PET_fromTemp: Priestley-Taylor Potential Evapotranspiration from...
In EcoHydRology: A Community Modeling Foundation for Eco-Hydrology

Description Usage Arguments Value Note Author(s) References Examples

Calculates potential Evapotranspiration (in meters) based on the Priestley-Taylor equation (1972). We use an estimation of net radiation based on temperature data.

PET_fromTemp(Jday, Tmax_C, Tmin_C, lat_radians, 
AvgT = (Tmax_C + Tmin_C)/2, albedo = 0.18, TerrestEmiss = 0.97, 
aspect = 0, slope = 0, forest = 0, PTconstant=1.26,
AEparams=list(vp=NULL, opt="linear"))

`Jday`	Day of the year
`Tmax_C`	Maximum daily temperature (degrees C)
`Tmin_C`	Minimum daily temperature (degrees C)
`lat_radians`	latitude (radians = decimal degrees*pi/180)
`AvgT`	Average daily temperature (degrees C) (if not known, will be taken as the averages of the daily extremes)
`albedo`	(-) average surface albedo. Can be expressed as a single value, or as a vector with the same length as Jday, Tmax_C and Tmin_C
`TerrestEmiss`	(-) Surface Emissivity - defaults to 0.97
`aspect`	(radians) Surface aspect
`slope`	(radians) average slope
`forest`	(-) Forest or shade cover (0-1). This modifies the amount of solar radiation reaching the location of interest. It should always set to zero for landscape-wide processes regardless of the amount of forest present. Only change this if calculating PET under a canopy.
`PTconstant`	(-) Priestley-Taylor Constant, often 1.26
`AEparams`	Atmospheric Emissivity options. Defaults to linear approximation. To use Brutsaert option, include vapor pressure values (kPa)

PET (potential evapotranspiration) in m

We are assuming that the Ground heat flux on a daily time-step is zero

Josephine Archibald, M. Todd Walter

Archibald, J.A. and M. T. Walter, 2013. Do energy-based PET models require more input data than T-based models? - An evaluation at four humid FluxNet sites. Journal of the American Water Resources Association (JAWRA)

Brutsaert, W., 1975. On a Derivable Formula for Long-Wave Radiation from Clear Skies. Water Resources Research 11(5):742-744.

Priestley and Taylor, 1972. On the assessment of surface heat flux and evaporation using large-scale parameters. Mon. Weath. Rev. 100: 81-92

data(OwascoInlet)
head(OwascoInlet)
attach(OwascoInlet)
PETapprox <- PET_fromTemp(Jday=(1+as.POSIXlt(date)$yday), Tmax_C=Tmax_C,
 Tmin_C=Tmin_C, lat_radians=42.45*pi/180)
plot(PETapprox*1000~date, type="l")
detach(OwascoInlet)