# Terminology The purpose of this page is to introduce some D-Claw-specific terminology used throughout the digger documentation. This is meant to support users who are not familiar with D-Claw in understanding what digger does and why. It is not a replacement for understanding the theory and practice of using D-Claw. D-Claw solves a set of depth-averaged differential equations for the motion of a granular fluid through time over a fixed basal surface, {math}`b` (George and Iverson, 2014; Iverson and George, 2014). As material of thickness {math}`h` moves over the basal surface, the surface elevation {math}`\eta` changes. The `digger` documentation refers to a number of different variables that correspond to information needed by D-Claw. The D-Claw solution (i.e., the solution to the differential equations through time) is referred to as **q**, and the set of auxiliary variables that are used as part of finding the solution is called **aux**. Accordingly, some of the terminology that `digger` uses will refer to elements of **q** and **aux**. The names of these variables, their units, their relationship to **q** and **aux**, and a description are provided here: | Variable Name | Element of q or aux | Units | Description | | --- | --- | --- | --- | | {math}`b(x,y)` | - | m | Spatially variable elevation of a basal surface over which material may move. This is typically topography or merged topography and bathymetry.| | {math}`h(x,y,t)` | q1 | m | Spatially and temporally variable thickness of material which may move.| | {math}`\eta(x,y,t)` | q8 | m | Spatially and temporally variable value for the surface altitude {math}`(b+h)`. Where {math}`h=0`, {math}`\eta=b` and {math}`\eta` represents the basal topobathymetric surface elevation. Where {math}`h>0`, {math}`\eta` represents the altitude of the flow surface.| | {math}`m(x,y,t)` | q4 | - | Spatially and temporally variable value of solid volume fraction. | | {math}`h_e(x,y)` | aux5 | m | Spatially variable initial thickness of erodible material.| This list is not a comprehensive list of the elements of **q** and **aux**, rather it is a list of the most relevant elements for `digger`. As [`digger.make`](#digger.make) creates input files, the files it generates represent {math}`h`, {math}`\eta`, and {math}`m` at simulation time = 0. **References** George, D.L., and Iverson, R.M., 2014, A depth-averaged debris-flow model that includes the effects of evolving dilatancy—II. Numerical predictions and experimental tests: Proceedings of the Royal Society of London. Series A, v. 470, no. 2170, p. 20130820, . Iverson, R.M., and George, D.L., 2014, A depth-averaged debris-flow model that includes the effects of evolving dilatancy—I. Physical basis: Proceedings of the Royal Society of London. Series A, v. 470, no. 2170, p. 20130819, .