It is useful to evaluate drought status over varying periods of time in order to predict different types of drought impacts. For example, short-term deficits in precipitation may cause impacts to vegetation, while longer-term deficits will impact water resources (e.g. groundwater levels and streamflow). The Standardized Precipitation Index (SPI), used to create the short-term drought status map above, makes it possible to conduct this type of evaluation.

The SPI was designed to evaluate precipitation amounts at multiple “timescales.” For example, during the past three months, the SPI may indicate “extreme” drought status, whereas the SPI for the past two years may indicate a “moderate” drought status, or even no drought at all. Both are valid and current drought statuses, but differ by the period of time considered.

For any given precipitation total and any given time period, the SPI gives an instant appraisal of how “rare” that precipitation value is. An SPI value of -1 for the time period February - April, for example, indicates that only 16% of all precipitation totals for the period February – April over the last 30 years were lower than the current total (a moderate drought status).

Arizona’s monthly short-term drought status map, shown above, incorporates 3-month, 6-month, and 12-month average SPI values from multiple weather stations in each of Arizona’s major watersheds. To arrive at these SPI values, precipitation levels for each period (3, 6 and 12 months) are compared to the 30-year historical record to determine the drought status for each watershed – normal, abnormally dry, moderate, severe or extreme.

Arizona’s Monitoring Technical Committee, established by the Governor’s Drought Task Force, produces the monthly drought status maps. These maps are intended to serve as a planning tool for resource managers around the state, as well as an information resource for the general public. The maps do not reflect an officially declared drought status. Click here to access the full-length Drought Monitor Reports produced by the Monitoring Technical Committee. Each of these reports also contains a long-term drought status map.