Calculate instantenous VPD from ambient conditions

Follows Abtew and Meleese (2013), Ch. 5 Vapor Pressure Calculation Methods, in Evaporation and Evapotranspiration

calc_vpd_inst(qair = NA, eact = NA, tc = NA, patm = NA, elv = NA)

Arguments

qair

Air specific humidity (g g-1)

eact

Water vapour pressure (Pa)

tc

temperature, deg C

patm

Atmospehric pressure (Pa)

elv

Elevation above sea level (m) (Used only if patm is missing for calculating it based on standard sea level pressure)

Value

instantenous VPD from ambient conditions