• hydrologic simulations;
  • parallel computing

[1] We present the results of a unique, parallel scaling study using a 3-D variably saturated flow problem including land surface processes that ranges from a single processor to a maximum number of 16,384 processors. In the applied finite difference framework and for a fixed problem size per processor, this results in a maximum number of approximately 8 × 109 grid cells (unknowns). Detailed timing information shows that the applied simulation platform ParFlow exhibits excellent parallel efficiency. This study demonstrates that regional scale hydrologic simulations on the order of 103 km2 are feasible at hydrologic resolution (∼100–101 m laterally, 10−2–10−1 m vertically) with reasonable computation times, which has been previously assumed to be an intractable computational problem.