estG {MeTo}

Estimate soil heat flux (G)

Description

Estimate soil heat flux (G) for periods shorter than a day.

Usage

estG(
  x,
  Rs,
  Tmean,
  Rhmean,
  lat.rad = NULL,
  lat.deg = NULL,
  long.deg = NULL,
  elev = 1,
  tl,
  control = list(albedo = 0.23, Lz = 345, est.ratio.Rs.Rso = NA)
)

Arguments

x

date-time object (see details)

Rs

incoming solar radiation [MJ/(m2 time)]

Tmean

Mean air temperature [degreeC]

Rhmean

Mean air humidity [percent]

lat.rad

latitude [rad]. Use either lat.rad or lat.deg. Latitude is positive for the northern hemisphere and negative for the southern hemisphere

lat.deg

latitude [degree]. Use either lat.deg or lat.rad. Latitude is positive for the northern hemisphere and negative for the southern hemisphere

long.deg

longitude of the measurement site (degrees east of Greenwich)

elev

station elevation above sea level [m]

tl

length of calculation period [hour] (1 for hourly period, 0.5 for a 30-minute period or 24 for daily period). Only needed if x is date-time object with length of 1.

control

list for control parameters and empirical factors defined in controlDefaults and constDefaults (see Details)

Details

during daylight periods G is estimated to be Rn x 0.1 During nighttime G = Rn x 0.5. Day is defined for extraterrestrial radiation > 0.

x:

must be provided as a common date-time object (e.g, POSIXct, POSIXlt or Date objects). All formats for which is.timepoint from the lubridate package returns TRUE can be used

control:

albedo:
default 0.23 for the hypothetical grass and alfalfa reference crops used in
the FAO-56 PM equations

Lz:
longitude of the centre of the local time zone (degrees west of Greenwich)
- 0 for Greenwich
- 345 for Germany
- 330 for Cairo (Egypt)
- 255 for Bangkok (Thailand)
- 75, 90, 105 and 120 for Eastern, Central, Rocky Mountain and Pacific time zones (United States)
Lz is only needed if calculation period is shorter 1 day.

est.ratio.Rs.Rso:
Rs/Rso is used to represent cloud cover. For hourly periods during the nighttime, the ratio Rs/Rso is set equal to the Rs/Rso calculated for a time period occurring 2-3 hours before sunset. If single values during nighttime are calculated Rs/Rso ration 2-3 hours before sunset can not be calculated and an approximation is needed. Following Allen (1999) one can assume Rs/Rso = 0.4 to 0.6 during nighttime periods in humid and subhumid climates and Rs/Rso = 0.7 to 0.8 in arid and semiarid climates. A value of Rs/Rso = 0.3 presumes total cloud cover.

Note

eq. 45 and 46 of reference

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, Rome, 300(9).

Examples

estG(x = as.POSIXct(c('2018-10-01 14:30', '2018-10-01 15:00')), Tmean = 38, Rhmean = 52, Rs = 2.450,
     elev = 8, lat.deg = 16.21, long.deg = 343.75, control = list(Lz = 15))

estG(x = as.POSIXct('2018-10-01 02:30'), Tmean = 28, Rhmean = 90, tl = 1, Rs = 0, elev = 8,
     lat.deg = 16.2, long.deg = 343.75, control = list(Lz = 15, est.ratio.Rs.Rso = 0.8))

estG(x = as.POSIXct('2018-10-01 14:30'), Tmean = 38, Rhmean = 52, tl = 1, Rs = 2.450, elev = 8,
     lat.deg = 16.21, long.deg = 343.75, control = list(Lz = 15))