Morton CRWE Formulation

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Description

Implementing the Morton CRWE formulation for estimating potential evapotranspiration or shallow lake evaporation.

Usage

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## S3 method for class 'MortonCRWE'
ET(data, constants, ts="monthly", est="potential ET", 
solar="sunshine hours", Tdew= T, alpha = NULL, ...)

Arguments

data

A list of data which contains the following items (climate variables) required by Morton CRWE formulation:
Tmax, Tmin, Tdew, Rs or n or Cd

constants

A list named constants consists of constants required for the calculation of Morton CRWE formulation which must contain the following items:
Elev - ground elevation above mean sea level in m,
lat_rad - latitude in radians,
PA - annual precipitation in mm, required when precipitation data is not available,
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,
epsilonMo - surface emissivity = 0.92 (Morton, 1986),
fz - A constant in Morton's procedure = 25.0 Wm^-2.mbar^-1 for T >= 0 degree Celcius, and = 28.75 Wm^-2.mbar^-1 for T >= 0 degree Celcius for CRWE model (Morton, 1986),
b0 - A constants in Morton's procedure, = 1.12 for CRWE model, (Morton, 1986) b1 - A constant in Morton's procedure, = 13 for CRWE model (Morton, 1986),
b2 - A constant in Morton's procedure, = 1.12 for CRWE model (Morton, 1986),
gammaps - Produce of Psychrometric constant and atmospheric pressure as sea level, = 0.66 mbar. degree Celcius^-1 for T >= 0 degree Celcius, = 0.66/1.15 mbar. degree Celcius^-1 for T < 0 degree Celcius (Morton, 1983),

alphaMo - a constant in Morton's procedure, = 17.27 when T >= 0 degree Celcius, = 21.88 when T < 0 degree Celcius (Morton, 1983),
betaMo - a constant in Morton's procedure, = 237.3 degree Celcius when T >= 0 degree Celcius, = 265.5 degree Celcius, when T < 0 degree Celcius (Morton, 1983),
sigmaMo - Stefan-Boltzmann constant in Morton's procedure, = 5.67e-08 W.m^-2.K^-4 (Morton, 1983),
lambdaMo - Latent heat of vaporisation in Morton's procedure, = 28.5W.day.kg^-1 when T >= 0 degree Celcius, = 28.5*1.15W.day.kg^-1 when T < 0 degree Celcius,

ts

Must be either monthly or annual, which indicates the disired time step that the output ET estimates should be on. Default is monthly.

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.

est

Must be either potential ET or shallow lake ET:
potential ET proceeds to estimating potential evapotranspiration;
shallow lake ET proceeds to estimating shallow lake evaporation.
Default is potential ET.

Tdew

Must be T or F, indicating if real data of dew point temperature is used for calculating the radiation in Morton's formulations, if T the data will be used and if F the dew point temperature will be calculated from data of daily vapour pressure. Default is T for using actual dew point temperature data.

alpha

Only needed if argument solar has value of data.
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 NULL in line with the default use of sunshine hours to estimate solar radiation (i.e. argument solar is sunshine hours.

...

Dummy for generic function, no need to define.

Details

The type of evapotranspiration calculated can be selected through argument est, please see Arguments for details. The alternative calculation options can be selected through argument solar and Tdew, please see Arguments for details.

Value

The function prints a calculation summary to the screen containign the following elements:
- ET model name and ET quantity estimated (i.e. the value of argument est)
- Option for calculating solar radiation (i.e. the value of argument solar)
- If the actual dew point temperature data are used (i.e. the value of argument Tdew)
- 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_MortonCRWE.csv in the working directory:

ET.Daily

Daily aggregated estimations of MortonCRWE potential evapotranspiration or shallow lake evaporation.

ET.Monthly

Monthly aggregated estimations of MortonCRWE potential evapotranspiration or shallow lake evaporation.

ET.Annual

Annually aggregated estimations of MortonCRWE potential evapotranspiration or shallow lake evaporation.

ET.MonthlyAve

Monthly averaged estimations of daily MortonCRWE potential evapotranspiration or shallow lake evaporation.

ET.AnnualAve

Annually averaged estimations of daily MortonCRWE potential evapotranspiration or shallow lake evaporation.

ET_formulation

Name of the formulation used which equals to MortonCRWE.

ET_type

Type of the estimation obtained which is either Potential Evapotranspiration or Shallow Lake Evaporation.

message1

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

message6

A message to inform the users about if actual dew point temperature has been used in the calculations or alternative calculations has been performed without dew point temperature data.

Author(s)

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.

Morton, F.I. 1983, Operational estimates of lake evaporation. Journal of Hydrology, vol. 66, no. 1-4, pp. 77-100.

See Also

data,defaultconstants,constants,ET.MortonCRWE

Examples

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

# Call ET.MortonCRWE under the generic function ET
results <- ET.MortonCRWE(data, constants, ts="monthly",
est="potential ET", solar="sunshine hours", Tdew= TRUE, 
alpha = NULL) 

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