# Rainfall-weighted flow accumulation

I am perfoming a geospatial process whereby I want to use the flow accumulation (as an input for discharge proxy) in calculating stream power. I am using ArcGIS 10.2

So far I have: 1. Valley/channel slope and width 2. drainage area 3. Terrain analysis grids (flow acc, flow dir etc) 4. rainfall surface grid

I just wanted to know what is the formula behind weighting the flow accumulation with for example, if facc is calculating the number of upstream cells accumulating in that point, when you weight it by rainfall (in mm), what does that mean? Sum? Multiplication of the two layers? I managed to do this step however, the values I got on the highest flow accumulation line is the same throughout the course of the river and that looks incorrect.

• Fill Dem first and derive flow direction. Apply flow accumulation with rain as weight. If rain in mm/day, the discharge at points equal that times cell size /86400. Divide by 1000 if cell size in metres. – FelixIP Jun 15 '18 at 19:46

## 1 Answer

I don't know for certain, but my guess is this: without weighting, each cell has a value of one before the flow accumulation is performed. With weighting, each cell is given a value that is proportional to the rainfall over that cell. So, many cells in the watershed will have weights near zero (i.e. the cells where very little precipitation falls). Consider, for example, an arid watershed that receives very little rainfall in the lower-elevation regions, but significant rainfall in the higher-elevations. In this case, as we move downstream, the contribution from the lower-elevation pixels is almost nothing compared to the higher-elevation ones, which would lead to a result similar to what you're seeing.