# ET.GrangerGray

##### Granger-Gray Formulation

Implementing the Granger-Gray formulation for estimating actual areal evapotranspiration.

##### Usage

```
# S3 method for GrangerGray
ET(data, constants, ts="daily", solar="sunshine hours",
windfunction_ver=1948, alpha=0.23, …)
```

##### Arguments

- data
A list of data which contains the following items (climate variables) required by Granger-Gray formulation:

*Tmax*,*Tmin*,*RHmax*,*RHmin*,*Rs*or*n*or*Cd*,*u2*or*uz*- constants
A list named

`constants`

consists of constants required for the calculation of Granger-Gray 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.*G*- soil heat flux in MJ.m^-2.day^-1, = 0 when using daily time step. 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`

.- 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`

.- 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, 1956) Default is`1948`

.- alpha
Any numeric value between 0 and 1 (dimensionless), albedo of evaporative surface representing the portion of the incident radiation that is reflected back at the surface. Default is 0.23 for surface covered with short reference crop.

- …
Dummy for generic function, no need to define.

##### Details

The alternative calculation options can be selected through arguments `solar`

and `windfunction_ver`

, please see `Arguments`

for details.
User-defined evaporative surface is allowed through argument `alpha`

, please see `Arguments`

for details.

##### Value

The function prints a calculation summary to the screen containing the following elements:
- ET model name and ET quantity estimated
- Evaporative surface with values of albedo
- Option for calculating solar radiation (i.e. the value of argument `solar`

)
- The version of Penman wind function has been used (i.e. the value of argument `windfunction_ver`

)
- 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_GrangerGray.csv* in the working directory:

Daily aggregated estimations of Granger-Gray actual areal evapotranspiration.

Monthly aggregated estimations of Granger-Gray actual areal evapotranspiration.

Annually aggregated estimations of Granger-Gray actual areal evapotranspiration.

Monthly averaged estimations of daily Granger-Gray actual areal evapotranspiration.

Annually averaged estimations of daily Granger-Gray actual areal evapotranspiration.

Name of the formulation used which equals to `Granger-Gray`

.

Type of the estimation obtained which is `Actual Areal Evapotranspiration`

.

A message to inform the users about how solar radiation has been calculated by using which data.

A message to inform the users about which version of the Penman wind function has been used.

##### 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.

Penman, H. L. 1956. *Evaporation: An introductory survey*. Netherlands Journal of Agricultural Science, 4, 9-29

##### See Also

##### Examples

```
# NOT RUN {
# Use processed existing data set and constants from kent Town, Adelaide
data("processeddata")
data("constants")
# Call ET.GrangerGray under the generic function ET
results <- ET.GrangerGray(data, constants, ts="daily",
solar="sunshine hours", windfunction_ver=1948, alpha=0.23)
# }
```

*Documentation reproduced from package Evapotranspiration, version 1.10, License: GPL (>= 2)*