Regional Frequency Analysis / Drought Atlas
ICIWaRM and the UNESCO Centro del Agua para Zonas Aridas y Semiaridas de America Latina y El Caribe (CAZALAC; Water Center for Arid and Semi-Arid Zones in Latin America and the Caribbean; English, Spanish) are collaboratively developing a method by which rainfall frequency maps (focused on extreme events) can be produced for countries throughout Latin America and elsewhere. In particular, drought, unlike floods, hurricanes and tornadoes, can last for indefinite periods of time and can spatially cover broad areas. The challenge is to predict the spatial and temporal aspects of a given drought, especially in arid and semi-arid regions.
As part of this overall effort, ICIWaRM is currently developing non-proprietary software that allows the estimation of rainfall intensity for a given frequency and duration using Regional Rainfall Frequency Analysis (RFA) and L-moments as presented in Hosking and Wallis (1997). The use of L-moments is preferred over regular moments because they are more robust in terms of accommodating extreme values and characterizing a greater number of frequency distributions. In addition, L-moments are less susceptible to bias in their estimation.
RFA assumes that if the frequencies of rainfall events are similar at several nearby locations within a “region”, a statistical analysis of all observations at all sites within the “region” will give a more accurate frequency distribution at each location compared to limiting the analysis to the individual at-site data. Using L-moments allows the development of a drought atlas in arid and semi-arid regions where data is sparse spatially and may be available only over a limited time period at individual sites.
The end result will deliver three outputs: storm frequency, duration, and intensity, and will consist of a series of digital maps that answer questions such as:
- “What is the probability that the current drought will end in X months?”
- “How long a drought should we prepare for?”
- “What is the frequency of maximum drought intensity observed?”
The project has also involved several workshops on regional frequency analysis using statistical techniques and on training scientists to use the software, as well as the networking of scientists from countries throughout Latin America who would like to apply the technology.
For further information, please contact Jason Giovannetone.
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