Weighted flow accumulation from flow direction [not DEM] in QGIS?

Weighted Flow Accumulation from Flow Direction in QGIS: A Human’s Guide

So, you’ve got a flow direction raster and need to figure out flow accumulation in QGIS, but without a DEM? No sweat! It’s totally doable. Flow accumulation is super important for understanding how water moves across the landscape. Think of it as tracing where raindrops end up – crucial for everything from predicting floods to finding good spots for fish habitat.

Now, flow accumulation basically counts how many cells drain into each location on your map. High numbers? That’s where the water’s really flowing, usually a stream or river. Zero? You’re probably on a hilltop. Flow direction tells you which way the water’s heading from each cell – downhill, naturally. Usually, you get this from a Digital Elevation Model (DEM), which is a fancy way of saying a map of elevation.

But what if you don’t have a DEM? Maybe you inherited the data, or you’re using some fancy algorithm that spits out flow direction directly. That’s where this guide comes in. It’s about doing weighted flow accumulation straight from that flow direction raster inside QGIS.

Why bother with flow direction alone? Well, sometimes you’re stuck with legacy data. I’ve been there – wading through old projects where the DEM is missing, but the flow direction raster is still kicking. Other times, it’s just faster. If you already have the flow direction, why re-process the DEM?

Okay, so how do we actually do this? QGIS has a few tricks up its sleeve.

GRASS GIS 7: The Workhorse

First up, GRASS GIS 7. It’s like the Swiss Army knife of GIS, and it’s built right into QGIS. The r.accumulate module is your friend here. What’s cool about r.accumulate is that it doesn’t need elevation data. It just follows the flow direction you give it.

What can it do? Oh, just about everything: weighted flow accumulation, figuring out subwatersheds, mapping stream networks, even finding the longest flow paths. It’s all in there.

But the real magic is the “weight map.” This lets you do weighted flow accumulation. Imagine you want to factor in rainfall. Areas with more rain should contribute more to the flow. That’s where the weight map comes in. You plug in a raster representing rainfall amounts, and r.accumulate does the rest. If you don’t have a weight raster, it just assumes every cell contributes equally (a weight of 1).

Using it is pretty straightforward. You tell r.accumulate where your flow direction raster is, what format it’s in (usually it can figure that out automatically), and then point it to your weight raster. Give it a name for the output, hit “Run,” and boom! Flow accumulation map.

PCRaster Tools: The Environmental Modeler

Next, we have PCRaster. This is more of a specialized tool, geared towards environmental modeling. You’ll need to install the PCRaster Tools plugin.

The PCRaster workflow is a bit different. You usually convert your data to PCRaster’s own format first. If you did have a DEM, you’d use lddcreate to make a “local drain direction” (LDD) map. But since we’re starting with flow direction, we can skip that. The key tool here is accuflux. It calculates flow accumulation from the LDD.

And yes, it handles weighted accumulation. You need a “material layer” – that’s where you put your weights. If you just want regular flow accumulation, make a raster where every cell is “1.” But for weighted stuff, this layer holds your weighting factors, like rainfall. PCRaster also has fancier tools like AccuFractionFlux for more complex routing scenarios.

SAGA GIS: The Algorithm Collection

SAGA GIS algorithms are also accessible through the QGIS Processing Toolbox. SAGA offers a range of tools, including “Flow Accumulation (One Step)” which is designed to work with DEMs, but can be adapted.

There are also “Catchment Area (Parallel)” or “Flow Accumulation (Top-Down)”. These SAGA functions can utilize a filled DEM as ‘elevation’ and a slope raster layer as ‘Accumulated material’ to achieve a weighted flow accumulation effect.

Whitebox Tools: The Geospatial Analyst

Whitebox Geospatial Analysis Tools offer capabilities for flow accumulation from flow direction rasters as well.

The D8 Flow Accumulation and D-infinity Flow Accumulation tools both take flow direction rasters as inputs. The D8 Mass Flux tool allows for a weight (loading) file and two types of losses (efficiency and absorption).

TauDEM tools

The TauDEM tools within QGIS provide algorithms for hydrologic terrain analysis.

The D8 Flow Directions tool creates a grid containing the flow direction from each grid cell to one of its adjacent or diagonal neighbors, calculated using the direction of steepest descent. The D-Infinity Transport Limited Accumulation function calculates the transport and deposition of a substance, which may be limited by both supply and the capacity of the flow field to transport it. It requires a D-Infinity Flow Direction Grid and a Supply Grid (the weighting raster) as inputs.

A Quick Example: GRASS GIS 7 in Action

Let’s walk through a GRASS GIS 7 example:

A Few Things to Keep in Mind

• Flow direction encoding: Know how your flow direction raster is encoded! Different tools use different systems.
• Coordinate system: Use a projected CRS (like UTM) with meters as units. Lat/long can mess things up. Trust me, I’ve learned this the hard way.
• Pit filling: If you made your flow direction from a DEM, make sure you filled the pits first. Otherwise, you’ll get weird results.
• Edge effects: The edges of your raster might have incomplete data, leading to underestimates of flow accumulation.

Wrapping Up

So, yeah, doing weighted flow accumulation from flow direction in QGIS is totally possible. GRASS GIS 7, PCRaster, SAGA GIS, Whitebox Tools, and TauDEM are your friends here. Just remember to double-check your data, understand your encoding, and watch out for those pesky edge effects. Now go forth and map some water!