The Offline Validation of Land Surface Models: Assessing Success at the Annual Timescale

The annual runoff rates generated by land surface models (LSMs) participating in the Global Soil Wetness Project (GSWP) are compared to observed rates in well-instrumented basins across the globe. Because such an offline evaluation can be clouded by an overwhelming influence of the atmospheric forcing itself (relative to the imposed land surface physics) on model output, we also estimate runoff rates with a decades-old climatological relation devised by M.I. Budyko, a relation that depends solely on annual precipitation and net radiation. The LSMs and the estimates derived with Budyko's relation are found to have standard errors of the same order (roughly 100mm/yr). Thus, we conclude that the complexities inherent in these state-of-the-art LSMs did not lead to increased accuracy in the simulated energy and water balances at the annual time scale. Proper LSM formulations are nevertheless recognized as essential for realistic land surface behavior at shorter (e.g., monthly or hourly) time scales, for which a simple Budyko-type equation would be in much greater error.