Default values for control. If necessary modify with control = list() in function call.
albedo: 0.23 [-] for hypothetical grass and alfalfa reference crops used in
the FAO-56 PM equations (e.g. in
Po: atmospheric pressure at sea level (101.3 [kPa])
z0: elevation at reference level (0 [m]) (e.g. in
Tko: reference temperature [degreeC] at elevation z0. Often assumed to be 20 degreeC (e.g. in
uz: height of windspeed measurement above ground surface (2 [m]) (e.g. in
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.
as: regression constant, expressing fraction of extraterrestrial radiation reaching earth on overcast days (default = 0.25) (e.g. in
bs: as + bs fraction of extraterrestrial radiation reaching earth on clear days (default = 0.5) (e.g. in
Rs/Rso is used to represent cloud cover. For hourly or shorter 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 ratio 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.
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).
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