I'm delighted to share the first publication by a WUNDERGRAD with our research lab at UWA - fabulous work by Aryan Puri, who undertook experiments and theoretical analysis of pendant rivulets as part of his Bachelor of Philosophy. You can read the full study in Hydrological Processes.
Pendant rivulets form when a thin stream of water trickles down the underside of a surface, holding on with surface tension. If you saw the last news item about pour points, you might recognise that if a pendant rivulet gets detached from the surface, it forms a pour point - just like in the picture here:
We wanted to understand what might cause pour points to detach and how these rivulets would behave for different flow rates and angles. But unlike in Dr. Ashvath Kunadi's pour point work, we didn't want to do the experiment with real tree branches (those things are inconveniently irregular and twisty), so we used that famous substitute for a tree branch -- a length of PVC pipe!
The actual experiments were quite simple in concept (although not always to do ...). A syringe pump delivered water onto the upper side of a pipe section. Water flowed around the pipe and made a rivulet below it, which then flowed down the pipe. Water that was delivered to the end of the pipe was caught in a beaker on a scale, and another bucket caught any pour points or drips along the way. Adjust flow rate, adjust pipe angle, rinse, repeat.
As usual when you start experimenting, you learn a few things.
The first thing we learned was this: straight pipes don't form pour points.
Instead, as water accelerates down the pipe, the rivulet gets more stable - less likely to detach.
Showing this theoretically took some nimble mathematical work by Aryan and mathematical MVP Prof. David Pfefferle, but we were able to show that our experimental findings were consistent with the hydrodynamcis of rivulets. Our theoretical work was all for linear, pendant rivulets, however, and we saw plenty of more complex hydrodynamic behaviour in the rivulets, including waves and meanders.
So - how do you make a pour point then? We could most reliably induce pour points by changing the angle of the pipe. We hypothesise that as the flow moves from a higher to a lower slope, the rivulet decelerates, becoming less stable and ultimately detaching - although measuring flow velocities in these rivulets was a bit beyond what our equipment could do!
There's a heap of work left to do to extend hydrodynamic understanding to the flow phenomena that occur as water moves around on the surface of leaves and branches and, eventually, whole canopies. But it's exciting to show that such theoretical interpretation is possible- and hopefully it will help us improve understanding of interception and throughfall processes.
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