Influence of Precipitation Changes on the SPI and Related Drought Severity. An Analysis Using Long-Term Data Series

Abstract

Abstract:Drought indices, such as the Standardized Precipitation Index (SPI) are used to 10 quantify drought severity. Due to the SPI probabilistic and standardized nature, a given value of SPI 11 computed in distinct time periods or locations indicates the same relative drought severity but 12 corresponds to different amounts of precipitation. Thus, the present study aims at contributing for a 13 comprehensive analysis of the influence of long-term precipitation variability on drought 14 assessment by the SPI. Long records of monthly precipitation, spanning from 1863 to 2007 in 15 several locations across Portugal, were divided into 30 years sub-periods and the SPI with 12-month 16 time scale (SPI-12) was computed for each sub-period and for the entire period of records. The 17 probability distributions adjusted to precipitation in those different time periods were compared 18 envisaging to detect the SPI sensitivity to the reference period and, therefore, to changes in 19 precipitation. Precipitation thresholds relative to the upper limits of SPI-12 drought categories were 20 obtained and the influence of the time period was investigated. Results have shown that when SPI 21 values derived from the full data record for a recent time period are lower/higher than the SPI 22 values derived from data of the considered time period a recent downward/upward shift of 23 precipitation has occurred. Coherently, a common pattern of drought aggravation from the initial 24 until the more recent period was not detected. However, in southern locations, lower precipitation 25 thresholds of the SPI drought categories were generally found in the more recent period, 26 particularly for more severe drought categories, whereas in the northern locations Porto and 27Montalegre, an increase was detected. The impacts of the reference period on the computed SPI 28 drought severity and frequency are shown, bringing to discussion the need for updating ´normal´ 29 conditions when long term precipitation records are available and precipitation changes are 30 observed. 31

Estimating uncertainty associated with the standardized precipitation index

“…It eliminates dependence on a specific reference period and, in fact, can show how a particular reference period strays from the characteristics of the longer record (as shown in some examples here and in Paulo et al . (). Most significantly, our analysis identifies some general rules for stability in parameter estimation that produce consistent, robust confidence intervals that account for trends, changes in variability, and outliers.…”

“…Similarly, Paulo et al . () used a long‐term data set to document drought severity changes in Portugal and to evaluate sensitivity to reference period. Others have made adjustments to account for trends (Wang et al ., ) or non‐stationarity associated with future climate change scenarios (Russo et al ., ).…”

“…In essence, they are using differences in the precipitation probability distribution across time or space to document changes. Similarly, Paulo et al (2016) used a long-term data set to document drought severity changes in Portugal and to evaluate sensitivity to reference period. Others have made adjustments to account for trends or non-stationarity associated with future climate change scenarios (Russo et al, 2013).…”

A Stakeholder Oriented Modelling Framework for the Early Detection of Shortage in Water Supply Systems

“…To compute the SPI values, a gamma distribution has been adopted [52,53]. As such, the data requirement for SPI-Q model application is limited to the monthly time series of precipitation, representative of the analyzed watershed basin and contemporary monthly inflows to the reservoir.…”

Editorial: Water Resources Management in a Changing World: Challenges and Opportunities

“…For this purpose, they use the GumbelHougaard copula from the Archimedean family. Paulo et al 2016 present BInfluence of Precipitation Changes on the SPI and Related Drought Severity^, where they carry out a comprehensive analysis of the influence of long-term precipitation variability on drought assessment by the SPI. In the article BOptimization of Hedging Rules for Reservoir Operation During Droughts Based on Particle Swarm Optimization B Spiliotis et al 2016 propose a methodology for identifying optimal hedging rules for operating a reservoir system, with the objective of mitigating drought impacts.…”