Implementing the Granger-Gray formulation for estimating actual areal evapotranspiration.
# S3 method for GrangerGray
ET(data, constants, ts="daily", solar="sunshine hours",
windfunction_ver=1948, alpha=0.23, …)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
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.
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.
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.
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.
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.
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.
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
# 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)
# }
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