It's possible that this is pumped storage. A lot of hydro stations are demand-based, they're used to compensate for renewables. So in the graph I would interpre

It's possible that this is pumped storage. A lot of hydro stations are demand-based, they're used to compensate for renewables. So in the graph I would interpret that as the reservoirs being refilled during periods of low demand (the amount of electricity this takes is a lot). From the official REE graph it looks like pumping usually takes place overnight and is powered by nuclear. https://demanda.ree.es/demandaGeneracionAreasEng.html https://demanda.ree.es/demandaGeneracionAreasEng.html If you look at this report, you can see in the annual report that pumping 'costs' around 6GW a year. http://www.ree.es/sites/default/files/downloadable/preliminary_report_2013.pdf http://www.ree.es/sites/default/files/downloadable/prelimina...