pyet.combination.penman

pyet.combination.penman(tmean, wind, rs=None, rn=None, g=0, tmax=None, tmin=None, rhmax=None, rhmin=None, rh=None, pressure=None, elevation=None, lat=None, n=None, nn=None, rso=None, aw=1, bw=0.537, a=1.35, b=-0.35, ea=None, albedo=0.23, kab=None, as1=0.25, bs1=0.5, clip_zero=True)

Potential evapotranspiration calculated according to Penman (1948).

Parameters:

• tmean (pandas.Series or xarray.DataArray) – average day temperature [°C].

• wind (float or pandas.Series or xarray.DataArray) – mean day wind speed [m/s].

• rs (float or pandas.Series or xarray.DataArray, optional) – incoming solar radiation [MJ m-2 d-1].

• rn (float or pandas.Series or xarray.DataArray, optional) – net radiation [MJ m-2 d-1].

• g (float or pandas.Series or xarray.DataArray, optional) – soil heat flux [MJ m-2 d-1].

• tmax (float or pandas.Series or xarray.DataArray, optional) – maximum day temperature [°C].

• tmin (float or pandas.Series or xarray.DataArray, optional) – minimum day temperature [°C].

• rhmax (float or pandas.Series or xarray.DataArray, optional) – maximum daily relative humidity [%].

• rhmin (float or pandas.Series or xarray.DataArray, optional) – mainimum daily relative humidity [%].

• rh (float or pandas.Series or xarray.DataArray, optional) – mean daily relative humidity [%].

• pressure (float or pandas.Series or xarray.DataArray, optional) – atmospheric pressure [kPa].

• elevation (float or xarray.DataArray, optional) – the site elevation [m].

• lat (float or xarray.DataArray, optional) – the site latitude [rad].

• n (float or pandas.Series or xarray.DataArray, optional) – actual duration of sunshine [hour].

• nn (float or pandas.Series or xarray.DataArray, optional) – maximum possible duration of sunshine or daylight hours [hour].

• rso (float or pandas.Series or xarray.DataArray, optional) – clear-sky solar radiation [MJ m-2 day-1].

• aw (float, optional) – wind coefficient [-].

• bw (float, optional) – wind coefficient [-].

• a (float, optional) – empirical coefficient for Net Long-Wave radiation [-].

• b (float, optional) – empirical coefficient for Net Long-Wave radiation [-].

• ea (float or pandas.Series or xarray.DataArray, optional) – actual vapor pressure [kPa].

• albedo (float, optional) – surface albedo [-].

• kab (float, optional) – coefficient derived from as1, bs1 for estimating clear-sky radiation [degrees].

• as1 (float, optional) – regression constant, expressing the fraction of extraterrestrial reaching the earth on overcast days (n = 0) [-].

• bs1 (float, optional) – empirical coefficient for extraterrestrial radiation [-].

• clip_zero (bool, optional) – if True, replace all negative values with 0.

Returns:

• pandas.Series or xarray.DataArray containing the calculated potential

• evapotranspiration [mm d-1].

Examples

>>> et_penman = penman(tmean, wind, rn=rn, rh=rh)

Notes

Following Penman (1948) and Valiantzas (2006).

\[PET = \frac{\frac{\Delta (R_n-G)}{\lambda} + \gamma (a_w + b_w u_2) (e_s-e_a)}{(\Delta +\gamma)}\]