Drought and Moisture Surplus for the Conterminous United States, Annual Data 3-Year Windows (Image Service)

The Moisture Deficit and Surplus map uses moisture difference z-score datasets developed by scientists Frank Koch, John Coulston, and William Smith of the Forest Service Southern Research Station. A z-score is a statistical method for assessing how different a value is from the mean (average). Mean moisture values were derived from historical data on precipitation and potential evapotranspiration, from 1900 to 2023. The greater the z-value, the larger the departure from average conditions, indicating larger moisture deficits or surpluses. Thus, the dark red areas on this map indicate a three-year period with extremely dry conditions, relative to the average conditions over the past century. For further reading on the methodology used to build these maps, see the publication here: https://www.fs.usda.gov/treesearch/pubs/43361