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U.S. Drought Monitor Statistics Explanation

Ways of computing Drought Monitor statistics

Statistics are calculated weekly for the U.S. Drought Monitor by two different methods that we refer to as “cumulative,” which is how we did it from the beginning, or “categorical,” meaning we report on one category at a time. Below is an explanation of each method.

  Absolute Values Cumulative Statistics Categorical Statistics
Area, Absolute Sq miles in drought Reports areas from D0 through D0-4 Reports areas one drought category at a time
Area, Proportional % of area Reports areas from D0 through D0-4 Reports areas one drought category at a time
Population, Absolute People in drought Reports population from D0 through D0-4 Reports population one drought category at a time
Population, Proportional % of people (in area) Reports population from D0 through D0-4 Reports population one drought category at a time

 

Cumulative Statistics

The cumulative U.S. Drought Monitor statistics combine drought categories for a comprehensive percent of area in drought. For example, the D0-D4 category shows the percent of the area that is classified as D0 or worse.

Someone using or reporting on this type of information might say that on May 26, 2015, 90 percent of Utah was in moderate drought or worse (D1-4) and 34 percent of the state was in severe drought or worse (D2-4). Various forms of drought response and assistance are often pegged to drought that is at or above a certain level.

Note that these proportions do not automatically add up to 100 percent.

Categorical Statistics

The categorical U.S. Drought Monitor statistic is the percent of the area in a certain drought category, and excludes areas that are better or worse. For example, the D0 category is labeled as such and only shows the percent of the area experiencing abnormally dry conditions.

Someone using categorical statistics might say that as of May 26, 2015, 9.32 percent of Utah was in extreme (D3) drought, 25.07 percent was in severe (D2) drought, 56.24 percent was in moderate (D1) drought, and 9.37 percent of the state was abnormally dry (D0).

Note that these statistics will add up to 100 percent for a given week.

Regular calculations

 

Up to four types of statistics are calculated for the drought monitor depending on the area in question.

  • PERCENT AREA STATISTICS provide the percent of the chosen area in each USDM category.
  • TOTAL AREA STATISTICS show the square miles of an area in each USDM category.
  • PERCENT POPULATION STATISTICS provide the proportion of the population affected in each USDM category.
  • TOTAL POPULATION STATISTICS provide the number of people affected in each USDM category.

Get more information about how the population in drought numbers are calculated.