Work out a daily PET to facilitate daily SPEI

[monocongo]

The Thornthwaite method being used for PET is based on monthly/calendar values. Work out another method to allow for daily SPEI, i.e. compute PET from daily temperature values so we can perform daily SPEI computations.

[Emmadd]

Hi @monocongo, would it be relatively straight forward to compute daily SPEI values if a time-independent PET calculation would be available? I've been using Makkink (https://nl.wikipedia.org/wiki/Referentie-gewasverdamping) which is based on global radiation and temperature and could contribute an implementation to climate_indices (I hope, my python skills are not great yet). Would you be interested? And what would be the next step to get to daily SPEI values?

[monocongo]

Yes, that sounds like a valuable contribution. Thanks for the idea, @Emmadd and let me know if I can help. There's no better way to improve your Python skills than writing good Python!

[Emmadd]

Here's a formulation for Makkink in case someone else wants to go ahead and implement it:

def makkink_KNMI(TG, Q):
    T = TG - 273.15  #[degC]
    #1) verzadigde dampspanning tov water
    e_s = 6.107 * 10**(7.5 * (T / (237.3 + T)))   #[hPa]
    #2) verzadigde dampspanningsgradient tov water
    delta = ((7.5 * 237.3) /  (T + 237.3)**2) * np.log(10) * e_s   #[hPa/degC]
    #3) psychrometerconstante (afhankelijk van T)
    gamma = 0.646 + (0.0006 * T)   #[hPa/degC]
    #4) verdampingswarmte van water
    labda = 1000 * (2501 - (2.38 * T))   #[J/kg]
    #5) soortelijke massa van water
    rho = 1000   #[kg/m3]
    #verdamping
    Ev = ( (1000 * 0.65 * delta) / ((delta + gamma) * rho * labda) ) * Q   #[mm/day] = [kg/m2/day]
    return Ev

Documentation (sorry, in Dutch) is available here for example https://www.knmiprojects.nl/projects/handboek-waarnemingen/documents/publications/2006/01/01/h10-verdamping