# 6.9. Mass balance accounting

The global mass balance of an ice sheet reads

where \(\mathrm{d}V/\mathrm{d}t\) is the rate of volume change, SMB the total surface mass balance, BMB the total basal mass balance and CALV the total calving rate (all counted as positive for a volume gain and negative for a volume loss). SICOPOLIS attempts at closing this balance as accurately as possible by applying the so-called “hidden ablation scheme” (Calov et al. [13]).

SICOPOLIS always employs a zero-ice-thickness boundary condition at the margin of the computational domain (\(i=0,\,i_\mathrm{max}\); \(j=0,\,j_\mathrm{max}\)). However, for accurate accounting of calving near the margin, the “hidden ablation scheme” also requires the next lines of grid points (\(i=1,\,i_\mathrm{max}-1\); \(j=1,\,j_\mathrm{max}-1\)) to be ice-free. Since this is not always desirable, it can be controlled by the run-specs-header parameter `MB_ACCOUNT`

:

`0`

: Glaciation of all inner points of the domain allowed (prevents accurate accounting of calving near the margin).`1`

: Outermost inner points of the domain (\(i=1,\,i_\mathrm{max}-1\); \(j=1,\,j_\mathrm{max}-1\)) not allowed to glaciate (required for accurate accounting of calving near the margin).

For real-world problems, the setting `MB_ACCOUNT = 1`

is usually fine. However, for some simple-geometry experiments that require the simulated ice sheet to cover the entire domain [e.g., the test simulation `repo_vialov3d25`

(3D Vialov profile)], `MB_ACCOUNT = 0`

must be chosen to allow that.