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In-Depth Information
main approaches for the hydrological representation of wetlands. One approach is con-
cerned with the redistribution of soil moisture in the model grid cell. A widely used
example is TOPMODEL (Beven and Kirkby
1979
). In this approach a topographical index
is computed that depends on the drainage of a given area routed through a point and its
slope. This index is then applied to determine the position of the local water table at that
point relative to the mean water table of the whole grid cell. The grid cell fraction where
the subgrid soil moisture exceeds the soil moisture storage capacity of the grid cell is then
regarded as a wetland. The TOPMODEL approach was used and improved in several
studies (e.g., Barling et al.
1994
; Gedney et al.
2004
; Bohn et al.
2007
; Kleinen et al.
2012
)
and is able to compute changes in wetland extent as well. While this approach is an elegant
solution, it has one major problem. As the wetland fraction depends on the redistribution of
the mean grid cell soil moisture, it follows that there is an upper boundary for the maxi-
mum water depth and wetland fraction. For the extreme case of a grid cell with zero slope,
no wetland can emerge because the mean soil moisture can obviously not exceed the
maximum soil moisture capacity. However, observations indicate that flat regions appear
to be more suitable for wetland formation.
The second approach is the explicit modelling of surface water. In this case depressions
in the topography are identified and filled with water that results from a positive water
balance. On the one hand, this can be done on a continental scale (e.g., Coe
1997
,
1998
,
2000
), but then the quality of the wetland representation is strongly limited by resolution of
the model. Alternatively, regional models allow for a higher resolution but then depend
strongly on detailed soil property information (e.g., Bowling and Lettenmaier
2010
;Yu
et al.
2006
) or are calibrated for specific catchments (e.g., Bohn et al.
2007
). Decharme
et al. (
2008
,
2011
) developed a global inundation model, but its focus is concentrated on
the representation of floodplains.
In contrast to these sophisticated approaches, Stacke and Hagemann (
2012
) developed a
somewhat simpler hydrological scheme that represents the global distribution and extent
variability of very different types of wetlands. The scheme was designed for the application
in complex ESMs on global scale with medium to coarse resolutions (50 km or coarser), as
the representation of surface water dynamics is—albeit important—not strongly developed
in such models. The global-scale hydrological scheme of Stacke and Hagemann (
2012
) has
been implemented in the Max Planck Institute for Meteorology Hydrology Model (MPI-HM).
It solves the water balance of wetlands and estimates their extent dynamically. The extent
depends on the balance of water flows in the wetlands and the slope distribution within the
grid cells. In contrast to most models, this scheme is not directly calibrated against wetland
extent observations. Using MPI-HM, the spatial distribution of simulated wetlands agreed
well with different global observations for present climate (Fig.
10
). The best results were
achieved for the northern hemisphere where not only the wetland distribution pattern but
also their extent was simulated reasonably well. However, the wetland fraction in the
tropical parts of South America and Central Africa was strongly overestimated, which
seems to be related to an underestimation of potential evapotranspiration over wet tropical
areas by the Penman-Monteith method used in MPI-HM. The simulated extent dynamics
correlated well with monthly inundation variations obtained from satellites for most
locations. Also, the simulated river discharge was affected by wetlands, resulting in a delay
and mitigation of peak flows. Compared to simulations without wetlands, locally increased
evaporation and decreased river flow into the oceans were generated due to the imple-
mented wetland processes.
