Parflow is an open source, object-oriented and parallel watershed scale model. It includes a fully integrated land flow with the ability to simulate complex topography, geology and heterogeneity, as well as land surface processes including land energy budget, biogeochemistry, and snow (through CLM). It can run on multiple platforms. Parflow is the result of a long, multi agency development history and is now a collaboration between CSM, LLNL, uni Bonn and UCB. Parflow is combined with mesoscale, weather code ARPS and NCAR code WRF.