pyet.combination.pm

pyet.combination.pm(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, ea=None, a=1.35, b=-0.35, lai=None, croph=0.12, r_l=100, r_s=None, ra_method=0, a_sh=1, a_s=1, lai_eff=0, srs=0.0009, co2=300, albedo=0.23, kab=None, as1=0.25, bs1=0.5, clip_zero=True)

Potential evapotranspiration calculated according to Monteith (1965).

Parameters:

• tmean (float 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 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].

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

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

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

• lai (float or pandas.Series or xarray.DataArray, optional) – leaf area index [-].

• croph (float or pandas.Series or xarray.DataArray, optional) – crop height [m].

• r_l (pandas.Series or float, optional) – bulk stomatal resistance [s m-1].

• r_s (pandas.Series or float, optional) – bulk surface resistance [s m-1].

• ra_method (float, optional) – 0 => ra = 208/wind 1 => ra is calculated based on equation 36 in FAO (1990), ANNEX V.

• a_s (float, optional) – Fraction of one-sided leaf area covered by stomata (1 if stomata are 1 on one side only, 2 if they are on both sides).

• a_sh (float, optional) – Fraction of projected area exchanging sensible heat with the air (2).

• lai_eff (float, optional) – 0 => LAI_eff = 0.5 * LAI 1 => LAI_eff = lai / (0.3 * lai + 1.2) 2 => LAI_eff = 0.5 * LAI; (LAI>4=4) 3 => see Zhang et al. (2008).

• srs (float or pandas.Series or xarray.DataArray, optional) – Relative sensitivity of rl to Δ[CO2].

• co2 (float or pandas.Series or xarray.DataArray, optional) – CO2 concentration [ppm].

• 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

>>> tet_pm = pm(tmean, wind, rn=rn, rh=rh)

Notes

Following Monteith (1965), Allen et al. (1998), Zhang et al. (2008), Schymanski and Or (2017) and Yang et al. (2019).

\[PET = \frac{\Delta (R_{n}-G)+ \rho_a c_p K_{min} \frac{e_s-e_a}{r_a}}{\lambda(\Delta +\gamma(1+\frac{r_s}{r_a}))}\]

, where

\[r_s = f_{co2} * r_l / LAI_{eff}\]

\[f_{co2} = (1+S_{r_s}*(CO_2-300))\]

ra_method == 0:

\[r_a = \frac{208}{u_2}\]

ra_method == 1:

\[r_a = log(\frac{(zw - d)}{zom}) * \frac{log(\frac{(zh - d)}{zoh})}{(0.41^2)u_2}\]