Locate wet patches using DEM

Question

I'm searching for a way to determine the location of wet patches, after rainfall, in a DEM. By this, I mean puddles.

We are investigating the best locations to locate paths for recreational use. Paths that remain wet for a longer time need a lot more maintainance.

I would like to visualise the wet areas, to take into consideration, while planning a new pathstructure.

Does anyone know how I can achieve this?

I would like it to look something like this: http://www.powersim.com/sitefiles/site4053/img/RainFallMadla3.jpg

Answer 1

The subject has a lot of atention among specialists and the number of tools available depends on the amount of work you wnat to put on it. The paradox is that you want to avoid wet areas while stormwater engineers are looking for recreational areas to use them as ponds and damping peak flows.

There are three levels at which you can take your analysis:

1. Mapping of low points, also known as bluespots mapping

2. Mapping of runoff

3. Mapping of runoff with the stormwater management system

Bluespots are easy found by identifying depression in a DTM, under QGIS you have many alternatives, I would use the GRASS command r.fill.dir from the Processing Toolbox, but there are more tools to do it.

I have just found a nice video from Danmark showing how to do the bluespot mapping with other QGIS tools: Point Cloud to Bluespot, Stream and Drainage Basin (Catchment Area)

I cannot resits to comment that there are a couple of projects to address the more complex calculations of the runoff and runoff with pipes problem, but they are probably far from your needs and avalable resources:

1. The project Itzí to create flood risk maps
2. The just born project hex-utils-grass that could be use in flood modelling with hexagons (interesting videos about it:https://www.youtube.com/watch?v=uLO4HDCVBp0 and https://youtu.be/4lfuNHmsWLg)
3. The Anuga project which has been used to simulate raifall on a 2d grid also

Further reading

• The Blue Spot Concept. Danish Road Institute, Report 181 - 2010

Answer 2

To fully answer your question I need more information on what exactly you define as "wet patches", but I believe that you are looking for the tool "Flow Accumulation", explained here by ESRI.

You could combine that with basins created with Watershed and other hydrologic characteristics like Flow Direction, or you could check the concavity of the DEM with Curvature, explained more in detail here. That should get you on the right track for starters.

Edit: I just saw that you tagged your question with QGIS. The links I provided are for ArcGIS, but I´m sure that after reading those you´ll have more of an idea on what tools you are looking for in QGIS.

Answer 3

Calculating the topographic wetness index should be of great use. Its a classic metric for determining likely wet and dry patches relative to the rest of the landscape based on the curvature and upslope drainage area. Without fully diving into incorporating land cover, water sources and quantities, and artificial drainage infrastructure, this would be a good start with just a DEM.

If OP is looking for other tools outside of QGIS as well, WhiteboxGAT (noted in a comment above) is very well designed for hydrologic analyses.