Please cite SAGA whenever you use it for your publications:

Calculate the volume under the grid's surface. This is mainly useful for Digital Elevation Models (DEM).

Name	Type	Identifier	Constraints
Input
Grid	grid, input	GRID
Options
Method	choice	METHOD	Available Choices: [0] Count Only Above Base Level [1] Count Only Below Base Level [2] Subtract Volumes Below Base Level [3] Add Volumes Below Base Level Default: 0
Base Level	floating point number	LEVEL	Default: 0