Development of a distributed biosphere hydrological model and its evaluation with the Southern Great Plains Experiments (SGP97 and SGP99)

A distributed biosphere hydrological model, the so-called water and energy budget-based distributed hydrological model (WEB-DHM), has been developed by fully coupling a biosphere scheme (SiB2) with a geomorphology-based hydrological model (GBHM). SiB2 describes the transfer of turbulent fluxes (energy, water, and carbon fluxes) between the atmosphere and land surface for each model grid. The GBHM redistributes water moisture laterally through simulating both surface and subsurface runoff using grid-hillslope discretization and then flow routing in the river network. The WEB-DHM was calibrated and validated for the Little Washita Basin using field observations from Southern Great Plains Hydrology Experiments (SGP97 and SGP99). For the SGP97 period, the model was calibrated and it shows an ability to reproduce point-scale energy fluxes (RMSE < 50 W m(-2)) as well as CO2 flux ( RMSE = 4.6 mu mol m(-2) s(-1)). At basin scale, the WEB-DHM can simulate a reasonable hydrograph (Nash = 0.956) and spatial soil moisture distribution with calibration of only a few soil hydraulic parameters for discharge. The model was then validated using SGP99 data sets and observed discharge. For the validation period, the model shows good performance in reproducing the soil surface temperature at 11 sites and the spatial distribution of surface soil moisture, as well as long-term discharges (Nash = 0.715) in the hydroyear from 1 September 1998 to 31 August 1999 that covers both the annual largest flood peak of 1999 and the SGP99 period. To our knowledge, this work is the first to undertake the development and evaluation of a distributed biosphere hydrological model using such comprehensive field observations.