EcoHydRology (version 0.4.12.1)

PET_fromTemp: Priestley-Taylor Potential Evapotranspiration from temperature

Description

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

Usage

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"))

Arguments

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)

Value

PET (potential evapotranspiration) in m

References

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

Examples

# NOT RUN {
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)

# }