Lake Chad may be shrinking because of the vast industrial-urban complex and huge agribusiness surrounding it, but Okeechobee is drying out because its not raining in the Kissimmee river valley that feeds it. South Florida is getting some reduced (about 50%) rain, but it has shifted south where it can't help the Lake. In spite of the increasing demands on Okeechobee, the problem has usually been too much water, not enough, and the water managers were always draining the Lake for flood control. That is no longer the case. The problem now is drought.

Unlike Aral or Chad, Okeechobee is not a basin, it is actually higher than the Everglades to the South, and in the past the vegetation in the 'Glades slowed the drainage to a crawl, and buffered extremes in rainfall. Today, levees hold in the water, and canals drain it as needed,
under human control. The problem is on the supply side, not the demand.

At the end of the rainy season on 1 November, average water levels are usually about 15 feet. Today they are about 10. This report was printed at the start of 2007 rainy season, which was a very disappointing one. The Lake, already low, only gained about a foot during the wet season from June to November. Not enough to get us through to next summer with a margin of safety.

http://www.dep.state.fl.us/secretary/news/2007/05/0530_01.htm

As for global warming, I didn't bring that up. But now that you mention it, I wouldn't be a bit surprised if it didn't have something to do with it. But even if global warming isn't responsible, there is still an unprecedented drought. I am not making this up.