What Is Drought?
Drought is a normal and recurring feature of climate. Although it occurs in virtually all of the world’s climatic zones, its characteristics vary significantly from one region to another. In some of the world’s most arid regions, a drought occurs when annual precipitation drops below 7 inches per year, while in the world’s most moisture-rich regions, a few weeks without rain might constitute a drought! Consequently, there is no universal definition of drought. In the most general sense, drought is a result of a deficiency of precipitation over an extended period of time, resulting in a water shortage, which impacts normal water usage. The severity of a drought depends upon the degree of moisture deficiency, its duration, and the size of the affected area. Because it is so hard to develop a quantitative definition for drought, it is difficult to determine precisely when a drought starts and ends.
In the United States, droughts are among the most financially burdensome of all weather-related disasters. In fact, in the 20 years preceding hurricane Katrina, the single largest U.S. weather-related disaster was the drought of 1988, which resulted in over $40 billion in damages throughout the central and northeastern portions of the country. Unlike impacts from flood, hurricane, tornado or other weather-related disasters, drought impacts are not always immediate. Failed crops can impact food prices well into the future. Devastated domestic livestock and wildlife herds can also take many years to recover.
Drought should not be viewed as merely a natural phenomenon or event. Its impact on society is often a result of the interplay between the natural event (reduced precipitation) and the way society responds with management of existing supplies. People often compound the impacts of drought through the mismanagement of available supplies. One example of this occurs when outdoor watering restrictions imposed by community leadership are actually perceived by the general public as being premature or unnecessary. Often in these instances, while people comply with the letter of the law, water use actually rises. Consequently, an informed and caring general public is an important ingredient to successful drought management.
Utah’s successful management of drought will in large part be a function of leadership providing the necessary information to the public, and people in turn, responding positively. Utah’s current Drought Response plan was written during the drought of the late ’80s and early ’90s. The plan uses the Surface Water Supply Index (SWSI) to describes five drought conditions. The SWSI is a relative scale with 0 representing average conditions. A positive number indicates that water supplies are above normal, while a negative number indicates that water supplies are below normal.
The first condition, Normal to wet, is really a non-drought condition and the plan merely calls for the periodic updating of SWSI data and maps. When the SWSI moves into the 0 to -1.0 range, conditions are still considered normal, with water supplies only slightly below average. Although this is not a true drought condition it is called “Emerging Drought” in the plan, primarily because of the potential for worsening conditions. During this phase, the plan merely calls for increased monitoring of data. When the SWSI moves below -1.0, the condition is a Phase I Drought. At this point, the state’s Water Supply and Availability Committee is activated and begins to closely monitor water supply data and initiates the dissemination of information to the media and general public. When the SWSI moves below -2.0, it is a Phase II drought condition. At this time, a Drought Review and Reporting Committee and Drought Task force are created with members of various state agencies to coordinate drought response actions and facilitate the timely dissemination of data and information. The Governor’s Proclamation of Drought Emergency, rather than a SWSI number, triggers drought Phase III. The Governor’s declaration of drought emergency is the initial and necessary step to make available many of the state and federal drought assistance programs.
The US Drought Monitor is a broad-scale national drought map. It is updated weekly and communicates any unusually dry conditions with 4 drought categories (moderate, severe, extreme, and exceptional drought) and an “abnormally dry” category, indicating areas that may be moving into or out of drought. These categories correspond to percentiles of different data (exceptional 0-2, extreme 3-5, severe 5-10, moderate 11-20, abnormal dryness 21-30), with the lower percentile numbers meaning drier conditions.
Different data that account for precipitation over multiple time scales, streamflow, groundwater, soil moisture, and other meteorological and hydrological variables, are placed into percentiles for different locations. Often, the different data will not all indicate the same drought category for a particular location. The author of that week’s map looks at all of this information and assign a category to an area based on what percentile range the different indicators and indices converge on.
Since the US Drought Monitor map is intended to show the entire nation, local conditions may vary from the categories shown on the map. Here in Utah, we rely upon snowmelt runoff stored in reservoirs to get through the hot summer irrigation season. If enough snow falls in the mountains during the winter months, dry summer weather conditions can be somewhat overcome in many areas of the state. While the US Drought Monitor is a useful tool with many applications, the Surface Water Supply Index (SWSI) is used to monitor drought status. The SWSI compares gauged steam flow and current reservoir storage with average stream flow and reservoir storage conditions. This comparison provides us with a more accurate assessment of our ability to deal with the drought situation indicated by the US Drought Monitor map.