ET.Penman: Penman Formulation

Description

Implementing the Penman formulation for estimating open-water evaporation or potential evapotranspiration

Usage

## S3 method for class 'Penman':
ET(data, constants, solar, wind, windfunction_ver, 
alpha = 0.08, z0 = 0.001, \dots)

Arguments

data

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

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,

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).

windfunction_ver

The version of Penman wind function that will be used within the Penman formulation. Must be either "1948" or "1956". "1948" is for applying the Penman's 1948 wind function (Penman, 1948); "1956" is for applying the Penman's 1956 wind function (Penman, 19

alpha

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. The default is 0.08 for open-water s

Any value (metres), roughness height of the evaporative surface. The default is 0.08 for open-water surface which is for the calculation of Penman open-water evaporation, all other values will trigger the calculation of Penman potential evapotranspriation

...

Dummy for generic function, no need to define.

Value

This function returns a object of class Penman. Such objects are list with components:
ET.DailyA zoo object containing daily aggregated estimations of Penman open-water evaporation or potential evapotranspiration.
ET.MonthlyA zoo object containing monthly aggregated estimations of Penman open-water evaporation or potential evapotranspiration.
ET.AnnualA zoo object containing annually aggregated estimations of Penman open-water evaporation or potential evapotranspiration.
ET.MonthlyAveA zoo object containing monthly averaged estimations of daily Penman open-water evaporation or potential evapotranspiration.
ET.AnnualAveA zoo object containing annually averaged estimations of daily Penman open-water evaporation or potential evapotranspiration.
ET_formulationA character string containing the name of the formulation used which equals to "Penman".
ET_typeA character string containing the type of the estimation obtained which is either "Open-water Evaporation" or "Potential Evapotranspiration".
message1A message to inform the users about how solar radiation has been calculated by using which data.
message2A 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, and which version of the Penman wind function has been used.

Details

The alternative calculation options can be selected through arguments solar, wind and windfunction_ver, please see "Arguments" for details. User-defined evaporative surface is allowed through arguments alpha and z0, 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. PENMAN, H. L. 1948. Natural evaporation from open water, bare soil and grass. Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences, 193, 120-145. VALIANTZAS, J. D. 2006. Simplified versions for the Penman evaporation equation using routine weather data. Journal of Hydrology, 331, 690-702. PENMAN, H. L. 1956. Evaporation: An introductory survey. Netherlands Journal of Agricultural Science, 4, 9-29.

Examples

Run this code

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

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

# Call generic function ET(data, ...) with class "Penman"
results <- ET(data, constants, solar="sunshine hours", wind=
"yes", windfunction_ver = "1948", alpha = 0.08, z0 = 0.001)

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