#include <math.h>

/*--------------------------------------------------------------------------*/

double opp_eppley( double chl,
		   double irr,
		   double sst,
		   double dayL ) {
/*
!C--------------------------------------------------------------------------*\

   !Description:     opp_eppley - computes daily primary productivity using
                     the Behrenfeld-Falkowski (BeFa) algorithm, but 
		     modifies the pb_opt function after Eppley (as  
		     implemented by Antoine and Morel).  The BeFa
                     algorithm estimates productivity using surface chl
                     (mg m-3), surface irradiance (Einsteins m-2 d-1),
                     sea surface temperature (C), and day length (hours).
		     Pb_opt is modelled as an exponential function of SST.

   !Input Parameters:  
      chl            Chlorophyll_a surface concentration in milligrams
                     chlorophyl per cubic meter
      irr            Photosynthetically available radiation in Einsteins per
                     day per square meter
      sst            Sea surface temperature in degrees Centigrade
      dayL           Length day in decimal hours.

   !Output Parameters: 
      <return>       Primary productivity in milligrams Carbon per square meter
                     per day

   !Revision History:  

      First programmed up by Monica Chen at Rutgers
      (1996)
      
      Revised by K. Turpie at NASA 
      (August 1997)
      
      Maintained by Don Shea at NASA
      
      Now maintained by Robert O'Malley at Oregon State University  
      (April, 2005 - present)

      changing z_eu calc to match updated Morel equation
      O'Malley    02.13.2023

   !References and Credits
   
      Behrenfeld,M.J; Falkowski,P.G.; 1997. Photosynthetic Rates Derived
      from Satellite-Based Chlorophyll Concentration.  Limnology and 
      Oceanography, Volume 42, Number 1
      
      Eppley, R.W.; 1972.  Temperature and Phytoplankton Growth in the Sea.
      Fishery Bulletin, Volume 79, Number 4
      
      Antoine, D.; Morel, A.; 1996.  Oceanic Primary Production
      1.  Adatptation of a Spectral Light-Photosynthesis Model
      in view of Application to Satellite Chlorophyll Observations
      
!END------------------------------------------------------------------------*\
*/
   double chl_tot,
     z_eu,
     pb_opt,
     irrFunc,
     npp;
     
   double X,
     log10z_eu;


   /* Calculate euphotic depth (z_eu) with Morel's 2007 model.            */
   /* Previously had used Morel's 1988 model                              */

   X = log10(chl);
   log10z_eu = 1.524 - 0.436*X - 0.0145*X*X + 0.0186*X*X*X;
   z_eu = pow(10,log10z_eu);
     

   /* Calculate the Pb_opt from satellite sea surface temperature (sst).    */

   pb_opt = 1.54 * pow(10, 0.0275 * sst - 0.07);


   /* calculate the irradiance function */
   
   irrFunc = 0.66125L * irr / ( irr + 4.1L );


   /* Return the primary production calculation.                            */
   
   npp = pb_opt * chl * dayL * irrFunc * z_eu;

   return npp;
}