cal_ET0_from_PM_for_hourly {simET}

Calculating reference evapotranspiration from Penman-Monteith method for hourly time step

Description

Calculating reference evapotranspiration from Penman-Monteith method for hourly time step

Usage

cal_ET0_from_PM_for_hourly(
  slopVapourPressureCurve,
  netRadiation,
  soilHeatFlux,
  psychrometricConstant,
  meanHourlyTem,
  windSpeed,
  saturationVapourPressure,
  actualVapourPressure
)

Arguments

slopVapourPressureCurve

saturation slope vapour pressure curve at Thr [kPa °C].

netRadiation

net radiation at the grass surface [MJ m-2 hour-1].

soilHeatFlux

soil heat flux density [MJ m-2 hour-1].

psychrometricConstant

psychrometric constant [kPa °C].

meanHourlyTem

mean hourly air temperature [°C].

windSpeed

average hourly wind speed [m s-1].

saturationVapourPressure

saturation vapour pressure at air temperature Thr [kPa].

actualVapourPressure

average hourly actual vapour pressure [kPa].

Details

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.

Value

A vector for reference evapotranspiration [mm hour-1].

Note

With the advent of electronic, automated weather stations, weather data are increasingly reported for hourly or shorter periods. Therefore, in situations where calculations are computerized, the FAO Penman-Monteith equation can be applied on an hourly basis with good results. When applying the FAO Penman-Monteith equation on an hourly or shorter time scale, the equation and some of the procedures for calculating meteorological data should be adjusted for the smaller time step.

For the calculation of radiation parameters, see P74-75

References

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.