Fix ET data for estimating recharge

[jnsebgosselin]

[copied from an email sent by a user]

I'm using your GWHAT model to estimate recharge and I have evapotranspiration data (estimated by the Bowen method at Montmorency Forest). I would like to know if there is a way to fix ET data, I mean, use my ET data in your model to estimate recharge. I was reading that your model estimate evapotranspiration after RAS calculations.

Is it possible to use ET data as constrain in your model? Maybe manipulating the input values or another approach.

[jnsebgosselin]

Yes it should currently be possible to use fixed ET data in GWHAT for calculating groundwater recharge instead of letting GWHAT calculate it with a modified Thornwaite approach.

To do so, simply add your daily PET (potential evapotranspiration) data in a column of your weather data input file. The name of the column containing your PET data must include either the term "ETP" or "Evapo" (it is case insensitive). GWHAT should then read these PET data from the file when importing the weather data and use them when calculating groundwater recharge.

So for example:

[jnsebgosselin]

The part of the code that is used to read input weather data in GWHAT can be found here:

https://github.com/jnsebgosselin/gwhat/blob/d2fee4559e9a30b6404821acf07f3580e3903b70/gwhat/meteo/weather_reader.py#L346-L389

[wantolinez4]

Thanks for your answer @jnsebgosselin. I have another question: is it possible to fix actual evapotranspiration to the final results? Here...

Thanks in advance for your great work!

[jnsebgosselin]

I don't understand what you mean by "fix actual evapotranspiration to the final results".

Do you mean that you want to be able to fix the real evapotranspiration in the model instead of letting the model calculate it from the daily potential evapotranspiration and readily available water in the soil?

[wantolinez4]

Exactly! I don't want that the model calculates real evapotranspiration from potential evapotranspiration. Instead, the model should use the input values of AET

[jnsebgosselin]

Ok I understand. It is not possible to do this explicitly in GWHAT though.

I think what you could do is add your AET data as PET data in your input weather datafile. Then, set the RASmax range to a single value sufficiently large so that the readily available water is not a limiting factor in the model when calculating the real evapotranspiration, so that effectively AET == PET.

Also, make sure that you have a couple of years of weather data prior to the start of your water level data to give some time to the model to initialize properly (to fill the readily available reservoir).

If this does not work, we can try to think of a way to add an option to force the AET in the model directly.

[wantolinez4]

That's a good idea. I'm going to try that approach. Merci beaucoup!