MATLAB: Calculating Discharge Over Multiple Years

maximum dischargetime series

I have a time series dataset which consists of the year in column 1 from 1883 – 2018, in column 2 there is month data and in column 3 each day. In column 4 there is a discharge value for each specific day. I need to calculate the maximum discharge for each year.
I am new to matlab and any help would be greatly appreciated
Thanks!

Best Answer

The first 3 lines re-create the input matrix named m. See inline comments to understand the rest.
It produces a table T summarizing the results. 
% recreate input data (m)
d = datetime(1883,9,1) : days(1) : datetime(1950,12,31);
[yr,mo,dy] = datevec(d); 
m = [yr(:),mo(:),dy(:),rand(size(yr(:)))*50+40];
% Identify winter months 
winterMonths = [11, 12, 1]; % any order but must be consecutive months
isWinter = ismember(m(:,2), winterMonths); 
% group the consecutive winter months
L = bwlabel(isWinter); %requires image processing toolbox
% -------------------------------------------------------



% % If you don't have image processing tool box:          %
% isWinterCS = cumsum(diff([0;isWinter]) == 1);           %
% L = zeros(size(isWinter));                              %
% L(isWinter) = isWinterCS(isWinter);                     %
% -------------------------------------------------------
% Compute max dischage per annual winter
maxDischarge = grpstats(m(:,4),L,'max'); %requires stats & ML toolbox

% Organize data by year (using the earliest year in each winter)
maxDischargeYear = grpstats(m(:,1),L,'min'); %requires stats & ML toolbox
% -------------------------------------------------------
% % If you don't have stats & ML tool box:                %
% groupID = findgroups(L);                                %
% maxDischarge = splitapply(@max,m(:,4),groupID);         %
% maxDischargeYear = splitapply(@min,m(:,1),groupID);     %
% -------------------------------------------------------
maxDischargeYear(1) = []; % Remove group 0 (non-winter months)

maxDischarge(1) = [];     % Remove group 0 (non-winter months)
T = table(maxDischargeYear(:),maxDischarge(:),'VariableNames',{'Year','MaxDischarge'});
% Show first few rows
head(T)
%     Year    MaxDischarge
%     ____    ____________
% 
%     1883       89.476   
%     1884       89.657   
%     1885       89.815   
%     1886       89.866   
%     1887       89.884   
%     1888       88.552   
%     1889       89.299   
%     1890       89.571
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