Calculating reference evapotranspiration from Penman-Monteith method for hourly time step
cal_ET0_from_PM_for_hourly(
slopVapourPressureCurve,
netRadiation,
soilHeatFlux,
psychrometricConstant,
meanHourlyTem,
windSpeed,
saturationVapourPressure,
actualVapourPressure
)A vector for reference evapotranspiration [mm hour-1].
saturation slope vapour pressure curve at Thr [kPa °C].
net radiation at the grass surface [MJ m-2 hour-1].
soil heat flux density [MJ m-2 hour-1].
psychrometric constant [kPa °C].
mean hourly air temperature [°C].
average hourly wind speed [m s-1].
saturation vapour pressure at air temperature Thr [kPa].
average hourly actual vapour pressure [kPa].
In areas where substantial changes in wind speed, dewpoint or cloudiness occur during the day, calculation of the ETo equation using hourly time steps is generally better than using 24-hour calculation time steps. Such weather changes can cause 24-hour means to misrepresent evaporative power of the environment during parts of the day and may introduce error into the calculations. However, under most conditions, application of the FAO Penman-Monteith equation with 24-hour data produces accurate results.
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. FAO Irrigation and drainage paper No. 56. Rome: Food and Agriculture Organization of the United Nations, 1998.