pyet.combination.pm_asce

pyet.combination.pm_asce(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, a=1.35, b=-0.35, cn=900, cd=0.34, ea=None, albedo=0.23, kab=None, as1=0.25, bs1=0.5, clip_zero=True, etype='os')

Potential evapotranspiration calculated according to Monteith (1965).

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 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].

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

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

• cn (float, optional) – numerator constant [K mm s3 Mg-1 d-1].

• cd (float, optional) – denominator constant [s m-1].

• ea (pandas.Series or float, 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.

• etype (str, optional) – “os” => ASCE-PM method is applied for a reference surfaces representing clipped grass (a short, smooth crop). “rs” => ASCE-PM method is applied for a reference surfaces representing alfalfa (a taller, rougher agricultural crop),).

Returns:

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

• evapotranspiration [mm d-1].

Examples

>>> et_pm = pm_asce(tmean, wind, rn=rn, rh=rh)

Notes

Following Monteith (1965) and Walter et al. (2000)

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