function read_file_extract_period

% specify file names
fname_co2 = 'c:/Ecuador/csvGrid/mote20.csv';
fname_stm1 = 'c:/Ecuador/csvGrid/mote22.csv';
fname_stm2 = 'c:/Ecuador/csvGrid/mote490.csv';

%fmt of the data
fmt = '%[^,] %f %f %f %f %f %f %f %f %f %f';

% read co2 input file
[co2.dt co2.ch1 co2.ch2 co2.ch3 co2.ch4 co2.ch5 co2.ch6 co2.ch7 co2.ch8 co2.ch9 co2.ch10] = ...
     textread(fname_co2,fmt,'delimiter',',');
[stm1.dt stm1.ch1 stm1.ch2 stm1.ch3 stm1.ch4 stm1.ch5 stm1.ch6 stm1.ch7 stm1.ch8 stm1.ch9 stm1.ch10] = ...
     textread(fname_stm1,fmt,'delimiter',',');
[stm2.dt stm2.ch1 stm2.ch2 stm2.ch3 stm2.ch4 stm2.ch5 stm2.ch6 stm2.ch7 stm2.ch8 stm2.ch9 stm2.ch10] = ...
     textread(fname_stm2,fmt,'delimiter',',');
 
% convert date string to a number 
% so that we can compare values
co2_num =  datetime2datenum(co2.dt, 'yyyy-mm-dd HH:MM:SS');
stm1_num = datetime2datenum(stm1.dt, 'yyyy-mm-dd HH:MM:SS');
stm2_num = datetime2datenum(stm2.dt, 'yyyy-mm-dd HH:MM:SS');

% find the overlap between these three files
t1 = max([min(co2_num) min(stm1_num) min(stm2_num)]);
t2 = min([max(co2_num) max(stm1_num) max(stm2_num)]);

% find the indexes which lie between t1 and t2
ind_co2 = find(co2_num > t1 & co2_num < t2);
ind_stm1 = find(stm1_num > t1 & stm1_num < t2);
ind_stm2 = find(stm2_num > t1 & stm2_num < t2);

% concatenate Date, CO2, Soil Temp, Soil Moisture
dt = co2.dt(ind_co2);
all_modes = [co2.ch1(ind_co2) co2.ch2(ind_co2) co2.ch3(ind_co2) ...
    stm1.ch1(ind_stm1) stm1.ch2(ind_stm1) stm2.ch1(ind_stm2) ...
    stm1.ch3(ind_stm1) stm1.ch4(ind_stm1) stm1.ch3(ind_stm2)];

% correct the co2 for each depth
NUM_DEPTH = 3;
PRESSURE = 1013.25 * 1.0E-5;
KELVIN_CORRECTION = 273.15;
GAS_CONSTANT = 8.314;

co2_corrected = [];
for i=1:NUM_DEPTH,
    denom = KELVIN_CORRECTION + all_modes(:,i+6);
    co2_corrected(:,i) = all_modes(:,i)*PRESSURE ./ denom;
end

[TIMES DEPTHS] = size(co2_corrected);
GRADIENT = zeros(TIMES,1);

x = [2, 8, 16];
CO2_DEPTHS = 1:3;
for i=1:TIMES
    y = co2_corrected(i,CO2_DEPTHS);    
    p = polyfit(x,y,2);
    k = polyder(p);
    GRADIENT(i) = polyval(k,0);
end

%% PLOT THE GRADIENTS
figure; 
subplot(2,1,1);
plot([1:TIMES],GRADIENT);
tk = floor([1:TIMES/8:TIMES]);
set(gca,'xtick',tk);
set(gca,'xticklabel',dt(tk));
title('Flux values');
subplot(2,1,2);
plot([1:TIMES],all_modes(:,5));
tk = floor([1:TIMES/8:TIMES]);
set(gca,'xtick',tk);
set(gca,'xticklabel',dt(tk));
title('Moisture');
ylim([0.3 0.4]);

DAO = 1.47 * 1.0E-5;
T_REF = 293.15;
P_RATIO = 1.028926;

% Compute Do for temperature values
data_do = DAO .
    
    



end