I would like to include dissolved solids as one of the parameters in my analysis (USGS surface water quality data 2005-2014). However, from looking at the time series data, it is evident that the temperature at which the dissolved solid samples were dried is not the same for the duration of the dataset.
For example, from 2005-2009, dissolved solids were dried at 105 degrees C.
From 2010-2014, dissolved solids were dried at 180 degrees C. (See attached example)
As this is the case for all the sites in my dataset, can the data from these two drying techniques be combined into one parameter? Or must they remain seperate?
(I’m looking for any resources/articles that may specifically address this issue. Thanks for any insights you may share)
I found this paper. The standard is explained and suggests that variation in temperature and time can produce different results. If you do any more testing of the suspended sediments within the specific watershed, analyze it both ways and it may give you an estimate of the difference to be expected. You may want to discuss with USGS as to why they deviated from the 105 C standard. Since this paper attached was in 2001, perhaps there is some newer discussion. I am sure you have found some piece of detail that most analysts would just pass over, so good job. The unfortunate thing about doing a good job is sometimes it creates more work.
If you decide to combine them for the analysis, I would use a footnote to mention this issue in any tables for comparison and briefly in the methods. I included a paper where they separated the mineral and organic components of suspended sediments from several streams that may interest you. I cannot imagine much of the organics being volatized at 180C, but certainly there is some possibility to remove a little more water from particles. My guess (I DO NOT KNOW) though would be the differences are likely to be small in going from 105 to 180 C.
I JUST SAW YOU HAD ATTACHMENT WITH ONE YEAR WITH TWO VALUES, ONE FOR EACH METHOD. This totally does not make sense if they are the same samples, as the weight of suspended sediment is significantly greater at the 180 C, than at 105C. So obviously, for that one year, something else has changed other than just the temperatures. By presenting both values, so different, in one year brings the data into question as to the reason for the differences beyond temperature, different sampling location, procedures, timing, equipment, etc.
If you have two ISCO programmable sediment samplers that take a sample from a probe, you could get some differences in location, where they are in the water column, etc. For that one year with two values, there is something beyond the analytical temperature difference going on.
Thank you William, I appreciate you taking the time. I think you're right about talking to USGS. Just didn't want to reinvent the wheel if there was a standard approach that I was missing when it comes to analyzing variables and water quality trends. Thanks again for the papers
Sorry for incomplete analysis. Since the values are not instantaneous data, but presented by year, there may be other differences between these as the one year had different values with two methods. Perhaps they did not only change analysis temperature, but also changed location, frequency, number of samples, etc. It is hard to read this simple table on understand the detail differences they used in the two approaches for data presentation. Suspended sediment presented is usually by date and time, or sometimes composited from regular samples. Since most sediment moves during storm events, it should be weighted by discharge if a mean annual sediment is desired. I think more searching may be needed to find out how this data was derived so it could be presented as an annual value.
I should have mentioned that the values I presented in the original snapshot of the table were the mean values. For some variables, data was only available once/year, others more frequently. I had just noticed the different drying temperatures for TDS and forgot to shift back to the more detailed table (attached) when I took a screenshot.
The jump isn't nearly as significant between the 105 and 180 values when not comparing means. Still, I'll need to research more on this topic.
Hi Stacey,
If the only difference between methods is the drying temperature, you could use it as a factor in an analysis of variance. This could provide an estimate of the size of the temperature effect and its associated uncertainty. I would suggest coding the drying temperature as a nominal variable instead of using temperature as a continuous variable as I suspect your interest is in whether drying temperature affects the outcome, not the relationship between temperature and sediment yield.
Martyn
The data presented suggests infrequent sampling. With enough readings over may years, you may have some connections between discharge rate and TDS, but the mean of irregular sampling may not be much more than the mean of irregular samples. It is information, but difficult to use for analyzing dissolved solid yield or loading projections beyond a coarse estimate. However, if you can establish a good relationship of flow with TDS, or the primary components, you may be able to apply with some better reliability. You may need to divide data between rising limb of hydrograph and falling limb of hydrograph. My old paper on springs of Missouri water quality (karst - 1975) may be an example to you of correlating instantaneous discharge rate with several types of dissolved components sampled in stream (springfed) water.
I don't know if the different temperature can have affect of the total dissolved solid
I think 180c is the highest temperature and the optimum one for TDS so 105c Willmott have impact on the TDS except it took long time only.
So you can ignore it.
Regards
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