Average moisture values were derived from historical data on precipitation and potential evapotranspiration. A z-score is a statistical method for assessing how different a value is from the average (here, compared with a 1900-2017 baseline): the greater the z-score, the larger the departure from average conditions, indicating larger moisture deficits or surpluses. Thus, the most negative areas indicate a 1/3/5-year period with extremely dry conditions, relative to the average conditions.