# 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.

I will try to bring you some insights. In my opinion, SCA is poorly named.

Actually, the width is not involved in the catchment area which is given by TCA. SCA introduces a diffusive flow factor by considering flow width. It is a one dimensional value so it is not representative of an area and that's why it might be confusing. It gives us an idea of how concentrated are the incoming flow.

Besides this, I think you are also confused by the outlet concept which is often defined in GIS or in some conceptual definitions by a point. But actually, in order to compute a flow at an outlet you need flow velocity and a cross section. If this cross section area is assumed to be rectangular, it can be defined by the width and the height will vary according to the incoming flow. So when the outlet is considered as a point, it is more like a targeter of the outlet that has some dimensional characteristics.

So, for studies at the plot scale or the hillslope scale, the outlet is often defined by a line which is representative of the flow width at the downslope.

For grid based data, a pour point targets a representative area defined by the spatial resolution of your terrain data. If a point has no flow width, this area has one and the flow width is generally assumed to be the cell size as mentionned in your quote.

If you understand that, you may understand that the broader the width, the more diffuse will be the overland flow and therefore the less wet regarding the TWI or the less erosive. Of course, it remains resolution dependent.