Mechanisms of Drought Occurrence in Rain‐Fed Winter Wheat (<scp><i>Triticum aestivum</i></scp> L.) Used the Standardised Precipitation Evapotranspiration Index (<scp><i>SPEI</i>_w</scp>) With Crop Coefficients in Arid Northwest China

ABSTRACT Drought is a significant meteorological disaster that affects winter wheat production in the arid region of northwest China. In order to understand the mechanisms and factors associated with rain‐fed winter wheat drought in the arid region of northwest China, the monthly meteorological data from 1987 to 2016 was employed to analyse the characteristics of both meteorological drought and the Standardised Precipitation Evapotranspiration Index ( SPEI w ) incorporating crop coefficients, respectively. In addition, we calculated the propagation time and probability of winter wheat drought caused by meteorological drought in different growth stages. The main factors that influenced different types of winter wheat drought were also clarified. The main results were as follows: The frequency, duration, severity and areas affected were greater for rain‐fed winter wheat drought than meteorological drought in each growing stage. The propagation time from meteorological drought to rain‐fed winter wheat drought was 1 month in the initial, developmental and late stages, and 2 months in the middle stage. The probability of propagating from meteorological drought to rain‐fed winter wheat drought was higher when the degree of meteorological drought was higher, and winter wheat drought was more likely to be severe. When the degree of drought was greater in rain‐fed winter wheat during different growing stages, a smaller SPI threshold was required to trigger it. Rain‐fed winter wheat drought induced by non‐meteorological drought was influenced mainly by the relative humidity, net surface radiation and sunshine hours on a short time scale (1 month), whereas winter wheat drought induced by meteorological drought was mainly affected by various meteorological factors over a longer time scale.