CropSyst User Manual - sipeaa

Penman-Monteith evapotranspirationmodelIf daily air temperature solar radiation, maximum and minimum relative humidity (RHmax and RHmin)(or dew point temperature from which relative humidity can computed) and wind speed are available,then the Penman-Monteith equation may be used to calculate potential evapotranspiration.Slope vpf · (Rad net - SoilHeatFlux) + (DayFract · VPD · C va ) / R aRefET pot =Slope vpf + γ · (1 + r c /r a )whereSoilHeatFlux ((MJ/m²)/day) is the soil heat flux.DayFract (0-1) is the fraction of the day that is in daylight.C va ((MJ/m3)/°C) is the volumetric heat capacity of air taken as 1.2x10 -3 .γ (kPa/°C) is the psychometric constant.Slope vpf (kPa/°C) is the slope of saturation vapor pressure function of temperature.VPD (kPa) is the Vapor pressure deficietRad net ((MJ/m²)/day) is the net radiation .r a , r care the aerodynamic and canopy resistances to vapor transfer of the(day/m)reference crop. For a short clipped grass (0.12m height), theses areln((hs - 0.744 · hp) / (0.026 · hp)) ·ln((hs - 0.640 · hp))/(0.013 · hp)r a =0.16 · wsr c = 0.000787034 (this may be adjusted in the case of CO 2 simulation).wherews (m/s) is the wind speed. (Note that wind speed is stored in daily weather files a m/daybut will be converted to m/s).