+McCarthy, Francis X., Analyst in Emergency Management Policy, Government and Finance Division, Congressional Research Service (CRS), FEMA's Disaster Declaration Process: A Primer (RL 34146)(March 18, 2010) (PDF — 265K)
"The amount of assistance provided through presidential disaster declarations has exceeded $100 billion. Often, in recent years, Congress has enacted supplemental appropriations legislation to cover unanticipated costs. While the amounts spent by the federal government on different programs may be reported, and the progress of the recovery can be observed, much less is known about the process that initiates all of this activity. Yet, it is a process that has resulted in an average of more than one disaster declaration a week over the last decade.
"The disaster declaration procedure is foremost a process that preserves the discretion of the governor to request assistance and the President to decide to grant, or not to grant, supplemental help. The process employs some measurable criteria in two broad areas: Individual Assistance that aids families and individuals and Public Assistance that is mainly for repairs to infrastructure. The criteria, however, also considers many other factors, in each category of assistance, that help decision makers assess the impact of an event on communities and states."—Summary.