1、统指干旱的时候是不可数名词.

The paucity of fruit was caused by the drought.

水果缺乏是由于干旱造成的.

2、但是具体指某一次旱灾的时候是可数的.比如:

He told a press conference that Spain was suffering one of the worst droughts of the century.

他在记者招待会上说西班牙正遭受着本世纪最严重的旱灾之一.