Reading the U.S. Drought Monitor

What it is

The U.S. Drought Monitor (USDM) has been produced every week since 1999 by the National Drought Mitigation Center at the University of Nebraska–Lincoln together with the U.S. Department of Agriculture and the National Oceanic and Atmospheric Administration. Each week a lead author — the role rotates among climatologists and meteorologists at those agencies — draws the map from many indicators and from reports by local experts across the country.

The schedule

New maps are released on Thursday mornings. Each one reflects data through 8 a.m. Eastern time on the Tuesday before. So a map released on a Thursday is “valid” for that Tuesday, and anything that happened after Tuesday morning — a big storm, say — shows up the following week. BasinMind labels drought figures with that valid date.

The categories

The percentiles describe how unusual conditions are compared with the historical record: D4 conditions are expected about two years in a hundred or less. D0 is not drought; it marks areas going into or coming out of drought. The map also labels drought as short-term (S), long-term (L) or both, depending on which impacts dominate.

How the map is drawn

The USDM uses what its authors call a convergence of evidence: precipitation, temperature, soil moisture, streamflow, snowpack, vegetation health and other indicators, combined with reports from people on the ground about drought impacts. No single index decides a category. That makes the map broadly reliable, but it also means boundaries are drawn by judgment and are coarse at the scale of a single ranch or valley.

What it is — and is not — used for

• It is not a forecast. The map looks back at recent conditions. Outlooks come from NOAA's Climate Prediction Center.
• It drives federal programs. USDA uses the Drought Monitor to help trigger disaster designations and emergency loans, and the Farm Service Agency uses it for eligibility in the Livestock Forage Disaster Program. The IRS also uses it for tax deferral on forced livestock sales due to drought.
• It is not water administration. A D4 county can have senior water rights in priority, and a county with no drought can have calls on its streams. Calls depend on streamflow and priority; see What is a water call?

Drought in Colorado

Colorado's water supply depends heavily on mountain snowpack, so a dry winter can lead to drought in the following summer even if spring rains are normal, and a big snow year can ease long-term drought quickly. The USDM is one input to Colorado's own drought planning, alongside snowpack, reservoir storage and streamflow forecasts. Drought.gov collects Colorado conditions and outlooks in one place.

What to watch alongside the map

For water users, the Drought Monitor is a summary, not the whole picture. In Colorado it helps to read it with four other signals, all on BasinMind: snowpack in winter and spring, which sets the coming runoff; reservoir storage, which carries water from wet years into dry ones; streamflow compared with the median for the date; and the calls on your stream, which show how far down the priority list the shortage has reached. A county can move between categories in a single week after a storm or a heat wave, while long-term deficits in soil moisture and groundwater recover slowly.

How BasinMind reports drought

BasinMind reads the USDM's county statistics for the counties in each water division and reports the most severe category present and the county where it occurs, with the map's valid date. The live box above shows that for the Arkansas basin (Division 2). The full map and county detail are always on the Drought Monitor's own site.