Evapotranspiration (version 1.6)

ET.PenmanMonteith: Penman-Monteith Formulation

Description

Implementing the Penman-Monteith formulation (including the method for FAO-56 hypothetical short grass and the method for ASCE-EWRI Standardised crop) for estimating reference crop evapotranspiration

Usage

## S3 method for class 'PenmanMonteith':
ET(data, constants, solar, wind, crop, \dots)

Arguments

data

A list of data in class "PenmanMonteith" which contains the following items (climate variables) required by Penman-Monteith formulation: "Tmax", "Tmin", "RHmax", "RHmin", "Rs, "n" or "Cd", "u2" or "uz"

constants

A list named "constants" consists of constants required for the calculation of Penman-Monteith formulation which must contain the following items: "Elev" - ground elevation above mean sea level in m, "lambda" - latent heat of vaporisation = 2.45 MJ.

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 th

wind

Must be either "yes" or "no". "yes" indicates that the calculation will use real data of wind speed; "no" indicates that the alternative calculation without using wind data will be used in Penman formulation (Valiantzas 2006, Equation33).

crop

Must be either "short" or "tall". "short" indicates that the method for FAO-56 hypothetical short grass will be applied (Allen et al., 1998, Equation 6); "tall" indicates that the method for ASCE-EWRI Standardised crop will be applied (ASCE, 2005, Equati

...

Dummy for generic function, no need to define.

Value

• This function returns a object of class PenmanMonteith. Such objects are list with components:
• ET.Daily
  A zoo object containing daily aggregated estimations of Penman-Monteith rerference crop evapotranspiration.
• ET.Monthly
  A zoo object containing monthly aggregated estimations of Penman-Monteith rerference crop evapotranspiration.
• ET.Annual
  A zoo object containing annually aggregated estimations of Penman-Monteith rerference crop evapotranspiration.
• ET.MonthlyAve
  A zoo object containing monthly averaged estimations of daily Penman-Monteith rerference crop evapotranspiration.
• ET.AnnualAve
  A zoo object containing annually averaged estimations of daily Penman-Monteith rerference crop evapotranspiration.
• ET_formulation
  A character string containing the name of the formulation used which equals to either "Penman-Monteith FAO56" or "Penman-Monteith ASCE-EWRI Standardised".
• ET_type
  A character string containing the type of the estimation obtained which is "Reference Crop Evapotranspiration".
• message1
  A message to inform the users about how solar radiation has been calculated by using which data.
• message2
  A message to inform the users about if actual wind data has been used in the calculations or alternative calculations has been performed without wind data.

Details

The alternative calculation options can be selected through arguments solar and wind, please see "Arguments" for details. User-defined evaporative surface is allowed through arguments crop, 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. ALLEN, R. G., PEREIRA, L. S., RAES, D. & SMITH, M. 1998. Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300, 6541. ALLEN, R. G. 2005. The ASCE standardized reference evapotranspiration equation, Amer Society of Civil Engineers.

Examples

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

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

# Call generic function ET(data, ...) with class "PenmanMonteith"
results <- ET(data, constants, solar="sunshine hours", wind="yes", crop="short")