Analysis and assessment of heat resource for winter wheat northward moving during 1961-2001 in Shaanxi Province

Global warming has caused northward moving of crop growth boundary. To determine the suitability of winter wheat in the northern region of Shaanxi Province, daily temperature data for 1961-2001 were collected from 5 stations (Yulin, Suide, Hengshan,Wuqi and Yan'an stations) in northern Shaanxi Province. The heat resource used in the study included the ≥0 ℃ accumulated temperature before winter, negative accumulated temperature during wintering, mean temperature in January, ≥0 ℃ accumulated temperature for the whole growth period and extreme low temperature. The results showed that ≥0 ℃ accumulated temperature before winter exceeded 400 ℃·d in all the stations. Only Yan'an station met the required minimum temperature of 450 ℃·d for negative accumulated temperature during winter. As temperature increased from year to year, negative accumulated temperature during winter in Suide and Wuqi stations in 1993 to 1994 reached threshold value of 450 ℃·d. For ≥0 ℃ accumulated temperature throughout the growth period, all the stations met the required 2 000 ℃·d. The only exception was Wuqi station, where ≥0 ℃ accumulated temperature was slightly below the threshold value. In terms of mean temperature of January (the coldest month of the year), frozen injuries were no longer a future in Yan'an station as mean temperature exceeded 6 ℃. In fact, mean temperatures for Suide and Wuqi stations were respectively 7.60 ℃ and 7.07 ℃. This suggested winter wheat survival rate of over 80%. In Yulin and Hengshan stations, however, mean temperatures of January were too low for wheat to grow. While the extreme low temperature value for winter wheat growth is 20 ℃, that for all the stations exceeded 17 ℃. This suggested that extreme low temperatures in all the stations were suitable for winter wheat growth. In northern Shaanxi Province, negative accumulated temperatures during winter period, mean temperature of January and ≥0 ℃ accumulated temperature for the whole growth period increased significantly throughout the investigated period. Negative accumulated temperature during wintering increased at the rate of 36.2~71.7 ℃·d·10a^-1. For ≥0 ℃ accumulated temperature during the growth period, it was 44.1~88.7 ℃·d·10a^-1 and 0.32~0.61 ℃·d·10a^-1 for extreme low temperature. Accumulated temperature before winter increased significantly by 20.3 ℃·d and 16.1 ℃·d per decade respectively for Yan'an and Wuqi stations. It, however, showed no increase for Yulin and Suide stations. Extreme low temperature increased across the entire Northern Shaanxi Province, though not significantly. In conclusion, heat resource in Yan'an was already high enough by 2001 to support the growth of winter wheat. Winter wheat growth was also suitable in Wuqi and Suide stations, but with some extreme low temperatures in January. In Yulin and Hengshan stations, however, negative accumulated temperatures during wintering period and mean temperature in January were unfit for winter wheat growth.