helper functions for potential evapotranspiration (ET0)

  • lambda: latent heat of vaporization, about [2.5 MJ kg-1].

  • slope: The slope of the saturation vapour pressure curve at certain air temperature Tair, [kPa degC-1].

  • gamma: psychrometric constant ([kPa degC-1]), Cp*Pa/(epsilon*lambda).

  • U2: 10m wind speed (m/s). According to wind profile relationship, convert U_z to U_2.

  • es: saturation vapor pressure (kPa)

  • ea: actual vapor pressure (kPa)

    1. RH_mean | Tmin, Tmax: (es(Tmax) + es(Tmin)) * RH_mean/200

    2. Tmin : es(Tmin)

  • cal_TvK: vitual temperature (K)

    1. Tair + q

    2. Tair + ea/Pa

    3. 1.01 * (Tair + 273) , FAO56 Eq. 3-7

  • cal_rho_a: air density (kg/m^3)

  • rH: resistance of aerodynamic (s/m), about 100 s/m.

cal_U2(Uz, z.wind = 10)

cal_ea(Tmin, Tmax = NULL, RH_mean = NULL)

cal_VPD(Tair, Tdew = NULL)

cal_lambda(Tair)

cal_slope(Tair)

cal_gamma(Tair, Pa = atm)

cal_bowen(Tair, Pa = atm)

cal_Pa(z = NULL)

cal_rH(U2, h = 0.12)

cal_rH2(U2, Tair, Pa = atm)

cal_TvK(Tair, q = NULL, ea = NULL, Pa = atm)

cal_rho_a(Tair, Pa = atm, q = NULL, ea = NULL)

cal_es(Tair)

Arguments

Uz

wind speed at height z.wind

z.wind

Height where measure the wind speed [m]. Default 10m.

Tmin

Daily minimum air temperature at 2m height [deg Celsius]

Tmax

Daily maximum air temperature at 2m height [deg Celsius]

RH_mean

daily mean relative humidity [%]

Tair

2m air temperature, ([deg Celsius])

Tdew

dew temperature, ([deg Celsius])

Pa

land surface Air pressure [kPa].

z

Elevation above sea level [m]. Must provided if pres are not provided.

U2

wind speed at 2m

h

canopy height (m), for the calculation of the following intermediate variables:

  • d: zero plane displacement height (m)

  • z_om: roughness length governing momentum transfer (m)

  • z_oh: roughness length governing transfer of heat and vapour (m)

  • z_h : height of humidity measurements (m)

  • z_m : height of wind measurements (m)

q

specific humidity (g/g)

ea

actual vapor pressure (kPa)

References

  1. Allen, R. G., & Luis S. Pereira. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. European Journal of Agronomy, 34(3), 144–152. doi:10.1016/j.eja.2010.12.001

Examples

cal_VPD(10, 5)
#> [1] 0.3556517

par(mfrow = c(2, 2), mar = c(3, 1.8, 2, 1), mgp = c(2, 0.6, 0))
Tair <- -10:50
plot(cal_es(-10:50), main = "es (kPa)", xlab = "Tair")
plot(cal_bowen(-10:50), main = "Bowen ratio", xlab = "Tair")
plot(cal_slope(-10:50), main = "slope (kPa/degC)", xlab = "Tair")
plot(cal_gamma(-10:50), main = "gamma (kPa/degC)", xlab = "Tair")