A global two-dimensional implementation of the simplified Simple Biosphere
(SSiB) land surface scheme is integrated offline for two years as part of t
he Global Soil Wetness Project (GSWP). A climatology of soil wetness and su
rface fluxes has been produced. This climatology is compared to a number of
sensitivity studies that have been performed to investigate how the partit
ioning of precipitation between runoff and evapotranspiration is affected,
when aspects of the soil parameterization and the treatment of convective p
recipitation are altered.
The control integration has a reasonable spatial distribution of the surfac
e hydrologic balance components, and shows realistic seasonal and interannu
al variations. Evaporation from the soil surface accounts for a majority of
the water fluxes from the soil over all but heavily forested areas, where
transpiration dominates. The sensitivity studies show that in general the m
ost sensitive terms on seasonal time scales appear to be runoff, direct eva
poration from the soil, and the seasonal change of water storage in the soi
l matrix. A realistic distribution of convective precipitation in space and
time is necessary to simulate at the grid scale adequately high values of
runoff, and to not over-represent direct evaporation of rainfall intercepte
d by the canopy. Sensitivity is found to the choice of thickness of the sur
face soil layer - a parameter often assigned arbitrarily in land surface mo
dels. Little sensitivity is found when imposing a vertical profile of soil
porosity intended to account for surface soil aeration and deep soil compac
tion.