It's a little pretentious to put this in the Cave Science forum, but hey, ho....

Partly inspired by Richardg's thread on "Great Whernside Resurgences" in which he discussed attempting to detect hitherto unidentified resurgences in the Wharfe using temperature measurements, I thought that I would have a play with a digital thermometer in Hebden Beck. Hebden Gill, which is really the continuation of the Mossdale valley has most of its water captured by the Grassington Moor mines and so has a relatively small flow when it enters the parish, mainly fed by water off the moors and pastures. It then has water from Duke's Level, an adit draining much of Grassington Moor, entering from the west. Below that, another tributary enters from the east. This is the outflow from a small reservoir on the moor built to supply energy to the mining operations.

The idea was to try to gauge the relative contributions from the three inlets - the main stream, Duke's Level, and Mossy Mere by taking the temperatures of the main flow and the tributaries above and below the point of confluence. The flow units are arbitrary, with the water entering the system at the top have a unit of 1.


It's a technique I last played with some 50 years ago. It is pretty accurate (the thermometer is not calibrated but is relatively accurate to within 0.1?C), and it could have some use underground.

Very interesting and instructive Langcliffe, thanks for posting.
Although I had to read it twice, on different days, and eventually I got it! 

Mike Wood said:

...Although I had to read it twice, on different days, and eventually I got it!

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Same here! The numbers seem to be qualitatively correct based on your description, but do you have a feel as to how close they are to being quantitatively correct?

MarkS said:

Same here! The numbers seem to be qualitatively correct based on your description, but do you have a feel as to how close they are to being quantitatively correct?

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A good question. The accuracy of the results depend on three factors:

1. The accuracy of the temperature measured.
2. How close to the confluence one can measure the temperature of the inlet waters.
3. How close to the confluence one can measure the temperature of the mixed water.

The first depends on the accuracy of the thermometer, and the experimental methodology. My thermometer is digital with 0.1?C display, and the ability to lock the reading. I took each reading three times, and got the same result each time, which was reassuring. However, if my figure for the first set of readings had been 0.1?C out in the worst possible directions, the calculated flow would have been 10% lower.

One can normally get close to where the inlet waters are about to combine, so the second factor shouldn't be a problem.

The third factor is potentially a greater problem, as one has to measure the temperature at the first point where one can guarantee that the waters have mixed. In the case of the Amazon, the Atlantic and the river water haven't properly mixed for over a 100 miles from the coast which would give plenty of opportunity for the temperature to adapt. Where Hebden Beck joins the Wharfe, in normal stage the waters probably haven't mixed for over a 100 metres.  In the case of the measurements above, the maximum distance was about 10 metres, which shouldn't be too much of a problem.

The results did look reasonable to me, but the only really practical way to assess them would be by using salt dilution techniques, and for that one would need a electrical conductivity meter, a big bag of salt, and a large bucket.