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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.
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.
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.
Simplified equations for estimating evaporation.
An empirical formula using vapor pressures and wind speed to estimate evaporation.
Incorporates air temperature, vapor pressures, and wind speed to refine evaporation estimation.
Used mainly for irrigation, this method estimates evaporation using temperature and daylight hours.
To estimate the loss from waters wide, use Meyer’s sprightly guide!
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!
Think of 'K-R-B' to remember K from Meyer’s, 'R' for Rohwer's, and 'B' for Blaney-Criddle.
M.E.R.B. = Meyer, Empirical, Rohwer, Blaney-Criddle (to remind about the formulas).
Term
Meyer’s Formula
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Rohwer’s Equation
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Blaney–Criddle Method
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A simplified mathematical equation used for estimation based on observed data.
An empirical formula used for estimating evaporation involving vapor pressures and wind speed.
An empirical formula that includes air temperature along with vapor pressures and wind speed to estimate evaporation.
An empirical method used primarily in irrigation engineering to estimate evaporation based on temperature and daylight hours.
Empirical formulas provide simplified methods for estimating evaporation when meteorological data is limited.
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.
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 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)
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)
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.