!! Compute gross primary production modifiers
!|author:  <a href="mailto:giovanni.ravazzani@polimi.it">Giovanni Ravazzani</a>
! license: <a href="http://www.gnu.org/licenses/">GPL</a>
!    
!### History
!
! current version  1.0 - 2nd May 2019  
!
! | version  |  date       |  comment |
! |----------|-------------|----------|
! | 1.0      | 2/May/2019 | Original code |
!
!### License  
! license: GNU GPL <http://www.gnu.org/licenses/>
!
!### Module Description 
! Routines to compute gross primary production modifiers
! Implemented modifiers:
!
! 1. Age modifier [[Agemod]]
!
! 2. CO<sub>2</sub> modifier [[CO2mod]]
!
! 3. Soil water content modifier [[SWCmod]]
!
! 4. Air temperature modifier  [[TEMPmod]]
!
! 5. Vapor pressure deficit modifier [[VPDmod]]
! 
MODULE PlantsModifiers

! Modules used:


USE DataTypeSizes, ONLY : &
   ! Imported Type Definitions:
   short, float

USE LogLib, ONLY: &
   !Imported routines:
   Catch

IMPLICIT NONE

!global routines:
PUBLIC :: SWCmod
PUBLIC :: AGEmod
PUBLIC :: TEMPmod
PUBLIC :: VPDmod
PUBLIC :: CO2mod


!local routines:



!=======
    CONTAINS
!=======

    
!==============================================================================
!| Description:
!  compute the age modifier. It modulates the maximum potential growth during the
!  different stages of the vegetation life cycle as trees in the early stages 
!  are not as vigorous as mature trees
!
!  Reference: 
!
!  Peng, C., J. Liu, Q. Dang, M. J. Apps, and H. Jiang, 2002: TRIPLEX: A generic 
!  hybrid model for predicting forest growth and carbon and nitrogen dynamics. 
!  Ecol. Modell., 153 (1–2), 109–130.
FUNCTION  AGEmod &
!
(age, agemax) &
!
RESULT (f)

IMPLICIT NONE

!arguments with intent(in):
REAL (KIND = float), INTENT(IN) :: age !!actual age (years)
REAL (KIND = float), INTENT(IN) :: agemax !! maximum age (years)

!local declarations:
REAL (KIND = float) :: f
!---------------------------------------end of declarations--------------------

!compute modifier
!IF ( age < 0.2 * agemax ) THEN
!    f = 0.7 + 0.3 * age / ( 0.2 * agemax)
!ELSE
!    f = 1. + MAX (0.,  ((age - 0.2 * agemax) / (0.95 * agemax) ) ** 3. )
!END IF



f = ( 1. / (1. + ( ( age / agemax ) / 0.95 ) ) )**4.


!final boundary check
IF ( f > 1.) THEN
    f = 1.
END IF

IF ( f < 0.) THEN
    f = 0.
END IF

RETURN
END FUNCTION AGEmod
    
    
!==============================================================================
!| Description:
!  soil water content modifier
!
!  Reference: 
!
!  Cox, P. M., C. Huntingford, and R. J. Harding, 1998: A canopy conductance 
!  and photosynthesis model for use in a GCM land surface scheme. 
!  J. Hydrol., 212–213, 79–94
!  
FUNCTION  SWCmod &
!
(swc, wp, fc, theta) &
!
RESULT (f)

IMPLICIT NONE

!Arguments with intent (in):
REAL (KIND = float), INTENT (IN) :: swc !! actual soil water content [m3/m3]
REAL (KIND = float), INTENT (IN) :: wp !! soil wilting point [m3/m3]
REAL (KIND = float), INTENT (IN) :: fc !! soil field capacity [m3/m3]
REAL (KIND = float), INTENT (IN) :: theta !! empirical parameter to compute soil water content modifier

!local declarations:
REAL (KIND = float) :: f
REAL (KIND = float) :: beta
!---------------------------------------end of declarations--------------------

!compute beta
IF ( swc <= wp ) THEN
   beta = 0.
ELSE IF ( swc > wp .AND. swc < fc ) THEN
    beta = ( swc - wp ) / ( fc - wp )
ELSE !swc >= fc
    beta = 1.
END IF

!compute modifier
f = ( 1. - EXP ( - beta * theta) ) /  ( 1. - EXP ( - theta) )

!final boundary check
IF ( f > 1.) THEN
    f = 1.
END IF

IF ( f < 0.) THEN
    f = 0.
END IF

RETURN
END FUNCTION SWCmod



!==============================================================================
!| Description:
!  compute air temperature modifier. The growth and dormant stages of 
!  vegetation are related to the annual cycle of air temperature. 
!  Maximum growth will happen at optimal temperatures Topt and
!  will stop when temperatures drop below or exceed certain temperature 
!  thresholds, Tmin and Tmax, respectively.
!
!  Reference: 
!
!  Landsberg, J. J., and R. H. Waring, 1997: A generalised model of forest 
!  productivity using simplified concepts of radiation-use efficiency, 
!  carbon balance and partitioning. For. Ecol. Manage., 95, 209–228.
FUNCTION  TEMPmod &
!
(Ta, Tmin, Tmax, Topt) &
!
RESULT (f)

IMPLICIT NONE

!Arguments with intent(in):
REAL (KIND = float), INTENT(IN) :: Ta ! current air temperature [°C] 
REAL (KIND = float), INTENT(IN) :: Tmin ! minimum temperature for vegetation growing [°C] 
REAL (KIND = float), INTENT(IN) :: Tmax ! maximum temperature for vegetation growing [°C]
REAL (KIND = float), INTENT(IN) :: Topt ! optimum temperature for vegetation growing [°C]

!local declarations:
REAL (KIND = float) :: f
REAL (KIND = float) :: Tair
!---------------------------------------end of declarations--------------------

IF ( Topt < Tmin) THEN 
    CALL Catch ('error', 'PlantsModifiers', 'Topt < Tmin cannot compute temperature modifier')
END IF

IF ( Topt > Tmax) THEN 
    CALL Catch ('error', 'PlantsModifiers', 'Topt > Tmax cannot compute temperature modifier')
END IF

!set tair
IF (Ta > Tmax ) THEN
    Tair = Tmax
    CALL Catch ('warning', 'PlantsModifiers', 'Tair > Tmax Tair set to Tmax')
ELSE IF (Ta < Tmin) THEN    
    Tair = Tmin
    CALL Catch ('warning', 'PlantsModifiers', 'Tair < Tmin Tair set to Tmin')
ELSE
    Tair = Ta
END IF
!compute modifier
f = ( Tair - Tmin ) / ( Topt - Tmin ) * &
    ( ( Tmax - Tair ) / ( Tmax - Topt ) ) ** ( (Tmax -Topt) / (Topt - Tmin) )

!final boundary check
IF ( f > 1.) THEN
    f = 1.
END IF

IF ( f < 0.) THEN
    f = 0.
END IF

RETURN
END FUNCTION TEMPmod


!==============================================================================
!| Description:
!  compute vapor pressure deficit modifier.
!
!  References: 
!
!  Landsberg, J. J., and R. H. Waring, 1997: A generalised model of forest 
!  productivity using simplified concepts of radiation-use efficiency, 
!  carbon balance and partitioning. For. Ecol. Manage., 95, 209–228.
!
!  Dingman, S. L., 2002: Physical Hydrology. Prentice Hall, 646 pp
!
FUNCTION  VPDmod &
!
(Ta, RH, kd) &
!
RESULT (f)

IMPLICIT NONE

!Arguments with intent(in):
REAL (KIND = float), INTENT(IN) :: Ta ! current air temperature [°C] 
REAL (KIND = float), INTENT(IN) :: RH ! air relative humidity [0-1] 
REAL (KIND = float), INTENT(IN) :: kd ! Stomatal response to VPD [mbar]  


!local declarations:
REAL (KIND = float) :: f
REAL (KIND = float) :: estar !!saturation vapor pressure [mbar]
REAL (KIND = float) :: vpd !!vapor pressure deficit [mbar]
!---------------------------------------end of declarations--------------------
!compute saturation vapor pressure in the air (Dingman, 2002) [mbar]
estar = 6.1076 * EXP ( (17.269 * Ta) / (Ta + 237.3) )

!compute vapor pressure deficit
vpd = estar * ( 1. - RH )

!compute modifier
f = EXP ( - kd * vpd)

!final boundary check
IF ( f > 1.) THEN
    f = 1.
END IF

IF ( f < 0.) THEN
    f = 0.
END IF

RETURN
END FUNCTION VPDmod



!==============================================================================
!| Description:
!  compute CO<sub>2</sub>2 modifier. 
!
!  References:
!
!  Veroustraete, F., Sabbe, H. and Eerens, H. (2002) ‘Estimation of carbon 
!  mass fluxes over Europe using the C-Fix model and Euroflux data’, 
!  Remote Sensing of Environment, 83(3), pp. 376–399. 
!  doi: 10.1016/S0034-4257(02)00043-3.
FUNCTION  CO2mod &
!
(co2, age) &
!
RESULT (f)

IMPLICIT NONE

!arguments with intent(in):
REAL (KIND = float), INTENT(IN) :: co2 !!CO2 concentration (ppm)
REAL (KIND = float), INTENT(IN) :: age !! plant age (year)

!local declarations:
REAL (KIND = float) :: f
REAL (KIND = float) :: fCalphax
!---------------------------------------end of declarations--------------------

IF (age == 2.) THEN   
    fCalphax = 1.
ELSE
    fCalphax = age / (2. - age)
END IF

f = fCalphax * co2 / (350. * (fCalphax - 1.) + co2)

RETURN
END FUNCTION CO2mod

    
END MODULE PlantsModifiers