We spent a sunny day last May working on the edge of the Ettrick Water at Philiphaugh, a stone’s throw from the hydro turbines. It was the first few days of a project working with dancers and movement improvisation to explore what an ecological approach to choreography might be.
The collaborators on this were Claire Pençak, choreographer, dancers Merav Israel and Tim Rubidge and environmental artist Kate Foster.
One of the questions that emerged from this process was – How long would it take for a droplet of water to travel from the source to the mouth of Tweed?
This was the fascinating email from Professor Chris Soulsby, Chair in Hydrology, University of Aberdeen in response to our question.
‘What may seem a simple question actually has a very complex answer!
I’ll start simple, for water molecules (water may enter the catchment as rain DROPS, but it’s better to think of the subsequent movement through the landscape as individual molecules) to travel from the source to the mouth of the Tweed RIVER SYSTEM will – as you say – depend on flow rates. In really wet conditions, it would probably take only a day or two. In dry weather, it might be a week or two.
But, water is only in the river for a very short proportion of its overall “residence time” in the Tweed CATCHMENT. Only a tiny (<0.01%) proportion of rain falls on the river channel, most will fall over the catchment land surface. There it may follow a bewilderingly complicated spectrum of possible flow paths to the river channel (depending on where you are geographically and whether it’s wet or dry). At one extreme end of the spectrum, rain may fall on a road surface, rapidly drain down the gutter in to storm drains and get to the river in a matter of minutes. The same rapid transport could happen in mountains if rain falls over bare rock and rapidly runs off in to mountain streams. At the other end of the spectrum it may fall on dry soils and slowly drain into groundwater in the underlying bedrock where (in the sedimentary sandstones in the lower Tweed) it make take decades or centuries to reach the river channel.
So the water in the river channel at any point in time is an integrated collection of water flows from these different sort of flow paths (specialists talk about a rivers “transit time distribution” to acknowledge this. In the winter and during wet periods, the water “age” will be biased to the rapid, short term flow paths and may be an average of several weeks/months old (in other words, most fell as rain in the previous few months). In summer low flows, the water in the river will be dominated from deeper groundwater contributions and might be – on average – decades old. But, these “ages” are averages of water that has a spectrum of ages. So, almost at any time SOME of the water in the river will have fallen in the past few months and SOME will have fallen before – say – the Battle of Flodden. The relative proportions shift depending upon how wet the weather is (when its wet the water is on average younger, as it dries the water is older).
In other words the water in the river is not only integrating everything that has happened over the large area of the catchment, but (to get back to your question) it’s integrating through history. That’s why protection of water is so important; for example agricultural or industrial pollutants input into the catchment today, may still be draining into the river in hundreds of years’ time!
June 1st 2014