Climate trends and behaviour of drought indices based on precipitation and evapotranspiration in Portugal

L S Pereira,R D Rosa,A A Paulo

doi:10.5194/nhess-12-1481-2012

Abstract

Abstract. Distinction between drought and aridity is crucial to understand water scarcity processes. Drought indices are used for drought identification and drought severity characterisation. The Standardised Precipitation Index (SPI) and the Palmer Drought Severity Index (PDSI) are the most known drought indices. In this study, they are compared with the modified PDSI for Mediterranean conditions (MedPDSI) and the Standardised Precipitation Evapotranspiration Index (SPEI). MedPDSI results from the soil water balance of an olive crop, thus real evapotranspiration is considered, while SPEI uses potential (climatic) evapotranspiration. Similarly to the SPI, SPEI can be computed at various time scales. Aiming at understanding possible impacts of climate change, prior to compare the drought indices, a trend analysis relative to precipitation and temperature in 27 weather stations of Portugal was performed for the period 1941 to 2006. A trend for temperature increase was observed for some weather stations and trends for decreasing precipitation in March and increasing in October were also observed for some locations. Comparisons of the SPI and SPEI at 9- and 12-month time scales, the PDSI and MedPDSI were performed for the same stations and period. SPI and SPEI produce similar results for the same time scales concerning drought occurrence and severity. PDSI and MedPDSI correlate well between them and the same happened for SPI and SPEI. PDSI and MedPDSI identify more severe droughts than SPI or SPEI and identify drought occurrence earlier than these indices. This behaviour is likely to be related with the fact that a water balance is performed with PDSI and MedPDSI, which better approaches the supply-demand balance.

Highlights

Drought is a natural feature of climate and occurs in almost all climatic regions with varying frequency, severity and duration (Wilhite, 1993)
This study aims at assessing the behaviour of the Standardised Precipitation Index (SPI), Standardised Precipitation Evapotranspiration Index (SPEI), Palmer Drought Severity Index (PDSI) and MedPDSI indices throughout Portugal and comparing them using data from several weather stations located in regions with different aridity
To better understand the behaviour of the SPI, SPEI, PDSI, and MedPDSI indices throughout Portugal, and sprecifically to verify if drought indices using potential evapotranspiration (PET) instead of real ET could be adequate to foresee impacts of climate change, a trend analysis for monthly precipitation and temperature was performed for all weather stations in the period 1941–2006

Summary

Introduction

Drought is a natural feature of climate and occurs in almost all climatic regions with varying frequency, severity and duration (Wilhite, 1993) It can be defined as a temporary imbalance of water availability consisting of a persistent lower than average precipitation of uncertain frequency, duration and severity, of unpredictable or extremely hard to predict occurrence, resulting in diminished water resources availability (Pereira et al, 2009). Drought is both a hazard and a disaster; a hazard because it is an accident of unpredictable occurrence, part of the naturally variable climate system; disaster because it corresponds to the failure of the precipitation regime, causing the disruption of the water supply to the natural and agricultural ecosystems as well as to other human activities. High temperatures and high evapotranspiration rates can aggravate drought effects

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