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Temperature Profile

Tue, 06/03/2008 - 12:29pm — Thomas
I have a reservoir model up and running and everything looks good except the temperature profile. It is quite a bit off from the measured data that we have. The temperatures at the top and bottom of the 50 ft deep reservoir are matching fine (+/- 1F), but the profile shows the high surface temperatures extending too far down into the reservoir before the thermocline is hit (deviating up to 13F at 28ft depth. The measured data show the thermocline at a depth of 18 feet, but the model puts it at 28 feet. The profile shows a near vertical line from the surface down to about 28 feet where it then starts to cool off. As per the manual, I played with the wind sheltering and didnt see much of a change, so I increased the light extenction coeff to 1, but that only helped to decrease the temperatures by about 0.5F. Further increases of the light extenction coef (from 2.0 to 100.0) had almost no effect. What other parameters may be resulting in this deep temperature penetration into the lake. Any help would be appreciated. Tom
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Water Quality
Wed, 06/04/2008 - 7:34am — nick

Typically reducing the wind sheltering coefficient has helped me with the type of temperature profile you have described. Under the HEAT EXCH card which surface heat exchange (SLHTC) option are you using? You may try using either ET or TERM though TERM is the default and I always use it. Also try adjusting the AFW, BFW, CFW, and WINDH values.
Wed, 06/04/2008 - 12:33pm — Thomas

I have been using ET. I will try TERM to see if that helps. I am not modeling Algae, but there is a lot of algae in the reservoir. It is bordering on eutrophic. Could that be having an effect on Temperature? I was going to try and simulate light attenuation due to algae by putting in a constant ISS concentration and not settleing it out, but I am not sure if that is a good approach. Tom
Wed, 06/04/2008 - 2:50pm — chris

Tom, How many withdrawal structures does the dam have? If reducing the wind sheltering doesn't raise the thermocline, then I'd make sure the elevations and flow rates of the dam outlets are correct. If there are multiple outlets and there is error in the amount of estimated flow for each outlet, the model predictions of the thermocline can be affected. chris
Wed, 06/04/2008 - 8:00pm — Thomas

Hi Chris, I have a single 3 cfs low flow withdrawal 4 layers above the bottom. On initial review of the output it looks ok, but I will check it a little closer. The temperature response seems a little strange. If I look at monthly profiles starting in January of the second year(I am running the model for a 2-year calibration period), I see the whole profile above the theromocline heating up at the same rate. So, it starts out with the whole lake at 52 degrees, then all layers in this vertical temperature profile warm up at the same rate. As this vertical profile moves the the right (heats up) it slowly leaves the lower layers behind as they reach their equilibrium temperature. In contrast to the way the model is responding, the observed profiles heat up at the top and you can see the heat penetrating downward as the summer progresses, which is what you would expect to happen. It seems that somehow I am getting full thermal mixing with no attenuation until the thermocline is reached. All layers above the thermocline are the same temperature except for the top layer which is slightly warmer due to EXH2O=0.45. Tom
Fri, 06/06/2008 - 10:06am — chris

Tom, If you haven't done so, you might run a sensitivity run with the wind sheltering coefficients set to zero. This will give you an idea how much the wind is affecting the depth of the thermocline. chris
Fri, 06/06/2008 - 12:44pm — Thomas

Hi Chris, I tried a wind sheltering coef of 0.00 and 2.00. The result is the profile moves right and left reflecting an overall heating and cooling respectivly of the profile, but basically the profile keeps its same shape with the thermocline at roughly the same depth. I then changed the input file to use the internally generated solar radiation with a cloud cover of 20% and 80%, and again the profile shifts left and right, but the thermocline depth really does not change much. I am going to try and change the vertical eddy viscosity and see if that has any effect. I am in a time crunch to get this finished. Any ideas? Tom
Fri, 06/06/2008 - 3:19pm — Thomas

Hi Chris, I changed the vertical Eddy Viscosity to a high and low values of 0.01 and 4, and did not see much of a change. But, I changed Longitudinal Eddy Viscosity and Diffusivity from 1.0 to 1,000.0 and the thermocline is much closer to the measured values. From this point it looks like I only need to do some fine tuning to get an acceptable fit. Are the values of 1,000 for AX and DX in an acceptable range for a lake? Tom
Sat, 06/07/2008 - 11:25am — chris

Tom, 1000 for AX and DX seems a little high. Another parameter that can affect the thermocline is PQC, which distributes flow at the upstream BC. If PQC is 'ON', the flow goes to the layer with corresponding density. If PQC is 'OFF', flow is distributed evenly amongst the active layers in the upstream segment. Are there any tributaries? The vertical placement of flows from a tributary can also affect the thermocline. chris
Mon, 06/09/2008 - 12:43pm — Thomas

Hi Chris, I have tried playing with PQC and it didnt change things much. The reservoir is in the SF Bay Area where we get most of our precip in the winter months and basically nothing from May through October, so I dont think inflow distribution will effect the thermocline very much. After changing AX and DX to 1,000 all of the other parameters started to have the effect that I originally expected. Wind sheltering started to become important and effect temperature. Also the light extinction coefficient started to have an effect on temperature as well. Previously, I had performed a sensitivity analysis on these parameters and they had little effect on water temperature. Now they are highly significant on absolute temperature as well as thermocline location. There is a lot of algae in the lake, could that be effecting the AX and DX parameters? I originally tried to account for the light extenction due to algae by introducing a high sediment concentration, but with AX and DX = 1.0, constant sediment concentrations as high as 10,000mg/l (and a high extenction coef) didnt really effect temperature much so i took the sediment parameter out. Tom
Tue, 06/10/2008 - 11:47am — chris

Hi Tom, What are the model predicted algae concentrations (in mg/l)? chris
Tue, 06/10/2008 - 3:36pm — Thomas

I am not predicting the algae concentration because I don't have the supporting data necessary to model it. I just noticed that my model was predicting high temperatures almost all the way to the bottom of the reservoir so I thought that algae may be effecting the light attenuation. So, I tried to simulate the light extinguishing characteristics of the algae by artificially putting in a sediment concentration and sediment extinction coefficient. I tried a variety of sediment concentrations (up to 10,000 mg/l) and found that they had very little effect on the temperature in the reservoir. (at least there was little influence when AX and DX were set to 1.0. I have not tried to see if sediment would have an effect with the high values of AX and DX. I think that it probably would, becuse after raising AX and DX, the light extenction coeff started having an influence on Temperature. Raising AX and DX to 500 brought the thermocline up about 8 feet. Increasing AX and DX to 1000 brought it up about another 8 feet so that the model was matching the measured data pretty well. I was comparing the model output to monthly temperature profiles collected over a 2-year period. The model as it sits now, has a mean error of prediction of about 1 degree C. This is with AX and DX set to 1000, the wind sheltering set to 1.5, and most of the other parameters near where the default values are. Tom