Model for calculation of the R-factor. The `R`-factor is a measure used in erosion and (overland) sediment modelling to quantify the effect of rainfall on soil erosion. It is typically defined in the context of the RUSLE equation, in which gross erosion for an agricultural parcel is estimated. Specifically, the R-factor is a measure for the total erosivity of a number of rainfall events within a defined timeframe (year, month, number of days). The factor is computed by calculating the erosivity for every rainfall event in a timeseries, and taking the sum of the erosivity of all events in one year. These yearly values can be used to compute an average value, the R-factor, presenting the rainfall erosivity for a given period.