Module Downslope Distance Gradient
Calculation of a new topographic index to quantify downslope controls on local drainage. 
References:
- Hjerdt, K.N., McDonnell, J.J., Seibert, J. Rodhe, A. (2004):   'A new topographic index to quantify downslope controls on local drainage',   Water Resources Research, 40
- Author: (c) 2006 by O.Conrad
 
- Specification: grid
 
- Menu: Terrain Analysis|Morphometry
 
Parameters
|   | Name | Type | Identifier | Description | Constraints | 
| Input | Elevation | Grid (input) | DEM | - | - | 
| Output | Gradient | Grid (output) | GRADIENT | - | - | 
 | Gradient Difference (*) | Grid (optional output) | DIFFERENCE | Difference to local gradient. | - | 
| Options | Vertical Distance | Floating point | DISTANCE | - | Minimum: 0.000000 Default: 10.000000 | 
 | Output | Choice | OUTPUT | - | Available Choices: [0] distance [1] gradient (tangens) [2] gradient (degree) Default: 2 | 
| (*) optional | 
Command-line
Usage: saga_cmd ta_morphometry 9 [-DEM <str>] [-GRADIENT <str>] [-DIFFERENCE <str>] [-DISTANCE <str>] [-OUTPUT <str>]
  -DEM:<str>       	Elevation
	Grid (input)
  -GRADIENT:<str>  	Gradient
	Grid (output)
  -DIFFERENCE:<str>	Gradient Difference
	Grid (optional output)
  -DISTANCE:<str>  	Vertical Distance
	Floating point
	Minimum: 0.000000
	Default: 10.000000
  -OUTPUT:<str>    	Output
	Choice
	Available Choices:
	[0] distance
	[1] gradient (tangens)
	[2] gradient (degree)
	Default: 2