Add a PET function using Hargreaves' equation

[monocongo]

Begueria et al (2014) as well as Allen et al (1998) suggest Hargreaves as a better PET approximation method compared to Thornthwaite's when solar radiation, wind speed, and relative humidity data are unavailable.

The Hargreaves equation is spelled out in Droogers and Allen (2002), Allen et al (1998) and Hargreaves (1994).

There is R code for this, look into that and see what can be re-used.

Thanks/credit to Rebecca Ward for suggesting this.

Related to #101

[monocongo]

Another R implementation

[s-m-t-c]

My colleague @erin-telfer has a notebook that takes netcdf as input (converts to xarray) to calculate PET using Hargreaves, this is how I calculated PET to use with the climate_indices module.

Could be a bit more helpful than the above R implementations.
https://github.com/GeoscienceAustralia/dea-notebooks/blob/ErinT/ewater/NB07_calculate_PET_grids.ipynb

[monocongo]

Thanks for your help on this. Unfortunately, I can't reach that URL. ANy chance you can copy/paste the code someplace else where I can get to it? I'm not actively maintaining this code lately but I'd like to get to this eventually, and if this is as you describe then it should be relatively painless.

[monocongo]

This implementation looks solid: https://pyeto.readthedocs.io/en/latest/api.html#pyeto.hargreaves

[SouhailAB]

My colleague @erin-telfer has a notebook that takes netcdf as input (converts to xarray) to calculate PET using Hargreaves, this is how I calculated PET to use with the climate_indices module.

Could be a bit more helpful than the above R implementations.
https://github.com/GeoscienceAustralia/dea-notebooks/blob/ErinT/ewater/NB07_calculate_PET_grids.ipynb

Could you please share another link to the notebook. Unable to reach the URL