cal_ET0_from_PM {simET}

calculating reference evapotranspiration from Penman-Monteith method

Description

The FAO Penman-Monteith method is maintained as the sole standard method for the computation of ETo from meteorological data.

Usage

cal_ET0_from_PM(delta, Rn, G, gamma, Tem, u2, es, ea)

Arguments

delta

slope vapour pressure curve (kPa °C). From cal_slopeOfSaturationVapourPressureCurve()

Rn

net Radiation at the crop surface [MJ m-2 day-1]. From cal_netRadiation()

G

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

gamma

psychrometric constant (kPa °C).

Tem

air temperature at 2 m height [°C].

u2

wind speed at 2 m height [m s-1].

es

saturation vapour pressure [kPa].

ea

actual vapour pressure [kPa].

Value

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

Note

Ten-day or monthly time step :

Notwithstanding the non-linearity in the Penman-Monteith equation and some weather parameter methods, mean ten-day or monthly weather data can be used to compute the mean ten-day or monthly values for the reference evapotranspiration. The value of the reference evapotranspiration calculated with mean monthly weather data is indeed very similar to the average of the daily ETo values calculated with daily average weather data for that month.

When the soil is warming (spring) or cooling (autumn), the soil heat flux (G) for monthly periods may become significant relative to the mean monthly Rn. In these cases G cannot be ignored and its value should be determined from the mean monthly air temperatures of the previous and next month.

Daily time step:

Calculation of ETo with the Penman-Monteith equation on 24-hour time scales will generally provide accurate results.

As the magnitude of daily soil heat flux (G) beneath the reference grass surface is relatively small, it may be ignored for 24-hour time steps.

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.