Evapotranspiration (version 1.7)

ET.ChapmanAustralian: Chapman Formulation

Description

Implementing the Chapman formulation for estimating equivalent Penmen-Monteith reference crop evapotranspiration.

Usage

## S3 method for class 'ChapmanAustralian':
ET(data, constants, Penpan, solar, alpha, \dots)

Arguments

data

A list of data in class "ChapmanAustralian" which contains the following items (climate variables) required by Chapman formulation: "Epan" or "Tmax" and "Tmin" and "RHmax" and "RHmin" and "u2" or "uz" and "Rs" or "n" or "Cd"

constants

A list named "constants" consists of constants required for the calculation of Penman 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,

Penpan

Must be TRUE or FALSE, indicating if the Penpan formulation is used for estimating Class-A pan evaporation required in Chapman formulation. If TRUE Penpan will be used and if FALSE the actual data of Class-A pan evaporation will be used.

solar

Must be either "data", "sunshine hours", or "cloud". "data" indicates that solar radiation data is used directly for calculating evapotranspiration, "sunshine hours" indicates that solar radiation is to be calculated using the real data of sunshine hours;

alpha

Must be defined if Penpan = TRUE. Any numeric value between 0 and 1 (dimensionless), albedo of evaporative surface (or surrounding surface for Penpan formulation) representing the portion of the incident radiation that is reflected back at the surface.

...

Dummy for generic function, no need to define.

Value

• This function returns a object of class ChapmanAustralian. Such objects are list with components:
• ET.Daily
  A zoo object containing daily aggregated estimations of Chapman equivalent Penmen-Monteith reference crop evapotranspiration.
• ET.Monthly
  A zoo object containing monthly aggregated estimations of Chapman equivalent Penmen-Monteith reference crop evapotranspiration.
• ET.Annual
  A zoo object containing annually aggregated estimations of Chapman equivalent Penmen-Monteith reference crop evapotranspiration.
• ET.MonthlyAve
  A zoo object containing monthly averaged estimations of daily Chapman equivalent Penmen-Monteith reference crop evapotranspiration.
• ET.AnnualAve
  A zoo object containing annually averaged estimations of daily Chapman equivalent Penmen-Monteith reference crop evapotranspiration.
• ET_formulation
  A character string containing the name of the formulation used which equals to "Chapman".
• ET_type
  A character string containing the type of the estimation obtained which is "Equivalent Penmen-Monteith Reference Crop Evapotranspiration".
• message1
  A message to inform the users about how solar radiation has been calculated by using which data.
• message5
  A message to inform the users about if the Class-A pan evaporation is from actual data or from Penpan estimation.

Details

The alternative calculation options can be selected through arguments Penpan and solar, please see "Arguments" for details.

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.

Examples

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

# Set data class to be "ChapmanAustralian" to call function
funname <- "ChapmanAustralian" 
class(data) <- funname 

# Call generic function ET(data, ...) with class "ChapmanAustralian"
results <- ET(data, constants, Penpan = TRUE, solar = "sunshine hours", 
alpha = 0.23)