Drought refers to the water shortage caused by the unbalanced
supply and demand of water. The occurrence of drought is
related to many factors, such as precipitation, evaporation,
temperature, soil moisture, irrigation conditions, planting
structure, drought resistance during crop growth period and
water use in industry and urban and rural areas, therefore,
the definition of drought varies in different disciplines and
fields, such as meteorological drought, hydrological drought,
agricultural drought and so on.
The degree of precipitation shortage and soil moisture status
are usually used as drought indicators, such as precipitation,
continuous rainless days, percentage of precipitation anomaly,
soil relative humidity, in addition, factors other than precipitation,
such as temperature, evaporation and other comprehensive drought index,
are also considered, for example, Z index, Palmer drought index, etc.
According to the degree of drought, it can be divided into four types,
as shown in Table 6-1.
Table 6-1 Number of consecutive no-rainfall days with different drought degrees (days)