ET.PriestleyTaylor {Evapotranspiration}

Priestley-Taylor Formulation

Description

Implementing the Priestley-Taylor formulation for potential evaporation

Usage

## S3 method for class 'PriestleyTaylor'
ET(data, constants, ts="daily", solar="sunshine hours", alpha=0.23,
message="yes", AdditionalStats="yes", save.csv="no", ...)

Arguments

data

A list which contains the following items (climate variables) required by Priestley-Taylor formulation:
Tmax, Tmin (degree Celcius), RHmax, RHmin (per cent), Rs (Megajoules per sqm) or n (hour) or Cd (okta)

constants

A list named constants consists of constants required for the calculation of Priestley-Taylor formulation which must contain the following items:
Elev - ground elevation above mean sea level in m,
lambda - latent heat of vaporisation = 2.45 MJ.kg^-1,
lat_rad - latitude in radians,
Gsc - solar constant = 0.0820 MJ.m^-2.min^-1,
sigma - Stefan-Boltzmann constant = 4.903*10^-9 MJ.K^-4.m^-2.day^-1,
alphaPT - Priestley-Taylor coefficient = 1.26 for Priestley-Taylor model (Priestley and Taylor, 1972)
G - soil heat flux in MJ.m^-2.day^-1, = 0 when using daily time step.

The following constants are also required when argument solar has value of sunshine hours:
as - fraction of extraterrestrial radiation reaching earth on sunless days,
bs - difference between fracion of extraterrestrial radiation reaching full-sun days and that on sunless days.

ts

Must be either daily, monthly or annual, which indicates the disired time step that the output ET estimates should be on. Default is daily.

solar

Must be either data, sunshine hours, cloud or monthly precipitation:
data indicates that solar radiation data is to be used directly for calculating evapotranspiration;
sunshine hours indicates that solar radiation is to be calculated using the real data of sunshine hours;
cloud sunshine hours is to be estimated from cloud data;
monthly precipitation indicates that solar radiation is to be calculated directly from monthly precipitation.
Default is sunshine hours.

alpha

Any numeric value between 0 and 1 (dimensionless), albedo of evaporative surface representing the portion of the incident radiation that is reflected back at the surface.
Default is 0.23 for surface covered with short reference crop, which is for the calculation of Priestly-Taylor reference crop evaporation.

message

Must be either yes or no, indicating whether message should be printed for calculation summary including the following elements:
- ET model name and ET quantity estimated
- Evaporative surface with values of albedo
- Option for calculating solar radiation (i.e. the value of argument solar)
- Time step of the output ET estimates (i.e. the value of argument ts)
- Units of the output ET estimates
- Time duration of the ET estimation
- Number of ET estimates obtained in the entire time-series
- Basic statistics of the estimated ET time-series including mean, max and min values.

AdditionalStats

"yes" or "no" indicating whether monthly averaged and annual averaged ET should be calculated.

save.csv

Must be either yes or no, indicating whether a .csv of ET estimates should be saved to working directory.

...

Dummy for generic function, no need to define.

Details

The alternative calculation options can be selected through argument solar, please see Arguments for details.
User-defined evaporative surface is allowed through argument alpha, please see Arguments for details.

Value

The function generates a list containing the following components, which is saved into a csv file named as ET_PriestleyTaylor.csv in the working directory:

ET.Daily

Daily aggregated estimations of Priestley-Taylor potential evaporation.

ET.Monthly

Monthly aggregated estimations of Priestley-Taylor potential evaporation.

ET.Annual

Annually aggregated estimations of Priestley-Taylor potential evaporation.

ET.MonthlyAve

Monthly averaged estimations of daily Priestley-Taylor potential evaporation.

ET.AnnualAve

Annually averaged estimations of daily Priestley-Taylor potential evaporation.

ET_formulation

A character string containing the name of the formulation used which equals to Priestley-Taylor.

ET_type

Type of the estimation obtained which is Potential Evaporation.

message1

A message to inform the users about how solar radiation has been calculated by using which data.

Author(s)

Danlu Guo

References

McMahon, T., Peel, M., Lowe, L., Srikanthan, R. & McVicar, T. 2012. Estimating actual, potential, reference crop and pan evaporation using standard meteorological data: a pragmatic synthesis. Hydrology and Earth System Sciences Discussions, 9, 11829-11910.

Priestley, C. & Taylor, R. 1972, On the assessment of surface heat flux and evaporation using large-scale parameters'. Monthly Weather Review, vol. 100, no. 2, pp. 81-92.

See Also

ET,processeddata,defaultconstants,constants

Examples

# Use processed existing data set and constants from kent Town, Adelaide
data("processeddata")
data("constants")

# Call ET.PriestleyTaylor under the generic function ET
results <- ET.PriestleyTaylor(processeddata, constants, ts="daily",
solar="sunshine hours", alpha=0.23, message="yes", AdditionalStats="yes", save.csv="no")