How DroughtOdds counts a drought year
The weekly map
Every Thursday the U.S. Drought Monitor publishes a map that sorts the country into D0 (abnormally dry) through D4 (exceptional drought). Its county statistics give, for every county, the share of land area in each category. DroughtOdds uses 1,401 of those weekly maps, from January 2000 on.
Rule one: a drought week
A week counts as drought when D1 (moderate drought) or worse over at least 50% of the county's area that week. D0 is left out on purpose: it is the "watch this" stage that most of the country passes through most years, and counting it would put almost every county in drought. Half the area is the line because a lawn sits in one place; a county where a sliver of the far corner is in D1 is not a county in drought.
Rule two: a drought year
A full calendar year counts as a drought year with 13 or more drought weeks, a quarter of the year, about one growing season. A two-week dry spell browns a lawn and then it recovers; a season of it is what kills turf and empties shallow wells. Only years with at least 50 weekly maps count, so the year in progress never skews the figure.
Why summer timing matters
A drought in January costs a lawn little; one in July costs a lot. So a county that sits in the middle on yearly count but has its droughts bunched into May to September is advised one step drier. That applies to 72 counties, among them Lamar County, GA, Bibb County, GA and Cedar County, NE. The drought-tolerant grass guide covers what that step means.
What it cannot see
Some counties are big enough to hold a desert and a mountain. One figure covers both. Irrigation districts, groundwater and local water rules are outside the data too. For the years that hit most of the country at once, see the worst drought years since 2000.