FLUXNET15 – how to convert latent heat flux to actual evapotranspiration?

Decoding Evapotranspiration: Turning FLUXNET15 Data into Real-World Water Loss

Evapotranspiration (ET)—sounds like something out of a sci-fi movie, right? But it’s actually the unsung hero of the water cycle, the combined process of water evaporating from surfaces and plants “breathing” it out. Knowing how much water is moving this way is super important, whether you’re trying to manage irrigation on a farm or build better climate models. That’s where FLUXNET15 comes in. Think of it as a global network of weather stations on steroids, tracking how ecosystems exchange carbon, water, and energy with the atmosphere . One of the key things FLUXNET15 measures is latent heat flux (LE). But what is that, and how do we turn it into something useful like actual evapotranspiration? Let’s break it down.

Latent Heat Flux and Evapotranspiration: A Match Made in Hydrology Heaven

Latent heat flux (LE) is basically the amount of energy it takes to turn liquid water into vapor . Evapotranspiration (ET) is that process of water changing state, whether it’s evaporating from a puddle or being released by a plant. So, LE is just ET expressed as energy—think of it as the “energy cost” of evapotranspiration. The connection is actually pretty straightforward:

ET = LE / λ

In plain English:

• ET is how much water is leaving the surface (usually measured in kg per square meter, or even simpler, millimeters).
• LE is the energy used in that process (measured in mega joules per square meter).
• λ (that’s lambda) is the latent heat of vaporization – we’ll get to that in a sec.

Basically, you divide the energy used by the “energy cost” per unit of water, and boom, you’ve got evapotranspiration! This conversion relies on the assumption that water’s density is around 1000 kg/m3.

Cracking the Code of Lambda: The Latent Heat of Vaporization (λ)

Okay, so what’s this “latent heat of vaporization” all about? It’s the energy needed to turn a kilogram of water from liquid to vapor without changing its temperature . Seems simple enough, but here’s the catch: it’s not exactly constant. It changes a little bit with temperature.

A common shortcut is to use 2.45 MJ kg-1, which works well around 20°C. But if you want to be more precise, especially if you’re dealing with a wide range of temperatures, you’ll want to use this formula:

λ = 2.501 – (2.361 x 10-3) x Ta

Where:

• Ta is the air temperature in degrees Celsius.
• λ is the latent heat of vaporization in MJ kg-1.

This equation, straight from the FAO Irrigation and Drainage Paper 56 (thanks, Allen et al.!), gives you a more accurate lambda based on the air temperature .

From FLUXNET to Field Data: A Step-by-Step Conversion

Alright, let’s get practical. Here’s how to turn that FLUXNET15 latent heat flux data into actual evapotranspiration numbers you can use:

A Few Bumps in the Road: Things to Watch Out For

• Missing Pieces: FLUXNET15 data isn’t always perfect. Sometimes there are gaps due to equipment glitches or other issues. You might need to fill in those gaps using some statistical techniques.
• Energy Imbalance: Here’s a fun fact: eddy covariance measurements (the tech behind FLUXNET15) often have trouble closing the energy balance. Ideally, all the incoming energy should equal all the outgoing energy, but that’s rarely the case in the real world. There are ways to correct for this, but it can get complicated.
• Quality Control is Key: Always read the FLUXNET15 documentation! It’ll tell you about any known issues with specific sites or time periods.
• Water Density: Remember that we’re assuming water density is 1000 kg/m3. That’s usually fine, but it does change a bit with temperature.
• Lambda Choice Matters: Using that temperature-dependent equation for lambda will give you better results, especially if you’re working in a place with big temperature swings.

The Bottom Line

Turning latent heat flux from FLUXNET15 into actual evapotranspiration opens up a world of possibilities for understanding our planet’s water cycle. By understanding the science, using the right formulas, and being aware of potential pitfalls, you can unlock valuable insights into how ecosystems are using water. FLUXNET15 is a powerful tool, and with a little know-how, you can put it to work!