There are a number of key variables related to the Gorona del Viento (GdV) project that we have no data for, basically because GdV has declined to send us any. One of the most important is the reservoir balance, which up until recently we have only been able to guess at. However, thanks to the valued participation of Rainer Strassburger, our man on El Hierro, we now have some recent dated photographs of water levels in the upper and lower reservoirs. Are they good enough to allow us to make meaningful reservoir balance estimates? This post looks briefly into this question and concludes that they are.
GdV upper reservoir at the pre-construction stage, image credit vistaire
First a summary of results. Note that all numbers are approximate.
1. Between February 20 and March 28, 2016, there was a net transfer of 20,000 cubic meters from the upper reservoir (UR) to the lower reservoir (LR), with 40,000 cubic meters transferred from the UR to the LR between February 20 and March 17 and 20,000 from the LR to the UR after March 17.
2. There is an encouragingly close balance between the transfer volumes, indicating that photos of water levels can be used to estimate reservoir volumes to within acceptable limits of error.
3. The two reservoirs between them presently contain about 90,000 cubic meters of water, or less than 20% of GdV’s total combined reservoir capacity of 535,000 cubic meters. As of March 28 both reservoirs were only 15-20% full.
And second a brief description of the procedures I used to obtain these results.
One of the notable things about the GdV reservoirs is that they contain narrow terraces of white-painted concrete (?) blocks that divide them into three segments:
Figure 1: Upper reservoir under construction, showing white concrete block terraces (click to enlarge)
The primary purpose of these blocks is probably to anchor the liner, but the spacing is intriguing. It’s almost as if they divide the reservoir into three layers of equal volume, with the top line of blocks representing 100% full, the second 67% full, the third 33% full and the lowest empty. Detailed geometric analysis of reservoir shapes would be needed to confirm this, but I didn’t have the time to do any so I accepted the 33/33/33 argument as a working hypothesis and went from there.
The next step was to measure water levels relative to the white lines. I did this by copying Rainer’s images into Microsoft “Paint”, identifying common measurement points on all of them (yellow line) and recording Y-axis pixel values. Here’s the latest result from the upper reservoir :
Figure 2: Y-axis pixel counts, upper reservoir, March 28 2016, 157 hours
The water volumes that correspond with the first three pixel counts are 385,000, 257,000 and 128,000 cubic meters according to my 33/33/33 assumption, so next I plot these volumes against the pixel counts to see whether I get a straight line, which if my 33/33/33 assumption is correct I should:
Figure 3. Pixel counts versus assumed 33/33/33 reservoir volumes, upper reservoir, March 28 2016, 1537 hours
The line shows a slight dogleg but isn’t bent enough to cause me to discard my 33/33/33 assumption. So I proceed.
The next step is to estimate what volume the 264 pixel count at the water level represents. I did this by adjusting the volume until I judged that the point fitted the projection of the Figure 3 plot as closely as possible. In this case 60,000 cubic meters (black) gives me a slightly better-looking projection than 50,000 cubic meters (blue) or 70,000 cubic meters (red), so I accept the 60,000 cubic meters number while recognizing that I’m probably looking at a +/-10,000 cubic meter estimation error.
Figure 4. Pixel count at water level fitted to 33/33/33 reservoir volume plot to estimate water volume, upper reservoir, March 28 2016, 1537 hours
And here’s the reservoir balance for the six days for which photos are available after all the volume measurements were to hand:
Figure 5: Volumes of water contained in lower, upper and both reservoirs combined, February 20 to March 28, 2016. The open red circle in the lower reservoir plot is interpolated. No photo is available for March 14.
I was pleasantly surprised by these results. The LR fills and the UR empties, the UR fills and the LR empties, the volumes are comparable and the combined stored volume stays the same within 10,000 cubic meters, or 2% of the combined reservoir volumes. With a more sophisticated estimation approach than the one I used it should be possible to come up with some fairly exact numbers.