Drought is summarized using the Cumulative Drought Severity Index (CDSI). This uses weighted frequencies of the self-calibrated Palmer Drought Severity Index (sc-PDSI) reported by the WestWide Drought Tracker (direct data download). For each month, drought is categorized as 'moderate' (sc-PSDI values from -2 to -3), 'severe' (-3 to -4), or 'extreme' (less than -4), and given scores of 1, 2, and 3, respectively. These weighted monthly values are then added together over the entire time period, such that an area that had experienced many extreme droughts over the 30 years receives a higher score than areas that experienced less frequent or less extreme droughts.