Drought is a natural phenomenon during which regions or communities experience shifts in the balance between precipitation and evapotranspiration (the processes of evaporation and transpiration)—a balance that is inherent to the earth’s water cycle (see the Understanding Natural Cycles in Water Distribution section). Several factors affect the impact of drought on humans and other life forms, including the timing of precipitation events, effectiveness of the rain that is falling (i.e., rainfall intensity and the number of rain events), characteristics of the built environment in the affected area, and local demand for water. Individual areas or communities can be affected differently by drought depending on several additional variables, including:
- the structure and capacity of existing water systems,
- economic development,
- the at-risk populations living within the affected area,
- local governance of water use, and
- other societal factors, such as the presence of local social networks.
Because the conditions that signify drought can vary substantially by U.S. region and locality, drought should ultimately be defined based on the context and location in which the water shortage is occurring.
Although drought most commonly is defined climatologically, drought can also be exacerbated by human activities. For example, even when precipitation is occurring at average rates within a specific area, urban expansion and development without regard to existing water supply and water system capacity can trigger a human-induced drought. Drought can occur anywhere in the world, and it is considered a transient environmental hazard except in arid geographical regions that historically receive very limited amounts of rainfall. In addition, because of the substantial amount of time that elapses between the warning signs of drought and any measurable negative consequences to human and environmental health, drought should be considered a chronic or “low rise” natural event rather than an acute emergency for public health preparedness and response purposes. Drought is unlike other natural emergencies such as hurricanes, floods, or earthquakes; drought-related conditions can take years to escalate to the point at which water supply becomes severely limited, and the length of time that drought conditions may persist and impact communities is unknown.
Water Recycling and Reuse
Because freshwater is only a minimal percentage of the total global water supply and water treatment and distribution are costly, water should be considered a scarce and valuable resource. Many groups are advocating new approaches to water use and distribution. For example, the U.S. Green Building Council has proposed that traditional water distribution systems could be modified to limit the distribution of potable water and that parallel systems be developed to enable the collection and redistribution of gray water. Some municipalities have installed distribution systems that encourage households and commercial operations to use recycled water in lieu of treated freshwater for specific applications (e.g., irrigation), thus conserving the freshwater that is available within their watersheds. However, this is an expensive option because it requires the establishment of separate piping systems for the recycled water. Alternative methods for using rainwater also are being developed. For example, buildings are increasingly being engineered with the capability to collect and use rainwater for non-potable applications (such as for flushing toilets and landscape irrigation).