Tricky Summation Operator Problem

Well at least for me anyway.

It seems that finding the sum for each row of the array (the hydrograph ordinate) is not as simple as I first thought. Does someone know how to do this (see attached MathCad 14 file)? Somehow one has to tell MathCad to access the next Area element whilst decrementing it within the row but also the Intensity element has to be incremented within each row. Is this possible just using the summation command?

I believe the formal name for such operations is a convolution thus I am sure the answer will be somewhat convoluted no pun intended.

Help would be very much appreciated.

Thanking you in advance.