varwg.meteo.meteox2y.hargreaves

varwg.meteo.meteox2y.hargreaves(tmax, tmin, date, in_format='%Y-%m-%dT%H:%M:%S')[source]

calculates the potential evapotranspiration rate [mm/d] following Hargreaves & Samani 1985 according to THE ASCE STANDARDIZED REFERENCE EVAPOTRANSPIRATION EQUATION

Parameters:
tmaxnp.array of floats

maximum of the daily air temperature [deg C]

tminnp.array of floats

minimum of the daily air temperature [deg C]

datenp.array of strings

date strings in format in_format

in_formatformat string

default: ‘%Y-%m-%dT%H:%M:%S’

Returns:
etpnumpy.array of floats

potential evapotranspiration [mm/d]

Notes

  • equation needs the extraterrestrial radiation Ra, which depends on:

    • inverse relative distance factor for the earth-sun dr []

    • solar declination delta [rad]

    • sunset hour angle [rad] with latitude Lauchaecker lat=48.738 deg => pi/180 * 48.738 deg = 0.8506 rad

  • conversion of date in day of year (j)

  • ETPmax = 7.0 mm/d due to energetic limit (Germany)