Source accounting is a representation of a specific user selected component's (a cell, feedlot, gully, point source, or reach) contribution to the watershed outlet over the simulation period. Variables for user selected components and all downstream reaches are accumulated during the simulation processing. After the simulation processing each variable accumulated is converted to a ratio at each end of all downstream reaches as well as a ratio for the selected component (if it is not a reach) where it first enters a reach. The contribution from the selected component to the watershed outlet is then determined from the computed ratios. Computed information on the source accounting computations is placed in the source accounting output file. The watershed outlet simulation accumulation data is also placed in the source accounting output file as a basis for ratios contained in the file.