation from soil and water surfaces and transpiration from plant surfaces.

Potential Evapotranspiration (PET)

The evapotranspiration that would occur if water is abundantly available.

Actual Evapotranspiration (AET)

The evapotranspiration that actually occurs, considering soil moisture limitations.

Penman Method

A physically-based approach to estimate potential evapotranspiration using energy balance and aerodynamic principles.

Blaney & Criddle Method

An empirical method for estimating evapotranspiration based on temperature and daylight hours, suitable for regions with limited data.

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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