Implementing the Penman-Monteith formulation. To estimate crop evapotranspiration, the formulation can take possible of either FAO-56 model for hypothetical short grass, the ASCE-EWRI Standardised model for tall grass. The model can also estimate ET from several other vegetation types based on Equation 6.70 in Dingman (2015).
# S3 method for PenmanMonteith
ET(data, constants, ts="daily", solar="sunshine hours", wind="yes",
vegetation="reference crop", …)A list which contains the following items (climate variables) required by Penman-Monteith 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 Penman-Monteith 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.
Must be either yes or no.
yes indicates that the calculation will use real data of wind speed;
no indicates that the alternative calculation without using wind data will be used in Penman formulation (Valiantzas 2006, Equation33).
Default is yes.
Must be either:
reference crop,
long grass,
deciduous broad-leaf forest,
evergreen needle-leaf forest,
open shrubland or
grassland.
reference crop indicates that the method for FAO-56 hypothetical short grass will be applied (Allen et al., 1998, Equation 6);
long grass indicates that the method for ASCE-EWRI Standardised crop will be applied (ASCE, 2005, Equation 1, Table 1).
All other options lead to the use of Equation 6.70 in Dingman (2015) to estimate ET for specific vegetation cover, with wind measured at instrument height adjusted to 2m following Federer er al. (1996) . Constants for each vegetation type are defined according to values suggested in http://ldas.gsfc.nasa.gov/nldas/web/web.veg.table.html.
Default is 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, crop height and roughness height
- Option for calculating solar radiation (i.e. the value of argument solar)
- If actual wind data has been used for calculation (i.e. the value of argument wind)
- 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_PenmanMonteith.csv in the working directory:
Daily aggregated estimations of Penman-Monteith evapotranspiration.
Monthly aggregated estimations of Penman-Monteith evapotranspiration.
Annually aggregated estimations of Penman-Monteith evapotranspiration.
Monthly averaged estimations of daily Penman-Monteith evapotranspiration.
Annually averaged estimations of daily Penman-Monteith evapotranspiration.
Name of the formulation used which equals to either Penman-Monteith FAO56 or Penman-Monteith ASCE-EWRI Standardised.
A character string containing the type of the estimation obtained depending on the vegetation type.
A message to inform the users about how solar radiation has been calculated by using which data.
A message to inform the users about if actual wind data has been used in the calculations or alternative calculations has been performed without wind data.
The alternative calculation options can be selected through arguments solar and wind, please see Arguments for details.
User-defined evaporative surface is allowed through arguments vegetation, 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.
Allen, R. G., Pereira, L. S., Raes, D. & Smith, M. 1998. Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage. paper 56. FAO, Rome, 300, 6541.
Allen, R. G. 2005. The ASCE standardized reference evapotranspiration equation. Amer Society of Civil Engineers.
Brian Cosgrove, Mapped Static Vegetation Data, available from http://ldas.gsfc.nasa.gov/nldas/web/web.veg.table.html.
Dingman, S.L., 2015, Physical Hydrology, Third edition, Waveland Press, Inc.
Federer, C. A., et al. (1996). Intercomparison of Methods for Calculating Potential Evaporation in Regional and Global Water Balance Models, Water Resources Research 32(7): 2315-2321.
# NOT RUN {
# Use processed existing data set and constants from kent Town, Adelaide
data("processeddata")
data("constants")
# Call ET.PenmanMonteith under the generic function ET
results <- ET.PenmanMonteith(data, constants, ts="daily", solar="sunshine hours",
wind="yes")
# Call ET.PenmanMonteith to estimate ET from different vegetation covers
# referecen crop (FAO-56)
results <- ET.PenmanMonteith(data, constants, ts="daily", solar="sunshine hours",
wind="yes",vegetation="reference crop")
# long grass (ASCE-EWRI)
results <- ET.PenmanMonteith(data, constants, ts="daily", solar="sunshine hours",
wind="yes",vegetation="long grass")
# other types etc. (Equation 6.70 in Dingman (2015))
results <- ET.PenmanMonteith(data, constants, ts="daily", solar="sunshine hours",
wind="yes",vegetation="deciduous broad-leaf forest")
# }
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