For each
pressure X an MSA_{X} map is calculated by applying the cause-effect relationship
to the appropriate input map. These calculations are done for the existing
water bodies according to the Global Lakes and Wetlands Database. Assuming no
interaction among the drivers, for each water type in a grid cell, GLOBIO
calculates the overall MSA_{i} value by multiplying the MSA_{X} values per (set of)
drivers:

where i is
a grid cell, MSA_{i} is the overall value for grid cell i, MSA_{Xi} is the relative
mean species abundance corresponding to the drivers LU (catchment land use and
eutrophication) and CH (change in hydrological regime).

Aggregation
of results

The MSA
values for the different water types (lakes, rivers and wetlands) are
aggregated per grid cell according to their relative area.

To obtain
the MSA_{r} of a region, GLOBIO calculates the area weighted mean of MSA_{i} values
of all relevant grid cells.

where A_{i} is
the surface water area of grid cell i.

Relative importance of drivers

The
relative contribution of each driver to a loss in MSA may be calculated from
formulas 1 and 2.