A drought or dry spell was a planetary condition where a lack of rain caused a shortage of water. In some locations, droughts were seasonally prevalent.
In the 14th century, Kataan suffered from a protracted drought. The inhabitants later realized that it wasn't a simple drought, but that their sun was about to go nova. (TNG: "The Inner Light")
As a six year old child, Kathryn Janeway once flooded the Forest of Forever by diverting a river, after Flotter suggested a dry spell was coming. (VOY: "Once Upon a Time")
In 2374, the Agricultural Consortium on Balancar reported a drought on the planet; Quark then discovered there actually was no drought, but that it was just a deception to raise the price of syrup of squill. (DS9: "The Magnificent Ferengi")