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Sean Czarniecki

SFR Package - Arc interpolation control

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This may end up under the GMS development request forum, but I'll first see if I'm missing something.  I'm using the SFR package for the first time.  I've taken one of my stream segment arcs and input the upstream and downstream elevations.  When I map to MODFLOW, it would appear that there is limited control over the interpolation.  This is in a mountainous region, so the terrain does not necessarily stay at a steady gradient.  This causes my stream to often be at a higher elevation than reality.  From what I can tell, my only option is to break up the segment into many smaller segments.  This is quite painful.  I had considered if I could do anything manually in the input files, but the SFR package information goes into a binary file.  Rather than breaking up the segment, wouldn't it be easier to change the interpolation to work with the vertices and allow the user to just change the vertex elevation?  Maybe it does this already, but I don't see it discussed anywhere....and since all my vertices are at a Z elevation of 0 (and the interpolation isn't affected by that), I'm thinking that they are ignored.

Any thoughts on making this process easier than breaking up my segments into many smaller segments?  I might seriously need to make segments go from one cell to the next in many locations.

Thanks!

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With SFR the upstream and downstream elevations are specified for each segment so there is no interpolation that occurs when you use Map->MODFLOW. The STR package had the elevation at each reach so that interpolation would happen. So if you use the SFR you will have to break up your arcs.

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Okay - I see what you are saying.  While there is no actual interpolation which applies a stream bottom elevation to a cell, there is an "internal" SFR2 interpolation which sets the stream bottom in the cell for use during calculations (as evidenced by errors which show up in the output file when a stream bottom elevation is below the bottom of a cell...and by the calculated depth of water in a reach [cell] in the istcb2 file). 

I might still consider putting this in the GMS development feature request if I have time in the near future.  Based on what you are saying, I would want Z elevations from the arc's nodes and/or vertices used to generate segment elevations, as that is the item which is painful to put into the dialog boxes (you can easily put in similar hydraulic properties for all of the arcs, but the elevations are all individual entries).  I'll think about what might work best.....there may not be a big push to get that done.  This is the first model I've worked on with a large number of streams.  I'm sure that most modelers don't go this route too often.

Thanks again!

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Hi Sean -- What we did recently to get the SFR2 package running was mainly a GIS exercise. We simplified and put all of the streams in a "blanket" type surficial layer that covers the entire active model area. Then we put a Digital Elevation Model (DEM) together of the land surface altitudes and and made it match the model grid - grid-wise. Next, use GIS magic to get the sinks (closed topographic) areas out of the DEM. Use more GIS magic to create flowlines from the DEM. We didn't have any luck with the National Hydrography Dataset - the flowlines don't match the topography and you get some arcs with the downstream ends higher than the upstream ends.  When all of the GIS work was done (it's fairly high-level work) we had a DEM and flowlines that world match and work in GMS and were able to get functioning SFR2 packages. There are several advantages to the SFR2 package over the Drain or River packages - the primary one being the removal of the "infinite" term in the equation that lets the Drain and River packages take or add as much water as the head differences and conductivity allows. The SFR2 budget is limited to the groundwater that has been discharged to the stream and any surface water volumes that are directly applied.

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Thanks, Bruce.  I totally agree that the SFR2 package is better than drains or rivers when you are actually trying to get the correct water balance in a watershed.  That's why we're using it.  The issue is that the region is mountainous, so using one segment for a 30,000 m long stretch of stream/river doesn't put the streambed at the right elevation for much of that length.  I like your thought of using GIS, but like you said, it would be pretty high-level work.....I may have to go that route though if I don't figure out a better way to do this in GMS.  Do we know if there is an upper bound to the number of SFR2 segments allowed?  Without accounting for elevation changes, I will have around 100 segments in the drainage basin.  If I wanted to be precise in accuracy, I could easily get to 1000 segments (I wouldn't likely if I did it manually, but if I went the GIS route, I would shoot for high accuracy).

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I'm running with about 8,000 segments in the larger of the 2 model I'm working on. I don't think the type of topography is that important, just get the closed depressions out and make the flowlines match the DEM

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Thanks again - I spoke with my GIS guy yesterday and it seems that getting the segment starting and ending elevations wouldn't be that difficult.  I'll just have to figure out how best to get them into GMS.  Did you do it via cut and paste into the dialog box for the segments, or through a MODFLOW input file?  Right now, GMS looks to a binary file for the array.  I think I was recently successful in getting an input file to ignore the binary file, but in another case, I was not successful (I don't recall the details on which package it was), so just curious as to which way you went.

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Hi Sean -- I did the SFR2 package construction all in GMS. The segments have to be in order - from upstream to downstream and the GMS MAP module programming does a good job of this. The trick I found was having a good DEM and matching flowlines to work with. Even with all the prep work we did, there we still some segments that had to fixed by hand.

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