Since you are trying to find the correlation between two one dimensional vectors, you should use corrcoef instead of corr.
Also, the value of ET_GWP is discarded in each iteration. One solution is to create a cell array to save all values.
% Correlate each row of a variable with rows of another variable
[rows,cols] = size(double_new_WB_prop);
ET_GWP = [];
for j = 1:rows
ET_GWP{j} = corrcoef(double_new_WB_prop(j,1:3700), double_new_WB_prop(j,70301:74000)');
end