Natural Disaster Monitoring

What qualifies as a natural disaster?

When is a drought a drought? And when is a flood a flood? The answers to these questions make the difference between Federal aid or no Federal aid, between one county's farmers being accorded special loan facilities and the next county missing out. Take a drought - the suburban resident first suffers a drought when the yard dries up and hose-pipe use is restricted, as it was this summer by Maryland's Governor. A meteorologist says there is a drought when there is less water in their rain gauges which happens long before the hose-pipe ban. The water utility gets worried when river and reservoir levels start to drop which happens with a different timing than rain. The farmer has a drought when crops and animals begin to suffer. So who is right? Well, everyone is right - from their own perspective. Disasters are "in the eye of the beholder".

Formally we can distinguish four broad categories; meteorological, hydrological, agricultural, and socioeconomic drought. The first two concern only physical parts of the system, in the case of meteorological drought we are referring to the rainfall, though in the case of hydrological drought, ground water and stream and lake levels are partly dependent on land use and water consumption by people. Agricultural drought incorporates the effects on crop growth and other biological systems. Finally, socioeconomic definitions of drought specifically address human perceptions of supply and demand, and greatly broaden the appropriate measurements.

We can think of these four types of drought as forming a continuum, with increasing relevance to the human population. But there is also a continuum of increasing difficulty of measurement and definition; rainfall is much easier to measure than its socioeconomic implications. For this reason, most widely-used drought indices use physical measurement, not socioeconomic factors, though at the cost of decreased relevance.

An index used by the government is the Palmer Drought Severity Index (PDSI). This index is scaled from -6 to +6, with the negative values representing drought. PDSI provides the basis for initiating the release of government relief funds for farmers to compensate for crop damage.

Satellite observations of the land surface are a very effective way of monitoring agricultural drought. Essentially we can look at the impact of the drought, rather than the rainfall (which may not always be directly or simply related to the effect on the crops).

| Main Drought Page |