Reservoir vs. pond

[Justin Goldstein]

Colleagues,
   I have a dam on the main stem of the main river in my watershed.  The sole purpose of the dam is to control flooding; no irrigation or municipal water is taken from it and there are no organized releases.  I have attached a picture of it.   I understand from the SWAT help pdf that main stem structures should be inserted as reservoirs, however since there are no releases I am wondering if I should instead call it a pond.    What are your thoughts?

   Thanks, 
   Justin

[Charles Ikenberry]

Justin,

If there are subbasins upstream of the dam (i.e., if it receives flow from more than just the subbasin it's located in), you'll have to model it as a reservoir.  Reservoirs receive flow from the surrounding subbasin and all upstream subbasins.  Ponds receive runoff only from the surrounding subbasin.  You could alter your subbasin configuration (delineation) so that there are no upstream subbasins - the feasibility of that will depend on the scale and purpose of the model you're developing. 

I'm not sure what you mean when you say it has "no releases."  I assume that floodwater does leave the system via drainage after the peak is attenuated.  If that's the case and you want to simulate the storage and eventual release of water back to the stream system, proceed with the reservoir or pond option as you prefer.  If there are truly no releases (i.e., it's 100 % volume retention), then just eliminate the drainage area from your model.

Hope that helps.

--Charles

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From: Justin Goldstein <gold...@gmail.com>
To: SWAT-user <swat...@googlegroups.com>
Sent: Tue, April 5, 2011 10:39:29 AM
Subject: [SWAT-user:2700] Reservoir vs. pond

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[Justin Goldstein]

   Thanks Charles!  I'll model it as a reservoir since the stream contains water from upstream subbasins.  There aren't any organized, planned releases like in larger reservoirs in which managers would plan to release a certain quantity of water; in this reservoir, water simply reaches the dam, the runoff peak is attenuated, and leaves.  There aren't any data quantifying the discharge before reaching the dam and afterwards, although I do have the area of the dam and its "storage" capacity.  Do you know if SWAT can still handle this? 

-Justin

[Charles Ikenberry]

Yes, SWAT can handle that.  You'll need two pairs of reservoir areas and volumes.  (1) The area and volume at the principal spillway, and (2) the area and volume at the emergency spillway.  You can find that info from design plans if you have them or you can estimate using contours or local topo data.

There are several methods for simulating flows that leave a reservoir.  You'll probably use the average annual release or the targeted release method.  Neither is completely satisfactory; however, they're more than adequate if you're interested in monthly flows.  If you want daily flows, you may have to be a bit more thorough in your efforts to develop reservoir parameters and live with a best approximation knowing that you're probably not simulating peak outflows very accurately.  I hope that SWAT's ability to route flows through ponds/wetlands/lakes will be improved as a result of some current research, but for now we have to live with what's already in the model.

Good luck,
Charles

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From: Justin Goldstein <gold...@gmail.com>

To: Charles Ikenberry <cdik...@yahoo.com>; SWAT-user <swat...@googlegroups.com>
Sent: Tue, April 5, 2011 11:33:41 AM
Subject: Re: [SWAT-user:2700] Reservoir vs. pond

[Jim Almendinger]

I've never used the average annual release method, so I can't speak to its usefulness.  If there is no manipulation of the outflow, then I'd say you'll almost certainly want the targeted release option, where release from the reservoir is dependent on the difference between the current reservoir volume and its "target" volume -- let's call that volume the "available volume" for release.  The NDTARGR parameter is just the inverse of the fraction of the available volume that is released each day.  If NDTARGR = 2, then half is released the first day, half of the remaining available volume is released the next day, half again on the 3rd day, and so forth, in a geometric series.  For some reason, NDTARGR is typed as an integer in the FORTRAN code; it would be useful in some cases to allow it to be real.  Allowing a stage-discharge table or equation specific to each reservoir would be an improvement, but this NDTARGR method seems to work in most cases.  

   SWAT will normally try to determine the "target" volume based on soil moisture, to manage the reservoir for water supply, but to minimize the prospect of flooding during the "flood" season.  Such SWAT-determined target volumes will be larger than the principal volume, to hold as much water as possible (unless it's flood season...).  These details just don't apply to many reservoirs, and certainly not to natural lakes and unmanaged reservoirs.  To avoid this soil-moisture stuff, give SWAT explicit monthly target volumes.  In your case -- set these all equal to your principal volume, which should be the reservoir volume right at the outlet threshold.  

Cheers,

-- Jim

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Dr. James E. Almendinger, Senior Scientist
St. Croix Watershed Research Station
Science Museum of Minnesota
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[Justin Goldstein]

Thanks, Charles and Jim!