Woodland Restoration Plot Network: Climatic Drought Index Data, Western Sydney Parklands (Western Sydney Regional Park), Australia, 1887-2014

Calculation of Hutchinson’s Drought Indices The Woodland Restoration Plot Network Drought data package was created from three different data sources: 1) the Bureau of Meteorology Station data (referred to as the bom_stn source in the data package), 2) the Australian Water Availability Project’s gridded climate data (labelled as awap in the data package) and 3) the TERN EMAST gridded climate data (labelled emast). The project conducted a comparative analysis of the different climatic data sources, which differ in terms of their temporal extent and spatial resolution. The Drought indices are calculated from the monthly rainfall totals. First these are integrated to rolling 6-monthly totals which are then ranked into percentiles by month and this is rescaled to range between -4 and +4 in keeping with the range of the famous Palmer Index. Mild drought is below -1 in the Palmer index. In the first index the duration of the drought is calculated. To do this consecutive months below the -1 threshold are counted. This is labelled duration_bom_stn when using the Bureau of Meteorology station data; duration_awap when using Australian Water Availability Project data; and duration_emast when using EMAST grid data. In the original working paper the authors calibrated their indicator to Government Drought Declarations. It was found that after 5 consecutive months a drought was often declared and so this threshold was defined as the beginning of a predicted drought declaration. This is termed the duration_xxx_declared, where xxx refers to the different data source being used to calculate the duration. This variable is set to TRUE when it is a predicted drought and FALSE otherwise. In the original method this cumulative counting step is continued until the rescaled percentiles exceed -1 again. The enhanced method implemented by Hutchinson and Hanigan imposes a more conservative threshold of zero (the median) to break a drought. It is this more stringent drought breaking criterion that is used in the Woodland Restoration Plot Network’s Drought data package. The second drought index uses a method devised by Hutchinson to calculate the severity of a drought. In this method the rescaled percentile values are integrated using conditional cumulative summation when they fall below the threshold of mild drought. This is labelled the severity_xxx variable in the data package. In the original calibration study the value of -17 was found to be the best predictor of drought declarations, and months below this value are cumulatively summed until the index value exceeds the median again (following Hutchinson and Hanigan’s enhanced implementation). This is labelled the severity_xxx_duration variable. The original 1992 working paper provides a technical description of the methods used for the original indices: Smith, D. I, Hutchinson, M. F, & McArthur, R. J. (1992) Climatic and Agricultural Drought: Payments and Policy. (Centre for Resource and Environmental Studies, Australian National University, Canberra, Australia). The computer software written by Hanigan to implement the enhanced algorithm is available at: https://github.com/ivanhanigan/HutchinsonDroughtIndex and fully described in the paper: Hanigan, I.C, Butler, C.D, Kokic, P.N & Hutchinson, M.F. (2012). Suicide and drought in New South Wales, Australia, 1970-2007. Proceedings of the National Academy of Sciences of the United States of America, Volume: 109, Issue: 35 (2012) doi: 10.1073/pnas.1112965109 Therefore in summary the variables are defined as: • duration_bom_stn = Station Data: Consecutive months where rescaled percentiles are below the -1 threshold are counted. The count is reset when the rescaled percentiles are greater than 0. • severity_bom_stn = Station Data: Consecutive months where rescaled percentiles are below the -1 threshold are summed The summation is reset when the rescaled percentiles are greater than 0. • duration_bom_stn_declared = Station Data: After 5 consecutive months of accumulation the beginning of a predicted drought is declared. • severity_bom_stn_declared = Station Data: After the accumulated total of index values reaches -17 the beginning of a predicted drought is declared. • duration_awap = AWAP Data: Consecutive months where rescaled percentiles are below the -1 threshold are counted. The count is reset when the rescaled percentiles are greater than 0. • severity_awap = AWAP Data: Consecutive months where rescaled percentiles are below the -1 threshold are summed The summation is reset when the rescaled percentiles are greater than 0. • duration_awap_declared = AWAP Data: After 5 consecutive months of accumulation the beginning of a predicted drought is declared. • severity_awap_declared = AWAP Data: After the accumulated total of index values reaches -17 the beginning of a predicted drought is declared. • duration_emast = EMAST Data: Consecutive months where rescaled percentiles are below the -1 threshold are counted. The count is reset when the rescaled percentiles are greater than 0. • severity_emast = EMAST Data: Consecutive months where rescaled percentiles are below the -1 threshold are summed The summation is reset when the rescaled percentiles are greater than 0. • duration_emast_declared = EMAST Data: After 5 consecutive months of accumulation the beginning of a predicted drought is declared. • severity_emast_declared = EMAST Data: After the accumulated total of index values reaches -17 the beginning of a predicted drought is declared.