Simple function to calculate melt rates in meter water equivalent per year for a digital elevation model (DEM) based on positive degree days factor (PDD) using seasonal variation.
User can specify factors: mean annual temperature, PDD_ice, seasonal variation in degrees and year length.
Approach following Marshall, SJ and Clarke, GKC 1999: Ice sheet inception: subgrid hypsometric parameterization of mass balance in an ice sheet model. Climate Dynamics 15:533-550
Does not include different PDDs for ice and snow yet, nor does it consider refreezing etc yet.