Summing Variable Data Across Rigid Boundaries

I shall use a basic example to illustrate my problem - see attached - as the spreadsheet im actually working on is huge and contains sensitive data. So, In the spreadsheet attached, the problem is: I am mowing the grass of a football pitch and getting paid by the m3 of grass i mow. however i'm getting paid more when i mow at greater distances - defined by certain 'reaches'. so i need to know how many m3 ive mowed in each reach. I make a note of where I start on the pitch and where i end, and also how much grass i mow.

so im looking for a solution that is intelligent enough to distinguish where Ive taken grass from and how much, given that i know where i started, where i finish and how much i take.