Precipitation thresholds for drought recognition: A further use of the standardized precipitation index, SPI

Abstract

Most of the methods used to analyse drought episodes include drought indices associated with quantitative scales and which are based upon instrumental variables, with particular emphasis for precipitation. Among such indices, one of the most common indices, if not the most widely used, is the standardized precipitation index (SPI), which assigns a numerical value to the monthly precipitation or to the cumulative precipitation in groups of consecutive months. Such a value can be compared across regions with very distinct climates. Despite the widespread use of the SPI, the interpretation of its values and drought monitoring based on the same are by no means trivial tasks, particularly due to the fact that are standardized values which cannot be easily related with the precipitation records from which they derived. For this very reason, an additional calculation method has been developed which returns back to the precipitation field the SPI values for any drought threshold. By assigning the drought thresholds directly to precipitation the meaning of the SPI results are clearer and the identification and monitoring of the drought occurrences are more expedite. Based on the new concept developed for the SPI, precipitation surfaces that provide precipitation thresholds for drought recognition on a monthly basis and across different timescales throughout the Portuguese mainland are presented. © 2013 WIT Press.