calculateETrefPM {DataMetProcess}

The FAO Penman–Monteith for calculating daily reference evapotranspiration

Description

Calculation of daily reference evapotranspiration using the PM method for a dataset stored in a data.frame (Allen et al., 1998).

Usage

calculateETrefPM(
  data = NULL,
  Lat = NULL,
  Alt = NULL,
  Alt_an = NULL,
  DAP = 1,
  Date = NULL,
  Temp = NULL,
  G = NULL,
  Humid = NULL,
  Rad = NULL,
  Press = NULL,
  Wind = NULL,
  Kc = NULL
)

Arguments

data

Data frame containing the data

Lat

Numeric, latitude in decimals

Alt

Numeric, altitude in meters

Alt_an

Numeric, anemometer height in meters

DAP

Numeric, days after planting for the first column date

Date

String with the column name containing date records (R default date: "%Y-%m-%d")

Temp

String with the column name containing temperature records in °C

G

Optional, if NULL will be considered as zero. String with the column name containing soil heat flux (MJ/m²/day)

Humid

String with the column name containing relative humidity records in %

Rad

String with the column name containing global radiation records in MJ/m²

Press

String with the column name containing atmospheric pressure records in hPa

Wind

String with the column name containing wind speed records in m/s

Kc

Optional, when not NULL the crop evapotranspiration ETc is calculated based on ETref. String with the column name containing crop coefficient (Kc) records

Details

The FAO Penman–Monteith method:

ETrefPM = \frac{0.408 \Delta(Rn-G) + \gamma \frac{900}{T+273}u_{2}(e_{s}-e_{a})}{\Delta+\gamma(1+0.34u_{2})}

where: ETref - reference evapotranspiration (mm/dia), delta - slope of the saturated water–vapor-pressure curve (kPA/°C), Rn - net radiation (MJ/m²/dia), G - soil heat flux (MJ/m²/day), y - psychrometric constant (kPA/°C), T - average daily air temperature (°C), u2 -wind speed at 2m height (m/s), es - saturation vapor pressure (kPa), e ea - actual vapor pressure (kPa)

Value

Data frame with: Date; ETref - reference evapotranspiration (mm/dia); LLI - irrigation level (mm/dia); DJ - julian day; DAP - days after planting; es - saturation vapor pressure (kPa); ea - actual vapor pressure (kPa); delta - slope of the saturated water–vapor-pressure curve (kPA/°C); y - psychrometric constant (kPA/°C); Rn - net radiation (MJ/m²/dia); ETc - crop evapotranspiration (mm/dia) (depends on supply of Kc)

References

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, 1998. ISBN 92-5-104219-5.

Examples

address <-
 base::system.file("extdata",
                    "ex2_daily.CSV",
                    package = "DataMetProcess")

df <- read.table(
address,
h = TRUE,
sep = ";"
)

#converting to Mj/m
df$radiacao_global_kj_m <- df$radiacao_global_kj_m/1000
colnames(df)[3] <- "radiacao_global_mj_m"

df.Eto <-
  calculateETrefPM(
    data = df,
    Lat = -21.980353,
    Alt = 859.29,
    Alt_an = 10,
    DAP = 1,
    Date = colnames(df)[1],
    Temp = colnames(df)[7],
    G = NULL,
    Humid = colnames(df)[15],
    Rad = colnames(df)[3],
    Press = colnames(df)[4],
    Wind = colnames(df)[18],
    Kc = NULL
  )