17.4.4 - Empirical Formulas
Enroll to start learning
You’ve not yet enrolled in this course. Please enroll for free to listen to audio lessons, classroom podcasts and take practice test.
Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Understanding Empirical Formulas
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Today we're discussing empirical formulas used for estimating evaporation. Can anyone tell me what an empirical formula is?
Is it a kind of formula that simplifies the process when we don't have all the data?
Exactly! These formulas help in estimation when we lack detailed meteorological data. Let's start with Meyer’s Formula. Who can explain it?
Meyer’s Formula is E = K (es – ea) (1 + u₉), where K is a constant based on the size of the water body.
Well done! Remember that K equals 0.36 for large deep water and 0.50 for small shallow water. That leads us to the significance of saturation and actual vapor pressures.
What are those vapor pressures, and how do they impact evaporation?
Great question! Saturation vapor pressure is the maximum pressure of water vapor in the air, while actual vapor pressure is the current state. The difference drives evaporation.
So if there's high saturation vapor pressure, evaporation is lower?
Correct. Let's summarize: Meyer’s Formula helps estimate evaporation by considering vapor pressures and wind speed at a specific height.
Exploring Rohwer’s Equation
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Now, let’s discuss Rohwer’s Equation. Who can provide the formula?
It's E = (0.771 + 0.06T) (es – ea) (1 + 0.1u).
Correct! T is important here; why do you think it’s included?
Probably because temperature affects the vapor pressure?
Exactly! Temperature directly influences saturation vapor pressure. This is key in estimating evaporation accurately.
What about wind velocity? How does that factor in?
Wind velocity increases evaporation by removing the damp air near the water surface. Remember that this formula helps us understand crucial relationships in your studies.
So it's quite practical for various conditions!
Exactly! Let's summarize. Rohwer’s Equation combines temperature and wind conditions to estimate evaporation effectively.
Applying the Blaney–Criddle Method
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Finally, we’ll discuss the Blaney–Criddle method. What do you know about it?
It's used for irrigation, right? E = p (0.46T + 8.13) summarizes it.
That’s correct! Why might p be significant in this formula?
P represents the percentage of annual daytime hours. Does that mean more sunlight increases evaporation?
Precisely! The more daylight hours there are, the higher the potential evaporation. Can anyone explain why temperature is multiplied here?
Because temperature affects how much water can evaporate?
Correct! Temperature plays a vital role in the water cycle. So let’s recap: the Blaney–Criddle method factors in both sunlight duration and temperature.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
This section discusses empirical formulas used for estimating evaporation from water bodies. It covers formulas like Meyer’s Formula, Rohwer’s Equation, and the Blaney–Criddle Method, which are applicable when limited data is available, and highlights their specific variables and context of use in water resource management.
Detailed
Empirical Formulas
Empirical formulas are critical tools in hydrology for estimating evaporation, particularly when comprehensive meteorological data is unavailable. The three main empirical formulas discussed in this section are:
Meyer’s Formula
Meyer’s Formula provides a way to estimate evaporation (E) based on the saturation vapor pressure ( s) and actual vapor pressure ( a) combined with the wind speed at a height of 9 meters (u₉):
E = K (es – ea) (1 + u₉)
- K varies depending on the water body's characteristics:
- K = 0.36 for large deep water bodies
- K = 0.50 for small shallow water bodies
Rohwer’s Equation
Rohwer's Equation refines estimation further by factoring in air temperature along with vapor pressures and wind velocity:
E = (0.771 + 0.06T) (es – ea) (1 + 0.1u)
- T is the air temperature in degrees Celsius; s and a are saturation and actual vapor pressures, respectively.
Blaney–Criddle Method
Mainly used in irrigation engineering, this method estimates evaporation using the mean daily percentage of annual daytime hours (p) and mean monthly temperature (T):
E = p (0.46T + 8.13)
These empirical methods are beneficial for water resource management, efficient irrigation, and water budget calculations, offering practitioners access to valuable tools when direct measurement data is scarce.
Audio Book
Dive deep into the subject with an immersive audiobook experience.
Introduction to Empirical Formulas
Chapter 1 of 4
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
Used when limited data is available. Examples include:
Detailed Explanation
Empirical formulas are mathematical relationships based on observed data, often used in situations where complete information may not be accessible. They allow scientists and engineers to estimate unknown parameters, such as evaporation rates, when only limited data is at hand. This section will cover three specific empirical formulas that can be utilized, each serving different conditions.
Examples & Analogies
Imagine trying to predict the dropping temperature outside based on your experiences from previous winters when you only have a few old records. Just like we use past experiences to guess future trends, empirical formulas use simplified assumptions based on previous observations to make estimations in science.
Meyer’s Formula
Chapter 2 of 4
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
a) Meyer’s Formula
E = K (es – ea) (1 + u₉)
- K = 0.36 for large deep waters, 0.50 for small shallow waters
- es, ea = saturation and actual vapor pressures
- u₉ = wind speed at 9 m height
Detailed Explanation
Meyer’s Formula is used to estimate evaporation (E) from water bodies based on the difference between saturation vapor pressure (es) and actual vapor pressure (ea), adjusted by wind speed at a specific height and a constant (K) that varies depending on the body of water. Specifically, larger and deeper bodies of water will have a different K value than smaller, shallower ones. This formula captures the relationship between environmental conditions and evaporation rates, making it useful in a variety of contexts.
Examples & Analogies
Think of Meyer’s Formula like a recipe for baking a cake. Just as you adjust the amount of flour depending on whether you're making a small cake or a large one, K helps adjust the formula based on water body size, allowing us to bake up an accurate estimation of evaporation!
Rohwer’s Equation
Chapter 3 of 4
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
b) Rohwer’s Equation
E = (0.771 + 0.06T) (es – ea) (1 + 0.1u)
- T = air temperature in °C
- es, ea = vapor pressures
- u = wind velocity in km/hr
Detailed Explanation
Rohwer’s Equation builds on the principles of Meyer’s Formula, incorporating air temperature (T) to adjust the evaporation estimate. It accounts for both the difference in vapor pressures and the wind velocity while also factoring in the ambient temperature, which influences how much moisture can be held in the air. The equation's components help in assessing conditions that affect evaporation more holistically.
Examples & Analogies
Imagine you're trying to determine how quickly a puddle will evaporate after a rain shower. If it’s warm outside (high T), the water will dry up faster, similar to how flavorful aromas are intensified when a dish is heated up. That's how Rohwer’s Equation shows the effect of temperature alongside other factors in the evaporation process.
Blaney–Criddle Method
Chapter 4 of 4
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
c) Blaney–Criddle Method
Used in irrigation engineering:
E = p (0.46T + 8.13)
- p = mean daily percentage of annual daytime hours
- T = mean monthly temperature in °C
Detailed Explanation
The Blaney–Criddle Method is particularly useful in irrigation engineering and provides a way to estimate evaporation based on mean daily percentage of annual daylight hours (p) and average monthly temperature (T). The formula emphasizes that both the amount of sunlight and temperature significantly influence evaporation, important for managing water resources in agricultural contexts.
Examples & Analogies
Consider how plants need both sun and warmth to thrive, similar to how they need water to grow. The Blaney–Criddle Method quantifies this relationship, just like a farmer decides when to water based on how sunny and warm it has been. It's a tool to optimize watering efficiently in agriculture.
Key Concepts
-
Empirical Formulas: Simplified equations for estimating evaporation.
-
Meyer’s Formula: An empirical formula using vapor pressures and wind speed to estimate evaporation.
-
Rohwer’s Equation: Incorporates air temperature, vapor pressures, and wind speed to refine evaporation estimation.
-
Blaney–Criddle Method: Used mainly for irrigation, this method estimates evaporation using temperature and daylight hours.
Examples & Applications
Using Meyer’s Formula, an engineer estimates evaporation from a small pond by measuring the current wind speed and vapor pressures.
Agriculturalists apply the Blaney-Criddle method during irrigation planning to assess water needs based on average monthly temperatures.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
To estimate the loss from waters wide, use Meyer’s sprightly guide!
Stories
Imagine you're a farmer checking the sun and temperature; with the Blaney-Criddle method, you know just how much water to give for thriving crops!
Memory Tools
Think of 'K-R-B' to remember K from Meyer’s, 'R' for Rohwer's, and 'B' for Blaney-Criddle.
Acronyms
M.E.R.B. = Meyer, Empirical, Rohwer, Blaney-Criddle (to remind about the formulas).
Flash Cards
Glossary
- Empirical Formula
A simplified mathematical equation used for estimation based on observed data.
- Meyer’s Formula
An empirical formula used for estimating evaporation involving vapor pressures and wind speed.
- Rohwer’s Equation
An empirical formula that includes air temperature along with vapor pressures and wind speed to estimate evaporation.
- Blaney–Criddle Method
An empirical method used primarily in irrigation engineering to estimate evaporation based on temperature and daylight hours.
Reference links
Supplementary resources to enhance your learning experience.