Guidelines for Setting Turbidity Thresholds          Rev 12/30/2003

General considerations

The following guidelines are designed to collect a few samples in small storms and more, but not too many, in large storms.  “Few” and “too many” are subjective terms and it is up to each investigator to define their desired range of sample abundance by setting the number of thresholds.  The guidelines are based on simulations with data from Caspar Creek (Lewis, 1996) and seven winters of experience suggesting that they meet our objectives (i.e. to accurately and economically estimate suspended sediment loads for, on average, the 6 largest storm events each year).  However, with different environments or different objectives, these guidelines may not be optimal.  For example, if one had a special interest in sampling the first few (likely small) events of the wet season, then an extra threshold or two might be temporarily added near the low end of each threshold scale.  If relatively more emphasis is to be placed on low flows in general, then the square root scale might be replaced with a logarithmic scale.  However, any alteration that places more emphasis on low turbidity conditions will result in more samples (and higher costs), unless the number of thresholds is reduced at the same time.

Using the threshold calculator applet

The TTS web page has a turbidity threshold calculator that can be used with any web browser.  For this to work properly you will need the Java 2 plug-in from Sun Microsystems. Click on "download the JRE".

1. Using the Sensor Maximum slider, set the maximum NTU reading that your sensor can record.  This value can be determined by calibration and may not be the same as the nominal range given by the manufacturer.  The manufacturer should be able to provide the necessary calibration information, however.
2. Set N, L, and U on the Rising Threshold sliders, based on the criteria described below under Rising thresholds.
3. Set N, L, and U on the Falling Threshold sliders, based on the criteria described below under Falling thresholds.
4. If desired, test the thresholds as described below under Simulating TTS.
5. Install the thresholds in the TTS Campbell program as described below under Entering thresholds in the Campbell TTS program.

Rising thresholds

1. Determine the lowest non-zero threshold, L.  This should be a value that is above typical inter-storm turbidity values.  In small streams it should also be a value that is expected to occur only after the stage rises enough to submerge both the turbidity sensor and the pumping sampler intake. 
2. Determine the highest threshold, U, within the range of your turbidity sensor.
3. Determine the number of thresholds, N, between L and U (including both L and U).
4. Use the threshold calculator applet, or manually calculate thresholds as follows. 
5. Compute d = (U^0.5–L^0.5)/(N-1)
6. The thresholds between L and U are  (L^0.5+d)², (L^0.5+2d)², … , (L^0.5+(N-2)d)²
7. Because of the way the algorithm is written, additional thresholds are needed at 0 and above the sensor measurement range, e.g. 9999.
8. The complete set of rising thresholds to be assigned is: 0, L, (L^0.5+d)², (L^0.5+2d)², … , (L^0.5+(N-2)d)², U, 9999

Falling thresholds

• The procedure is similar to that for rising thresholds, except (1) guidelines for determining L, U, and N are slightly different, (2) no threshold is needed above the sensor’s range, and (3) thresholds are assigned in descending order in the Campbell TTS program.
• L should be a value that is at or above typical inter-storm turbidity values.  In small streams it should be a value that is expected to occur before the stage falls enough to expose either the turbidity sensor or the pumping sampler intake.  It is best to choose a different L for falling turbidity than for rising turbidity.  Otherwise it is likely that, in a small storm event where only the lowest rising and falling thresholds are exceeded, only two samples would be collected, both at nearly the same turbidity.
• N should be higher than that chosen for rising thresholds. 
• N and U can be altered in a trial-and-error fashion to minimize redundancy between rising and falling thresholds.  However, because samples are not taken precisely at the threshold turbidity, but occur only when the threshold has been passed for two intervals, the risk of re-sampling the same turbidity is not all that great a concern.  Samples are least likely to occur precisely at thresholds in rising turbidity conditions and at high turbidity levels in general, i.e. when turbidity tends to change most rapidly.
• The complete set of falling thresholds to be assigned is: U, (L^0.5+(N-2)d)², …, (L^0.5+2d)², (L^0.5+d)², L, 0

Examples

Sensor range 0-500

Rising (L=10, U=450, N=10):

0 10 27 51 84 125 174 231 296 369 450 9999

Falling (L=15, U=475, N=17):

475 427 382 340 300 262 227 195 165 137 112 90 70 52 37 25 15 0

Sensor range 0-1000

Rising (L=15, U=900, N=10):

0 15 46 94 158 240 338 453 585 734 900 9999

Falling (L=20, U=950, N=17):

950 851 758 670 587 510 439 372 311 256 206 161 122 89 60 37 20 0

Sensor range 0-2000

Rising (L=20, U=1850, N=10):

0 20 77 170 300 467 670 910 1187 1500 1850 9999

Falling (L=30, U=1900, N=17):

1900 1698 1507 1328 1160 1004 858 724 602 491 391 302 225 159 105 62 30 0

Simulating TTS

TTS can be simulated with any existing Campbell TTS data file and a few functions written in the R programming language.  If you have already installed R for making data plots, then it is simple to add the simulation functions.  These functions and instructions for installation and usage can be obtained from TTS web page.  The package provides the capability to read a raw TTS data file and determine when sampling would have occurred for a hypothetical set of thresholds.  A plot can be produced that shows the record of stage, turbidity, and simulated pumped samples.  Many of the other program parameters specified in subroutine 7 of the TTS program can be altered as well.

Entering thresholds in the Campbell TTS program

1. Copy the thresholds into subroutine 7 (lines 196-200 in TTS Rev 4.1).  The rising thresholds are entered from lowest to highest and must include the values 0 and 9999.  The falling thresholds are entered from highest to lowest and must include 0 (but not 9999).  Eight thresholds are entered per Bulk Load statement in the Campbell program.  The ninth parameter of the Bulk Load statement is the starting address in memory where the previous 8 values will be stored.
2. The number of thresholds must be entered in subroutine 7, where maxrindex and maxfindex are assigned (lines 194 and 195 in TTS Rev 4.1).   The total number of non-zero thresholds is entered as parameter 2 in these statements,  i.e.  N+1 for rising thresholds and N for falling thresholds.  In all of the above examples, the values entered are 11 and 17, respectively.