With MATLAB, I calculated the concentrations of Algae, Zooplankton, Oxygen, & Carbon in a river as they interact and are consumed.

Sediment.matlab

%% Matthew Widjaja
% Environmental Modeling -- Program 1
% Parking Lot & Lake

clear t c
format short

global volumeV areaA baseFlowQ sediFlowM sediVelocityVs

%% Obtain Data
% Lets the user select from Assignment 1 Data or Alternate Data.

stepSizeN = input('Declare a Stepsize (ex. 0.1) = ');
sediDiameterD = input('Declare Diameter of Sediment (m) = ');
sediVelocityVs = (9.81 * ((2650/1000)-1) * sediDiameterD^2) ...
	/ (18 * .000001103);
dataUse = input('Should Assignment 1 Data be used? (y/n) = ','s');
if dataUse == 'y'
	volumeV = 200;
	areaA = 20;
	baseFlowQ = .2;
	stormTime = 3600;
	finalTime = 14400;
	sediFlowM = 50;
else
	volumeV = input('Declare Volume (m3) = ');
	areaA = input('Declare Plan Area (m2) = ');
	baseFlowQ = input('Declare Flow through Lake (m3/s) = ');
	stormTime = input('Declare Duration of Storm (secs) = ');
	finalTime = stormTime * 4;
	sediFlowM = input('Delcare Sediment Flow during Storm (mg/s) = ');
end


%% Euler Variables
% Automates Variables for Eulers & Starting Parameters

initialTime = 1;
finalTime = finalTime + 1;
stepCountH = (finalTime-initialTime)/stepSizeN;
t(1) = 0;
c(1) = 0;


%% Special Conditions
% If sediDiameterD = 0, we set sediFlowM = 0.

if sediDiameterD <= 0;
	sediFlowM = 0;
end


%% Test Steady State
% This uses a Formula to determine Theoratical Steady State

steadyStateC = (sediFlowM/volumeV) /...
	((baseFlowQ/volumeV) + (sediVelocityVs/(volumeV/areaA)));
fprintf('Theoratical Steady State = %g\n',steadyStateC);


%% Euler Method
% This performs Eulers Method using the function in f.m

for i=initialTime:stepCountH;
	if i <= stormTime/stepSizeN
		c(i+1) = c(i) + (stepSizeN * f(c(i)));
		t(i+1) = t(i) + stepSizeN;
	else
		sediFlowM = 0;
		c(i+1) = c(i) + (stepSizeN * f(c(i)));
		t(i+1) = t(i) + stepSizeN;
	end
end 


%% Data Presentation
% This produces a graph & confirms that the script is complete.

plot(t,c);
	title('Sediment Concentration in a Lake');
	xlabel('Time (Sec)');
	ylabel('Concentration (mg/m3)');
fprintf('Process Completed\n\n');