et_pot Subroutine

subroutine et_pot()

Uses

  • proc~~et_pot~~UsesGraph proc~et_pot et_pot module~basin_module basin_module proc~et_pot->module~basin_module module~climate_module climate_module proc~et_pot->module~climate_module module~hru_module hru_module proc~et_pot->module~hru_module module~hydrograph_module hydrograph_module proc~et_pot->module~hydrograph_module module~plant_data_module plant_data_module proc~et_pot->module~plant_data_module module~plant_module plant_module proc~et_pot->module~plant_module module~hydrograph_module->module~basin_module module~time_module time_module module~hydrograph_module->module~time_module 
~ ~ ~ PURPOSE ~ ~ ~
this subroutine calculates potential evapotranspiration using one
of three methods. If Penman-Monteith is being used, potential plant
transpiration is also calculated.

~ ~ ~ INCOMING VARIABLES ~ ~ ~
name       |units          |definition
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
albday     |none           |albedo for the day in HRU
gsi(:)     |m/s            |maximum stomatal conductance
ihru       |none           |HRU number
vpd2(:)    |(m/s)*(1/kPa)  |rate of decline in stomatal conductance per
           |               |unit increase in vapor pressure deficit
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ OUTGOING VARIABLES ~ ~ ~
name        |units         |definition
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
ep_max      |mm H2O        |maximum amount of transpiration (plant et) 
                           |that can occur on current day in HRU
pet_day     |mm H2O        |potential evapotranspiration on current day in
                           |HRU
vpd         |kPa           |vapor pressure deficit
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ LOCAL DEFINITIONS ~ ~ ~
name        |units         |definition
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

~ ~ ~ SUBROUTINES/FUNCTIONS CALLED ~ ~ ~
Intrinsic: Log, Sqrt, Max, Min
SWAT: Ee

~ ~ ~ ~ ~ ~ END SPECIFICATIONS ~ ~ ~ ~ ~ ~

calculate mean barometric pressure calculate latent heat of vaporization calculate psychrometric constant calculate saturation vapor pressure, actual vapor pressure and vapor pressure deficit calculate the slope of the saturation vapor pressure curve DETERMINE POTENTIAL ET

PRIESTLEY-TAYLOR POTENTIAL EVAPOTRANSPIRATION METHOD

net radiation calculate net short-wave radiation for PET calculate net long-wave radiation

net emissivity equation 2.2.20 in SWAT manual cloud cover factor equation 2.2.19 net long-wave radiation equation 2.2.21 calculate net radiation net radiation PENMAN-MONTEITH POTENTIAL EVAPOTRANSPIRATION METHOD

net radiation calculate net short-wave radiation for PET calculate net short-wave radiation for max plant ET calculate net long-wave radiation net emissivity equation 2.2.20 in SWAT manual cloud cover factor equation 2.2.19 net long-wave radiation equation 2.2.21 calculate net radiation net radiation

potential ET: reference crop alfalfa at 40 cm height maximum plant ET determine wind speed and height of wind speed measurement adjust to 100 cm (1 m) above canopy if necessary calculate canopy height in cm calculate roughness length for momentum transfer calculate roughness length for vapor transfer calculate zero-plane displacement of wind profile calculate aerodynamic resistance adjust stomatal conductivity for low vapor pressure this adjustment will lower maximum plant ET for plants sensitive to very low vapor pressure calculate canopy resistance calculate maximum plant ET HARGREAVES POTENTIAL EVAPOTRANSPIRATION METHOD

extraterrestrial radiation 37.59 is coefficient in equation 2.2.6 !!extraterrestrial 30.00 is coefficient in equation 2.2.7 !!max at surface READ IN PET VALUES

Arguments

None