Drought - WorldWeatherAttribution/wwa-wiki GitHub Wiki

Notes on drought analysis

• Start by considering both temperature/PET and precipitation, rather than jumping straight to SPI/SPEI - it's easier to communicate changes in precipitation than changes in SPI/SPEI

• Remember to compare the magnitudes of the PET and precipitation and interpret what changes mean in terms of the effective precipitation

• Precipitation is assumed to increase exponentially with GMST, while temperature, effective precipitation, SPI and SPEI all shift linearly

• However, SPI and SPEI classifications are still useful tools for communicating the severity of droughts so these should be included in the final report: eg. "the 10% decrease in precipitation caused what would have been an extreme drought (category D3) to become exceptional (category D4)"

• Different methods for calculating PET should always be compared to determine the most appropriate method (unless we've already done this for the area being studied) - ERA5 provides Penman-Monteith PEV, which should always be treated as the gold standard

• If only one dataset provides all the variables required to compute the SPEI, consider including the different PET estimates in the analysis as extra 'observations' - then the observation 'representation error' (white bars) will characterise the uncertainty arising from the choice of method

• Independent time series of SPEI are useful to validate our results and help to choose the most appropriate PET method