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,
  za = NULL,
  DAP = 1,
  date = NULL,
  Ta = NULL,
  G = NULL,
  RH = NULL,
  Rg = NULL,
  AP = NULL,
  WS = NULL,
  Kc = NULL
)

Arguments

data

Data frame containing the data

lat

Numeric, latitude in decimals

alt

Numeric, altitude in meters

za

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")

Ta

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)

RH

String with the column name containing relative humidity records in %

Rg

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

AP

String with the column name containing atmospheric pressure records in hPa

WS

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); 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,
    za = 10,
    DAP = 1,
    date = colnames(df)[1],
    Ta = colnames(df)[7],
    G = NULL,
    RH = colnames(df)[15],
    Rg = colnames(df)[3],
    AP = colnames(df)[4],
    WS = colnames(df)[18],
    Kc = NULL
  )