On May 22nd, 2013, Zach Herrmann and Stephanie Johnson of Houston Engineering presented a webinar about hydrologically conditioned DEMs (hydro DEMs), considerations when creating them, and determining non-contributing areas of a watershed. Then they talked about how to use hydro DEMs and products derived from them to prioritize conservation management areas, plan large scale retention sites, and prioritize restorable wetlands. Check out the videos at [url=]z.umn.edu/lidar[/url].
Questions and answers that came up during the discussion are shown in additional posts attached to this one. The final question of the discussion:
Q: What additional observations or advice do you have?
A: Reconditioning is a labor-intensive process, but there are a lot of ways to use the data once you have it.
A: There are different ways to do hydro-conditioning. It can be overwhelming to keep track of the steps involved, but it is important to understand and document all the steps because there are different processes for different goals. Folks at the state level are trying to standardize and document the processes to ensure the documentation lives with the layers.
A: In the video, I showed BMP prioritization for a small watershed. However, working on larger areas (8 digit HUC) actually involves multiple prioritizations within different subregions of the area. This depends on the needs of the local management agency and how they want to divide the watershed into sub-regions or subwatersheds for implementation planning.
Hydrologic conditioning and tiles and open inlets
Q: Any thoughts on dealing with tile and open inlets when conditioning your DEM?
A: We’ve been able to do a decent job of predicting where those inlet locations are based on land use, topography, and depth of depressions, but it is hard to account for the drainage preferences of landowners. So for use in planning, we’ve had to work closely with clients to field check locations of inlets – it is an iterative process.
Another aspect of this: when defining watersheds in drained landscapes, we differentiate areas that contribute water to tile from areas that are undrained and contribute overland flow to streams. This can be important when using the information for conservation management planning.
How to do hydrologic conditioning?
Q: Where do we get more information about running these processes?
A: From the WRC project page at z.umn.edu/lidar go to the lectures and exercises page. From there, look for the exercises under terrain analysis and hydrologic applications. For a workflow for using the NRCS lidar tools, go to the education section of the MnGeo LiDAR page; click on “download the tools”.
Existing Hydro DEMs
Q: Where can we access to reconditioned DEMs that have been prepared and paid for with CW legacy funds?
A: Currently, there is no single repository of hydro DEMs. BWSR has funded numerous projects to create hydro DEMs over the past few grant cycles. The IWI is working on a project that will provide public access to hydro DEMS in the Red River Valley. (Zach talks about these in Part 3 of the Hydro DEM videos.)
Using hydrography (stream) data to automate hydrologic conditioning
Q: Can you use USGS hydrography data to automatically identify and burn through digital dams?
A: It depends on the intended scale and the topography of the project area. If the intended use of the hydroDEM is for larger scale watershed delineation, particularly in regions where topography is fairly well defined from the base elevation data, and when potential for small errors are not critical then hydrography data could be used as you described. In regions of flat topography, such as the Red River Valley, this method does not work very well. Our experience has been that small errors compound quickly in these regions and can cause very large errors in watershed delineation, even at larger scale applications. Also, regardless of topography there are few, if any, hydrographic datasets available that trace flow paths far enough upstream to account for “digital dams” to develop an accurate reconditioned DEM to the field scale. This field scale reconditioning is usually associated with water quality and BMP prioritization projects that are occurring within MN.
We often do use available hydrography and transportation datasets to determine potential locations of culverts. This serves as a guide for review and an initial iteration of reconditioning before user review. We have found several instances, particularly in flatter topography, where hydrography data has been several hundred feet off from the true stream alignment.
NRCS LiDAR Tools and hydrologically conditioned DEMs
Q: When using the NRCS lidar tools do you need to work though the toolkit?
A: The NRCS lidar tools (available from the MnGeo lidar page) are a set of ArcGIS tools to help with hydrologic and terrain analyses and in the design of water and sediment control basins. They can be used by anyone with ArcGIS. They can be used with hydro DEMs.
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