For better or worse, relief organizations often chalk up their biggest fund raising successes during major humanitarian crises like the famine in east Africa.

The Center on Philanthropy estimates that American nonprofit aid groups received $1.9 billion in cash and in-kind gifts after the Asian tsunami of 2004, and $1.4 billion during the year after the earthquake that decimated Haiti in January 2010.

But aid groups say that raising money to address the famine has been more like that for the flooding in Pakistan last year, when dollars trickled into nonprofit coffers slowly and never came close to reaching the amounts donated to address other disasters.

“It’s even slower for us than Pakistan was,” said Jeremy Barnicle, a spokesman for Mercy Corps, a relief and development group based in Portland, Ore. “The slow onset emergencies are always hard because they lack the immediacy of an act of nature, but they are just as devastating.”