ET0_helper: helper functions for potential evapotranspiration (ET0)

In rpkgs/hydroTools: Tools for hydrological model

helper functions for potential evapotranspiration (ET0)

Description

• 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.

Usage

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)

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")

rpkgs/hydroTools documentation built on Oct. 8, 2024, 7:47 p.m.