Assessment of drought occurrence in Bei-bu wheat districts, China between 1960 and 2012

Bei-bu winter wheat region (BBWR) is located in the northern boundary of the winter wheat area in China. The ecological environment is relatively poor. Drought is a major natural hazard that harms winter wheat growth in this region. Based on the daily rainfall data recorded at 45 national meteorological stations within the BBWR between 1960 and 2012, we calculated the Standardized Precipitation Index (SPI), a measure of drought. We also addressed changes of drought occurrence across the BBWR using drought intensity, drought frequency and drought coverage. Results show that: (1) there were three drought periods, which occurred around 1966, 1973, and 2000 respectively. (2) From 1960 to 2012, whole drought or regional drought appeared in most years, except 1964, 1969, 1990 and 2003. In addition, drought index presented a slightly upward trend until 1999, when it started to present a downward trend. (3) Mild drought occurred more frequently than moderate drought and severe drought. In BBWR, mild drought occurred between 31.1% and 37.8%, while both moderate drought and severe drought occurred less than 13%. The frequency of occurrence of moderate drought and mild drought was both higher in the Jing-jin region, northwest of BBWR. In addition, the most serious severe drought appeared in Shanxi province.