The data that supports the scientist's decision to issue a drought warning for the region would be:
Reservoirs and lake levels continue to drop significantly.
This information directly relates to water availability, which is a key indicator of drought conditions. The decrease in water levels in reservoirs and lakes suggests that there is not enough rainfall or water supply, which can lead to drought situations.
The other options, while they may indicate environmental changes or challenges, do not specifically point to drought conditions as clearly as the drop in water levels does.