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Salinity Calculations

Wed, 08/29/2007 - 12:48pm — bobolsn
I resolved the file reading issue (with Rob's help). The model appeared to read the TDH and CDH properly, but the preprocessor would produce errors. Come to find out, the CDH and TDH had a blank line on the end and the preprocessor read jday as 0.0 and produced error messages. The model is running with only temperature and TDS (salinity) as active constituents. The highest input concentration for salinity is 37 ppt for the downstream boundary. The upstream boundary has "freshish" water with 5 ppt tds. The less dense upstream water stays near the top with calculated TDS of 10 to 15 ppt. The bottom layer increases to as much as 51 ppt. Concentrations in the layers just above the bottom also increase, but not as much (39-41 ppt). The layers are 1 meter in thickness. According to the manual, salinity is conservative, therefore I am perplexed as to why the increase. I looked through the CON file to see if there were any sediment releases that could potentially increase the TDS and set to 0.0 for nutrients and CO2. I tried smaller time steps as well. Does it sound like it could be a numerical issue or a flag/coefficient/rate not set properly? Thanks, Bob
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Wed, 08/29/2007 - 2:48pm — chris

Hi Bob, One way to check for numerical instability is to turn the variable MBC to "ON" in the control file. This turns on the mass balance error calculation option that is then output to the snapshot files (which also have to be turned ON). If the model is not conserving mass, it would show up here. chris
Wed, 08/29/2007 - 3:37pm — bobolsn

Chris, I turned on the mass balance and noted that the percent error for the TDS calculations was typically 10e-10 indicating a stable solution. In looking at the file I did note that the upstream TDS was 5 g/m^3 and not the 5 kg/m^3 as noted in the manual when SALT is on. This may be just a "typo". I have not been able to readily see a downstream TDS concentration with flood tides, but I assume that the units are as I entered 37 kg/m^3 as is the 5 kg/m^3 for the upstream. Is there an internal units conversion (one for upstream and another for downstream)? Thanks Bob
Fri, 08/31/2007 - 9:34am — Rob

Bob, Saliity is modeled as ppt or kg/m3 whereas TDS is modeled as g/m3. There is no internal units conversion for the upstream or downstream boundary conditions. The graph.npt file lists g/m3 but these units shoudl be changed is you are modeling salinity to kg/m3. I hope this helps. Rob
Fri, 08/31/2007 - 9:46am — bobolsn

Rob, Thanks for the reply. I have noticed an increase salinity concnetrations above the input concentrations (as high as 51 for 37 input). Do you have any thoughts as to why a conservative substance concnetration would increase? Bob