Better Descriptions of Residue Production in Perennial Systems to Improve RUSLE2.

In most USDA erosion models (e.g. WEPP, WEPS, RUSLE1), residue production occurs only during senescence of a crop and is calculated from the decline in live biomass.  This is equivalent to the assumption that there is no death (no dead biomass production) during periods of increasing biomass, and no additional growth (birth) after the peak biomass is reached. This is probably an acceptable assumption for the treatment of annual crops.  However, in mixed stands where different components mature at different times, and in perennial systems, death and birth usually occur simultaneously.  In the newest version of RUSLE2, residue production is continuous, based on the assumption that live biomass has an effective life span. In the absence of forage harvest or biomass removal, the daily change in live biomass is calculated as the difference between birth and death. Live biomass that is not harvested is added to a dead biomass pool after its lifespan is reached. Users input monthly potential growth patterns and vegetation shoot and root lifespans and RUSLE2 calculates corresponding potential birth patterns. Birth patterns are altered in response to management operations involving biomass removal. Daily changes in residue biomass are calculated as the difference between death and decomposition or residue harvest.  RUSLE2’s new routines simplify the creation of vegetation descriptions for perennial systems, provide more realistic estimates of residue creation throughout the year, and improve runoff and erosion estimation.  RUSLE2’s residue production scheme should also be adaptable to models that simulate crop growth with process-based equations.