For many, the word drought conjures images of parched ground, wilted trees and receding lake levels. Despite these easy associations, drought may be harder to define than many people think. This page will examine how we define and measure drought, its impacts and some mitigation strategies.
At its most basic, drought is a lack of precipitation over an extended period of time. However, what constitutes drought varies from region to region, depending on the local climate. Some areas naturally receive more rainfall than others, and other climatic factors such as temperature, humidity and wind speed play a role as well. Prolonged droughts are characterized by large-scale anomalies in atmospheric circulation patterns that persist for months, seasons or even longer. Meteorologists generally compare current rainfall patterns to historic averages to determine when a region is experiencing a drought and how severe it is.
Drought is also based on need. Drought occurs when water supply does not meet water demanded by people, animals and plant life. In addition, different groups of people view drought differently. Growers, for example, generally consider drought in terms of crop impacts, as soil moisture may indeed diminish quickly during periods of low rainfall.
The source of water used may also determine the severity of drought. For example, groundwater supplies may not be impacted as quickly as surface water supplies. On the other hand, groundwater may take longer to recover from a severe drought.
Drought is sometimes man-made. For centuries, we have altered our surroundings to protect ourselves from danger—storms, cold and even drought. By building dams and reservoirs, we can control flooding and provide a more reliable source of water for surrounding communities. However, sometimes increased use upstream can cause a deficit in supply for downstream stream neighbors.
Measuring and Monitoring Drought
Drought is measured in a variety of ways, usually with various drought indices. The Palmer Drought Index is one example of a widely used index that indicates prolonged and abnormal moisture deficiency or excess. The index is based on an equation that uses precipitation, temperature and soil moisture to calculate drought severity. The result is a number that indicates drought classifications. For instance, -3 to -3.99 is considered severe drought.
Scientists generally cannot predict drought more than a month in advance for most locations. Predicting drought depends on the ability to forecast two fundamental weather factors, precipitation and temperature. Anomalies in precipitation and temperature may last from several months to several decades, but how long drought lasts is dependent on a variety of complex climate factors, some local and others global.
The US Drought Monitor detects and tracks drought conditions and development on a weekly basis. Find more information on the Drought Monitor, provided by the National Drought Mitigation Center.
Four Stages of Drought
- Phase 1 – Watch 5 to 10 percent shortage (voluntary reductions)During this stage of drought, your city government is likely to explain the drought situation, forecast future actions and suggest voluntary conservation actions.
- Phase 2 – Warning 10 to 20 percent shortage (voluntary or mandatory reductions) Voluntary conservation actions initiated in Phase 1 are ongoing. The city may require that consumers reduce their water consumption by 10-20 percent at this stage.
- Phase 3 – Emergency 20 to 35 percent shortage (mandatory reductions) In order to provide water for everyone, your city government may institute rationing programs by issuing fixed allotments based on per capita or per household data, such as lot size, past usage, or other data. They may also require all homes to have low-flow showerheads and toilet displacement devices or ultra low flush toilets before granting an increased allotment. In addition, all municipal pools may be required to cover their pools to prevent evaporation.
- Phase 4 – Critical 35 to 50 percent shortage (mandatory reductions) All steps implemented in Phase 3 are likely to intensify, and local law enforcement will monitor for compliance. Outdoor water use may be cut completely.
Utilities enforce and promote water restrictions through rates, municipal ordinances, federal laws and regulations, and financial incentives. For example some municipalities:
- Offer financial incentives to install efficient plumbing or water efficient landscaping.
- Allow the public to water only on certain days by enforcing municipal ordinances.
- Ban fountains unless they run on recirculated water.
In contrast to drought management plans, some cities are embracing drought preparedness by putting programs and projects in place to minimize or avoid drought impacts. This has been a major endeavor at the national level.
Drought preparedness activities include ongoing conservation programs, water reuse applications, water storage and management projects, using both surface and groundwater, storing water in aquifers, and more.
El Niño-Southern Oscillation and other similar climate patterns affect global climate variability. These patterns tend to reoccur periodically with enough frequency and with similar characteristics to offer long-range climate prediction.
Global climate change may cause some areas to receive less rain while others receive more.In addition, climate change may impact the intensity of rainfall, so that more rain falls over a shorter period of time. Climate change could also impact drought by increasing temperature, which could increase demand and evaporation.