Implementing the Abtew formulation for estimating actual evapotranspiration.
# S3 method for Abtew
ET(data, constants, ts="daily", solar="sunshine hours",…)A list of data in class "Abtew" which contains the following items (climate variables) required by Abtew formulation: Tmax, Tmin, Rs or n or Cd
A list named constants consists of constants required for the calculation of Abtew 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.
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.
Must be either daily, monthly or annual, which indicates the disired time step that the output ET estimates should be on.
Default is daily.
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.
Dummy for generic function, no need to define.
The function prints a calculation summary to the screen containing the following elements:
- ET model name and ET quantity estimated
- 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.
The function also generates a list containing the following components, which is saved into a csv file named as ET_Abtew.csv in the working directory:
Daily aggregated estimations of Abtew actual evapotranspiration.
Monthly aggregated estimations of Abtew actual evapotranspiration.
Annually aggregated estimations of Abtew actual evapotranspiration.
Monthly averaged estimations of daily Abtew actual evapotranspiration.
Annually averaged estimations of daily Abtew actual evapotranspiration.
Name of the formulation used which equals to Abtew.
Type of the estimation obtained which is Actual Evapotranspiration.
A message to inform the users about how solar radiation has been calculated by using which data.
The alternative calculation options can be selected through argument solar, please see Arguments for details.
Abtew, W. 1996. Evapotranspiration measurements and modeling for three wetland systems in south florida. Wiley Online Library.
# NOT RUN {
# Use processed existing data set and constants from kent Town, Adelaide
data("processeddata")
data("constants")
# Call ET.Abtew under the generic function ET
results <- ET.Abtew(data, constants,ts="daily", solar="sunshine hours")
# }
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