What is drought?

Extreme weather such as heat wave, rainstorm and drought is expected to be accompanied by climate change. Recent research shows that this situation has begun to happen. Drought occurs in almost all types of climate. According to the National Climate Data Center, among all weather-related phenomena that may have a serious impact on the US economy, drought is second only to hurricanes. But unlike hurricanes, hurricanes can be easily identified and classified according to wind speed, and the definition of drought is much more difficult.

The definition of drought Most people think that drought is an unusually dry weather that lasts long enough to cause problems such as crop damage and water shortage. However, there is more than one definition of drought, because there are different reasons for the development of drought.

David Miskus, a drought expert and meteorologist at the Climate Prediction Center of the National Oceanic and Atmospheric Administration (NOAA), said: "Drought is not only due to insufficient precipitation and high temperature, but also due to overuse and overpopulation.

In the 1980s, two researchers found more than 150 open definitions of drought in the international journal of water resources. In order to bring some order to drought measurement, scientists divide these definitions into four basic categories: meteorology, hydrology, agriculture and social economy. The first three descriptions think that drought is a physical phenomenon. The last category regards drought as a supply and demand problem through the influence of water shortage.

These definitions usually explain the beginning, end and severity of drought by comparing the precipitation in a certain period with the historical average. Scientists include both rainfall and snowfall in the measurement of precipitation, because some areas in the United States, such as the western mountainous areas, rely on winter snowfall for most of their water resources every year.

Four main types of drought are described here:

Meteorological drought is aimed at different regions and depends on the annual average precipitation in the region. For example, according to the data of the United States Department of the Interior, the average annual precipitation in the southwest of the United States is less than 3 inches (7.6 cm), while the annual precipitation in the northwest exceeds 150 inches (38 1 cm). Compared with the historical average level in this area, the decrease of precipitation will be regarded as meteorological drought.

Agricultural drought explains the water demand of crops at different growth stages. For example, insufficient water during sowing may hinder germination, resulting in a decrease in plant number and yield.

Hydrological drought refers to the continuous lack of water in rivers and reservoirs. Human activities such as reservoir water level drop will aggravate hydrological drought. Hydrological drought is often associated with meteorological drought,

When the demand for water exceeds the supply, socio-economic drought will occur. Examples of this drought include over-irrigation or low river flow, forcing hydropower station operators to reduce energy production.

Tracking American drought "KDSP", Palmer Drought Severity Index (PDSI, weekly index displayed by CPC) designed by 1965 is the first comprehensive drought index. It is considered to be the most effective way to irrigate farmland. PDSI synthesizes the data of temperature, precipitation, evaporation, transpiration, soil runoff and soil recharge in a given area, and produces a negative number representing drought conditions. The index is used as an estimate of soil water shortage, which is roughly related to the severity of drought. PDSI is the most commonly used index in drought monitoring and research. In the past, it was widely used in tree ring reconstruction in North America and other regions.

This article was updated by Alina Bradford, a writer of Life Science, on September 28th, 20 18.