<p>Negative values for the ET, both potential and actual, usually refer to <a href="https://en.wikipedia.org/wiki/Condensation" rel="noreferrer">condensation</a>.
See also &quot;<a href="https://earthscience.stackexchange.com/questions/17839/era5-mean-evaporation-rates-data-why-is-that-negative/17854#17854">ERA5 mean evaporation rates data - Why is that negative?</a>&quot;</p>
<p>Regarding the ability of the physically-based hydrological models to handle condensation, I'd say that it's difficult to give you a general answer: it will depend on the specific math behind the model, but my general guess would be that they do.
Even a simple <a href="https://en.wikipedia.org/wiki/File:Tank_Model.jpg" rel="noreferrer">Bucket Model</a>, would integrate condensation in the <a href="https://en.wikipedia.org/wiki/Water_balance" rel="noreferrer">water balance equation</a>. While a more sophisticate Earth System Model would probably process it in the soil moisture and/or canopy component, in order to better represent these dynamics in the water-energy balance of the specific system. See for instance <a href="https://gmd.copernicus.org/articles/12/4309/2019/gmd-12-4309-2019.pdf" rel="noreferrer">Longo et al 2019</a></p>

CC BY-SA 4.0
2020-09-22T07:21:02.357Z
2020-09-22T07:21:02.357Z
Nemesi
<p>I am exploring FLUXNET sites for actual evapotranspiration (ETP) data (I convert the energy balance closure-corrected latent heat to ETP).</p>
<p>I am interested in the half-hourly data. During the night we see negative values. I would like to know the reason or physical process behind this.</p>
<p>Can physically-based hydrological models handle this situation? (: estimating negative half-hourly ETPs)</p>
<p>There are some discussion in web about how to handle the negative values of potential ETP, but not easy to find a solid discussion about the actual ETP in this case.</p>

<meteorology><hydrology><evaporation><water-vapour>
How to interpret the negative (actual) evapotranspiration values?
Alireza Amani
# How to interpret the negative (actual) evapotranspiration values?

(Asked by: username_0 on 2020-09-21)

I am exploring FLUXNET sites for actual evapotranspiration (ETP) data (I convert the energy balance closure-corrected latent heat to ETP).

I am interested in the half-hourly data. During the night we see negative values. I would like to know the reason or physical process behind this.

Can physically-based hydrological models handle this situation? (: estimating negative half-hourly ETPs)

There are some discussion in web about how to handle the negative values of potential ETP, but not easy to find a solid discussion about the actual ETP in this case.

**Tags:** meteorology, hydrology, evaporation, water-vapour


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## 1 answers

### Answer by username_1 on 2020-09-22. Score: 5 | Accepted answer

Negative values for the ET, both potential and actual, usually refer to [condensation](https://en.wikipedia.org/wiki/Condensation).
See also "[ERA5 mean evaporation rates data - Why is that negative?](https://earthscience.stackexchange.com/questions/17839/era5-mean-evaporation-rates-data-why-is-that-negative/17854#17854)"

Regarding the ability of the physically-based hydrological models to handle condensation, I'd say that it's difficult to give you a general answer: it will depend on the specific math behind the model, but my general guess would be that they do.
Even a simple [Bucket Model](https://en.wikipedia.org/wiki/File:Tank_Model.jpg), would integrate condensation in the [water balance equation](https://en.wikipedia.org/wiki/Water_balance). While a more sophisticate Earth System Model would probably process it in the soil moisture and/or canopy component, in order to better represent these dynamics in the water-energy balance of the specific system. See for instance [Longo et al 2019](https://gmd.copernicus.org/articles/12/4309/2019/gmd-12-4309-2019.pdf)


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