ET.ChapmanAustralian {Evapotranspiration} R Documentation

Chapman Formulation

Description

Implementing the Chapman formulation for estimating potential evapotranspiration.

Usage

```## S3 method for class 'ChapmanAustralian'
ET(data, constants, ts="daily", PenPan= T,
solar="sunshine hours", alpha=0.23, message="yes", AdditionalStats="yes",
save.csv="no", ...)
```

Arguments

 `data` A list of data which contains the following items (climate variables) required by Chapman formulation: Tmax, Tmin (degree Celcius), RHmax, RHmin (per cent), Rs (Megajoules per sqm) or n (hour) or Cd (okta), u2 or uz (meter per second) `constants` A list named `constants` consists of constants required for the calculation of Chapman 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, z - height of wind instrument in m, sigma - Stefan-Boltzmann constant = 4.903*10^-9 MJ.K^-4.m^-2.day^-1, lat - latitude in degrees, alphaA - albedo for Class-A pan, ap - a constant in PenPan = 2.4. 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`. `PenPan` Must be `T` or `F`, indicating if the PenPan formulation is used for estimating Class-A pan evaporation required in Chapman formulation. If `T` PenPan will be used and if `F` the actual data of Class-A pan evaporation will be used. Default is `T` for using the PenPan formulation. `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 the evaporative surface incident radiation that is reflected back at the surface. Default is 0.23 for surface covered with short reference crop. `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, and the value of pan coefficient (only for when potential ET is estimated) - Evaporative surface with values of albedo - Option for calculating solar radiation (i.e. the value of argument `solar`) - If the PenPan formulation is used for estimating Class-A pan evaporation required in Chapman formulation (i.e. the value of argument `PenPan`) - 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 arguments `PenPan` and `solar`, please see `Arguments` for details.

Value

The function generates a list containing the following components:

 `ET.Daily` Daily aggregated estimations of Chapman potential evapotranspiration. `ET.Monthly` Monthly aggregated estimations of Chapman potential evapotranspiration. `ET.Annual` Annually aggregated estimations of Chapman equivalent Penmen-Monteith evapotranspiration. `ET.MonthlyAve` Monthly averaged estimations of daily Chapman potential evapotranspiration. `ET.AnnualAve` Annually averaged estimations of daily Chapman potential evapotranspiration. `ET_formulation` Name of the formulation used which equals to `Chapman`. `ET_type` Type of the estimation obtained which is `Potential 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.

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.

Chapman, T. 2001, Estimation of evaporation in rainfall-runoff models, in F. Ghassemi, D. Post, M. SivapalanR. Vertessy (eds), MODSIM2001: Integrating models for Natural Resources Management across Disciplines, Issues and Scales, MSSANZ, vol. 1, pp. 293-298.

`ET`,`processeddata`,`defaultconstants`,`constants`,`ET.PenPan`

Examples

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

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

[Package Evapotranspiration version 1.15 Index]