EDIT after you edited the question: it seems that there are 4 columns and not 2, and the discharge is in column4. I am updating my answer accordingly.
Here is an example
data = [1915, 7, 1, 6; ...
1915, 10, 3, 8; ...
1915, 12, 8, 7; ...
1916, 4, 22, 3; ...
1916, 7, 13, 2] ;
years = unique( data(:,1) ) ;
yearId = data(:,1) - min( data(:,1) ) + 1 ;
Qmean = accumarray( yearId, data(:,4), [], @mean ) ;
[Qmean_sorted, ix] = sort( Qmean ) ;
years_sorted = years(ix) ;
Running this, you get
>> Qmean
Qmean =
7.0000
2.5000
>> Qmean_sorted
Qmean_sorted =
2.5000
7.0000
>> years_sorted
years_sorted =
1916
1915
Best Answer