Monthly to annual totals

Hello,

I am hoping to convert monthly totals to annual totals using a loop. However, instead of using the calendar year (Jan-Dec) I want to use the water year which runs from October to September. I have a number of time series organised in a structure (468 rows, 1 column) starting from Oct 1960 and ending in Sept 1999.

I have attempted to sum up each 12 consecutive rows (1:12, 13:24 and so on) to get the annual totals but my code (below) is not doing the job (I am not sure what exactly it is doing) and I was hoping I could get some help fixing it. Many thanks!

for k=1:length(varnames)
    p=0;
    for i=1:39
        for months=1:12
            p=p+1;
            annual_flow.(varnames{k})(i,1)=flow_monthly.(varnames{k})(p,1);
        end
    end
end