The University of Arizona

Drought Monitoring | CLIMAS

Drought Monitoring

Prolonged dry conditions are a natural part of life in the Southwest. Past records from tree-rings indicate that the Southwest has experienced protracted “mega” droughts that lasted about 50 years, a duration unlike the 1950s and 2000s droughts that caused economic losses to agriculture, ranching, industry and many other sectors. Because of recurring drought, many efforts have been made to minimize its impacts on human activity. The Colorado River, for example, stores about four years of water supply to buffer the affect of several consecutive years of below-average streamflows. However, some sectors are more vulnerable to drought than others, and drought can appear rapidly after just one season of below-average precipitation. Summer grasses, for example, that provides fodder for livestock do not grow in abundance or with as much health if the monsoon rains fizzle.

It is critical to monitor drought to help local governments, resource managers, and many other groups make effective decisions. Several products produced at the national and regional level help inform people living in the region about current and future conditions.

U.S. Drought Monitor

Perhaps the most widely used drought status assessment is produced by the U.S. Drought Monitor (USDM; Figure 1). The USDM is a weekly synthesis of many types of climate information, including precipitation, snow, temperature, impact reports, and expert assessments made by scientists. The USDM focuses on broad-scale conditions which may differ from local conditions.

For each state and region, the USDM tabulates the percent area affected by different drought statuses and compares the current conditions to previous weeks, months, and specific dates. For Arizona, the Arizona Drought Monitoring Technical Committee helps define the drought classification in the state. The USDM is housed on the National Integrated Drought Information System (NIDIS) Web site.

NCDC State of the Climate Drought

The National Climatic Data Center (NCDC) provides a detailed discussion on drought conditions in different regions of the U.S. The State of the Climate–Drought summary is published monthly and corresponds to the conditions of the previous month.

The State of the Climate–Drought summarizes many different drought monitors and indices, including the U.S. Drought Monitor, Palmer Drought Severity Index (PDSI), precipitation anomalies, and soil moisture conditions, among other variables.

Arizona Long-term Drought Status Reporting

Monitoring drought status is the backbone of Arizona’s Department of Water Resources (ADWR) Drought Preparedness Plan. ADWR produces a quarterly long-term drought status map and summary based on weekly input from Arizona Drought Monitoring committee (Figure 2). ADWR also generates a monthly commentary that summarizes the four drought status maps issued each month by the US Drought Monitor. Drought statuses are determined at the watershed level.

ADWR’s calculates long-term drought status by comparing the 24-, 36- and 48-month precipitation and streamflow totals to all these same periods in the previous 30-year record. Therefore, only gages with a 30-year record of data are used. If precipitation and streamflow are less than the 40th percentile, meaning 40 percent of the time values for the 24-, 36-, and 48-month period in the last 30 years are equal to or less than the current period value, than conditions are classified as abnormally dry or worse. This method has been applied to other regions with success.

New Mexico Drought Status Report

In New Mexico, a 12-member Drought Task Force recommends strategies for reducing the state's vulnerability to drought. The task force is chaired by the State Engineer and includes experts in financing, water project construction, water rights, water conservation, and water quality as well as officials who understand drought's impact on agriculture, wildlife, economic development, tourism, and wildfire.

The drought status report is issued monthly and includes assessment of snow conditions, soil moisture, agricultural crop conditions, and precipitation. The report is housed at the New Mexican Office of the State Engineer.

Arizona Droughtwatch

Az DroughtWatch amasses, summarizes, and displays timely observations of drought impacts around Arizona. The tool was developed to support drought monitoring efforts spearheaded by county-level Local Drought Impact Groups (LDIGs) organized in support of the Arizona State Drought Preparedness Plan. Observations of local drought impacts are invaluable in properly monitoring and characterizing drought across Arizona's complex landscape. Information collected through AZ DroughtWatch aids ADWR’s drought status reports, the U.S. Drought Monitor, the preparation of community-level drought response plans, and longer-term risk assessments and mitigation plans.


Figure 1. The U.S. Drought Monitor describes broad-scale drought conditions and is published weekly

Figure 2. Arizona Department of Water Resources published long-term drought status reports every four months.