>>211901363I googled it and many sources say that the reason is climate change with rising hot summer temperatures which affect evaporation. And it started going down around 30 years ago.
Maybe it's also the reason in general, when we get less snow and our rivers are dried out too in some years, so even less water flows to Caspian sea. Last year wasn't so dry, even a bit rainy, but rivers couldn't restore their usual level after previous dry years. Only this year they returned to the usual level.