4.1 Relationship between precipitation and yield in fallow period in dryland wheat.
In the dryland wheat region of the Loess Plateau, precipitation indirectly affects the yield by affecting the formation of wheat yield components in the dryland (Liu et al., 2005). The impact of precipitation on wheat has a persistent and lagging effect. Soil water shortage in the key growth period of wheat is often caused by insufficient precipitation in the early stage (Cao et al., 2017; Shangguan et al., 2002).Studied that precipitation in fallow period determines soil moisture before sowing, and sufficient soil moisture can not only ensure good emergence of dryland wheat, but even supply water to the jointing stage of dryland wheat (Hao et al. (2003). The results showed that the panicle per unit area of wheat at maturity was 304×104, 410×104 and 459×104 ha− 1, and the grains per spike were 19.5, 24.4 and 24.8, respectively, in dry, normal and wet precipitation years. The 1000-grain weight was 43.3, 44.9 and 46.6 g, respectively, and the yield components in dry years were the lowest, but the dry years had the greatest influence on the number per spike unit area, followed by the number of grains per spike and 1000-grain weight (Cao et al., 2017). Study, such as in dry land wheat, except in abnormal years, dryland wheat water requirement will not be late to restrict, but leisure period precipitation volatile and unpredictable, leisure period precipitation by influencing the soil before planting effects of wheat emergence rate, group tillers spike, indirectly affect production. The results of this study showed that the dryland wheat region was in the precipitation climate zone with annual precipitation of 300–700 mm, and the average precipitation in the fallow period was 296 mm, and the coefficient of variation (CV) was 10%. Precipitation in leisure period accounted for 58% of annual precipitation, and CV = 6%. More than half of the precipitation period in dryland wheat fields is misplaced with the growth period, which is the main limiting factor for low and unstable yield of dryland wheat (Sun et al., 2018). Moreover, the impact of precipitation on wheat has a persistent and lagging effect, and precipitation in fallow period will have a persistent influence on the growth and development of dryland wheat (Cao et al., 2017). In this study, five published long-term positioning experiments of wheat fields in dryland of the Loess Plateau were selected, and the sample data of literature data n = 57 were extracted (Table 2). The relationship between annual precipitation, precipitation in fallow period and yield in dryland wheat field was analyzed. The results showed that with the gradual increase of precipitation in fallow period. The grain yield of wheat in dryland increased gradually (Y = 2526.5LNX-10148, R2 = 0.42 P < 0.05). The increase of annual precipitation, the grain yield of dryland wheat increased gradually (Y = 3824LNX-19816.5, R2 = 0.45, P < 0.01). In conclusion, the precipitation during fallow period, annual precipitation and grain yield in dryland wheat area showed a significant nonlinear positive correlation. In rain-fed agricultural areas, different studies also showed that precipitation in fallow period had a significant impact on wheat yield (Dai et al., 2015; He et al., 2016; Cao et al., 2017).
4.2 Annual classification of precipitation based on fallow period
There are many methods to classify precipitation year types. The National Crop climate year type classification Standard (GB/T 21986 − 2008) is based on the percentage of precipitation anomaly R% to classify precipitation year types. If R% is less than ± 15%, it is perennial type, + 15% to + 30% is more precipitation 15–30% is less precipitation. More than ± 30% of precipitation is more or more juvenile. This standard is not suitable for the actual production of dryland wheat in the Loess Plateau because of the large difference in annual precipitation between precipitation year types (Zhang et al., 2008).Proposed the classification method of precipitation year types based on the dryland wheat region of the Loess Plateau. Wet years: Pi > Pm + 0.33δ; Pm-0.33 δ ≤ Pi ≤ Pm + 0.33δ; Dry years: Pi < Pm-0.33 δ, where Pi is annual precipitation (mm); Pm is annual average precipitation (mm); δ is the mean square error (mm) of precipitation over many years. The above precipitation year classification method is based on the annual precipitation as the standard, and the input of nitrogen fertilizer is based on the annual precipitation, which obviously has no timeliness. Therefore, the annual classification method based on fallow precipitation is more suitable for rational fertilization management. In this study, based on the annual classification method of precipitation in fallow period, the precipitation in fallow period is less than 220 mm, the precipitation in fallow period is 220–440 mm, and the precipitation in fallow period is more than 440 mm, and the precipitation in fallow period is wet (Cao et al., 2017; Guo et al., 2012). Directly carried out linear regression between precipitation and yield in fallow period, and then predicted yield and calculated fertilizer application amount through regression relationship, this study established a classification method based on precipitation in fallow period after removing abnormal years, which was more time-efficient and reliable. It has practical reference significance for the fertilizer input of dryland wheat according to the precipitation strain in fallow period.