# In hydrology, what's the difference between specific and total catchment area?

I am getting stuck on the concepts of Total Catchment Area (TCA) and Specific Catchment Area (SCA) as I try to learn more about TOPMODEL and hydrology modeling. What do these metrics mean in hydrology? The context I encounter these in is in calculating Topographic Wetness Index (TWI) values.

My understanding so far is that: TCA is basically equivalent to a watershed for any given pour point; SCA helps determine saturation and saturation excess flow and it is the ratio of a local catchment area for a pour point divided by the width of the contour where that pour point is. I'm especially uncertain about SCA because it's unclear to me how the width of the contour is involved in a catchment area (based on my conceptual framework of a watershed a/k/a catchment area being defined by a single pour point). So, maybe I also need "Flow Width" (a term I think I've seen used to describe that contour width used in SCA) to be explained in order to understand SCA.

To give some context to what I'm referring to, here's a passage which references both TCA and SCA:

"Upslope area is the total catchment area above a point or length of contour and the specific catchment area is the upslope area per width of contour or cell size, L (Moore et al., 1991). Specific catchment area is used to calculate saturation and saturation excess overland flow in hydrological models such as TOPMODEL (Beven and Kirkby, 1979) and, along with other topographic indices, to calculate erosion and landsliding in many other models. Upslope area is commonly used for mapping channels on the basis of threshold upslope areas for channel initiation.

J. Wainwright & M. Mulligan. Environmental Modelling: Finding Simplicity in Complexity. Oxford: Wiley. 2012. Ebook.

This was originally posted in GIS.SE where it was suggested it be moved here.