function dPdt=phos5box(t,P);
% A function file that calculates the phosphorus time differentials for
%   a 5-box, global zero-dimensional phosphorus model.
%
% Started 2004:10:20 D. Glover, WHOI
% Modif'd 2004:10:21 DMG to get it running, add documentation
% Modif'd 2004:10:28 DMG to futz with Far and Fpr
% Modif'd 2008:11:06 DMG to change names to PS06_1

% Declare some parameter values with the following naming scheme:
%   SAT = 1 (i.e. box number one)
%   ANT = 2
%   SIP = 3
%   DIP = 4
%   DAT = 5
%
% Fluxes are given as Fnm, i.e. flux from box n to box m

% Declare some GLOBAL variables (set in PS06p1_main.m)

global TAU REFF V SPYR;

% Set some parameters that only dy/dt cares about

F=[ 0  0  0  0 20;                                  % Flux
   12  0  0 32  4;                                  % Matrix
    0 24  0  0  0;
    0  8 24  0  0;
    8 16  0  0  0];
F=F.*1e6;

Far=2.5e5; Fpr=7.5e5;   % River water fluxes
Pr=1.3;                 % River P concentration

dPdt=zeros(5,1);        % initialize output column vector

% Calculate Productivity  surface boxes

Prod=[P(1)*V(1)/TAU P(3)*V(3)/TAU];

% Differential equations

dPdt(1)=((Far*Pr-F(1,5)*P(1)+F(5,1)*P(5)+F(2,1)*P(2)-Prod(1))*SPYR)/V(1); %SAT

dPdt(2)=((-F(2,1)*P(2)+F(3,2)*P(3)+F(4,2)*P(4)-F(2,4)*P(2)-F(2,5)*P(2)+F(5,2)*P(5))*SPYR)/V(2); %ANT

dPdt(3)=((Fpr*Pr-F(3,2)*P(3)+F(4,3)*P(4)-Prod(2))*SPYR)/V(3); %SIP

dPdt(4)=((REFF*Prod(2)-F(4,3)*P(4)-F(4,2)*P(4)+F(2,4)*P(2))*SPYR)/V(4); %DIP

dPdt(5)=((REFF*Prod(1)+F(1,5)*P(1)-F(5,1)*P(5)-F(5,2)*P(5)+F(2,5)*P(2))*SPYR)/V(5); %DAT