Implementing the Matt-Shuttleworth formulation for reference crop evapotranspiration
# S3 method for MattShuttleworth
ET(data, constants, ts="daily", solar="sunshine hours", alpha=0.23, r_s=70, CH=0.12, …)A list which contains the following items (climate variables) required by Matt-Shuttleworth 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 Matt-Shuttleworth 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,
Roua - mean air density = 1.20 kg.m^-3,
Ca - specific heat of air = 0.001013 MJ.kg^-1.oC^-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.
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, which is for the calculation of Matt-Shuttleworth reference crop evaporation.
Any value (seconds per metre), surface resistance depends on the type of reference crop. Default is 70 for short reference crop.
Any value (metres), crop height depends on the reference crop. Default is 0.12 for 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, surface resistance and crop height
- 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_MattShuttleworth.csv in the working directory:
Daily aggregated estimations of Matt-Shuttleworth reference crop evapotranspiration.
Monthly aggregated estimations of Matt-Shuttleworth reference crop evapotranspiration.
Annually aggregated estimations of Matt-Shuttleworth reference crop evapotranspiration.
Monthly averaged estimations of daily Matt-Shuttleworth reference crop evapotranspiration.
Annually averaged estimations of daily Matt-Shuttleworth reference crop evapotranspiration.
Name of the formulation used which equals to Matt-Shuttleworth.
Type of the estimation obtained which is Reference Crop 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.
User-defined evaporative surface is allowed through arguments alpha, r_s and CH, please see Arguments for details.
Shuttleworth, W. & Wallace, J. 2009. Calculating the water requirements of irrigated crops in Australia using the Matt-Shuttleworth approach. Transactions of the ASABE, 52, 1895-1906.
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.
# NOT RUN {
# Use processed existing data set and constants from kent Town, Adelaide
data("processeddata")
data("constants")
# Call ET.MattShuttleworth under the generic function ET
results <- ET.MattShuttleworth(data, constants, ts="daily",
solar="sunshine hours", alpha=0.23, r_s=70, CH=0.12)
# }
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