## The RiverTools Mass Flux MethodThe image at the top of this page shows a side-by-side comparison of total contributing area grids computed using the Rivix Mass Flux Method, the D-Infinity method and the D8 method, for the Keystone Ski Resort. Unlike the D8 and D-Infinity methods, there are no pixellated artifacts in the Rivix Mass Flux Method image. Note that no smoothing of any kind has been performed in any of these images. Our method just works, even in complex topography. See our Image Gallery for more images that compare these methods, especially the one for Mount Sopris. ## The Problem: Divergent vs. Convergent TopographyOn a hillslope flowlines may first diverge (spread out) before
converging into channels at the bottom of the hill. RiverTools provides
routines for analyzing both convergent and divergent flow. For convergent
flow and the identification of individual channels, channel networks and
basin boundaries, the well-known The In RiverTools 3.0, a new method for partioning flow between neighbor
pixels was introduced, called simply the - divides each pixel into four quarter-pixels,
- computes a continuous flow angle for each, and then
- computes total contributing area (TCA) and specific contributing area (SCA) using the actual fraction of flow that would pass through each of a pixel's four edges under a uniform rainrate.
Note that we are speaking in terms of flows and mass balance even though our objective is to compute contributing areas on divergent surfaces as accurately as possible. The new Mass Flux algorithms are available in RiverTools via the Extract → Mass Flux Grid menu. In order to handle peaks and linear ridges as accurately as possible, grids based on quarter-pixels are created and saved (with QP for quarter-pixel in their name, along with their own RTI files) as an intermediate step. These have twice as many columns and rows as the original DEM, but are then integrated to create flow angle and area grids that have the same dimensions as the DEM. Continuous angle grids from the D-Infinity and Mass Flux methods look quite similar, but there can be significant differences between the area grids computed by the two methods as seen in the figure at the top of this page. Both the D-Infinity and Mass Flux algorithms use a D8 flow grid to resolve ambiguous flow situations in flats and pits. You can experiment with these different algorithms by using DEMs in
the Test_Surfaces folder in the The Carbondale story on our Stories Page describes the role that the mass flux method played in protecting the water supply for the town of Carbondale, Colorado. More information on the D8 and D-Infinity algorithms can be found
Here are two images that compare the interactive Vector Zoom tools for both the D8 and Rivix Mass Flux methods. ## ReferencesGruber, S. and S.D. Peckham (2009) Land-surface parameters and objects specific to hydrology, In: Hengl, T. and Reuter, H.I. (Eds), Geomorphometry: Concepts, Software and Applications, Chapter 7, Developments in Soil Science, vol. 33, Elsevier, 171-194, http://dx.doi.org/10.1016/S0166-2481(08)00007-X. Peckham, S.D. (2013) Mathematical surfaces for which specific and total contributing area can be computed: Testing contributing area algorithms, Proceedings of Geomorphometry 2013, Nanjing, China, pp. O-11-1 to O-11-4, http://geomorphometry.org/Peckham2013, October 7. Peckham, S.D. (2011) Monkey, starfish and octopus saddles, Proceedings of Geomorphometry 2011, Redlands, CA, pp. 31-34, http://geomorphometry.org/Peckham2011b. You can download some of these and other papers from our Downloads Page. |

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