SWMM-USERS Digest - 14 Mar 2012 to 15 Mar 2012 (#2012-27)
SWMM-USERS automatic digest system
Topics of the day:
1. Evaporation rate and initial event time
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Date: Thu, 15 Mar 2012 17:01:09 -0700
From: "Pang, Joseph" <Josep...@SEATTLE.GOV>
Subject: Re: Evaporation rate and initial event time
Which version of SWMM5 are you using? There have been updates on how evaporation was handled since v19. If you are using SWMM5 v18 or older, you would most likely get issues from evaporation depending on which methods you use to input evaporation data into the model (calculated from temperature or user input time series). Your AMC issue sounds like a v18 issue as well. In v18, 0mm rainfall still equals raining. From information given in your email, I would suggest that if you're not using SWMM5 v22 engine, try to use v22 and see what you get.
Hope this helps.
Joseph Pang
-----Original Message-----
From: SWMM-USERS [mailto:SWMM-...@LISTSERV.UOGUELPH.CA] On Behalf Of Gerald Krebs
Sent: Tuesday, March 13, 2012 8:40 AM
To: SWMM-...@LISTSERV.UOGUELPH.CA
Subject: [SWMM-USERS] Evaporation rate and initial event time
I'm in the process of calibrating an urban watershed (~95% impervious) in SWMM for 6 independent storm events. The watershed is rather small 5.87ha, but it is subdivided into 690 subcatchments (to allow for estimation of on-site BMP effects). I'm planning to use multi-rainfall-event multi-objective optimization for calibration. To minimize the model run time (the events are from 2 different years) I want to rearrange (create a continuous series) my calibration events to get them in an as short as possible time span for one simulation. During this process two questions were coming up:
1) I used SWMM's routine to compute evaporation based on daily
temperatures. When moving events (4 events stay at their correct date and 2 events move from July 2010 and 2009 to June 2010) the computed evaporation rates change due to the difference in day length. My idea was to overcome this issue by computing the evaporation rates first for the original event dates and use the output as an input evaporation rate (mm/d) in my climate file. However, my discharge curves differ (between 0.1 and 1 LPS) depending whether I use SWMM to compute evaporation during simulation or use the evaporation rates from the climate file. As my model setup is complicated I used a very simple model (one 100% impervious catchment without any depression storage and an outfall) to understand (or not ;-)) the issue.
Also this setup shows similar behavior. The water balance in the status report is the same, losses for the subcatchment are the same, but the discharge is slightly different (smaller though than in my complex model setup). Any ideas where that comes from?
2) The second point concerns the initial time periods I need before the
events to maintain the same discharge behavior (initial catchment condition). I will try to explain the issue using one event. The event dates
6/10/10-6/12/10 (~10mm precipitation). Before the event there was slight precipitation (~1mm) on 6/10/10. The last rain before that was in the end of May (~2mm). If I start the simulation on 6/10/10 (including the previous rain), I get the same discharge as if I start the run 6/1/10. That's still clear as there was no rain between 5/30/10 and 6/10/10. But when I still include the rain from end May (simulation start at 5/29/10) I get a different discharge for the event. The surface depression storage is empty before the event with any initial run time. Besides surface depression storage I have only the network and soil moisture to store water. I don't believe a 2mm event can have an impact on soil moisture for 10 days in June and affect the runoff (plus my pervious area allowing infiltration is very small ~5%). I realized that the system storage is never getting empty after a rain event (in fact it stays at 0.02m3 at the minimum). Also any ideas on this subject are appreciated. Does somebody have experience with creating a continuous sequence of independent events in SWMM?
Thanks, I hope I was able to explain somewhat understandable
Gerald
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End of SWMM-USERS Digest - 14 Mar 2012 to 15 Mar 2012 (#2012-27)
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