# Winter Wheat Mapping in Shandong Province of China with Multi-Temporal Sentinel-2 Images
## 4 Discussion
### Factors Influencing the Distribution of Winter Wheat  
The spatial distribution of crops is affected by various elements, such as topographic relief, temperature and precipitation, irrigation water source, soil type, etc. [64; 65]. Considering the geographic location and cultivation patterns, this study found that the topographic relief and irrigation water sources have a significant effect on the winter wheat cropping structure in Shandong.  
At first, topographic relief is an important restrictive element for crop cultivation, especially for provinces with a special topography like Shandong. It was found that the spatial distribution of winter wheat is closely related to the topography. As shown in Figure 8, winter wheat is mainly distributed in the plains, and it is most densely and evenly distributed in the area below 30 m in elevation. Winter wheat is also distributed in the area with an elevation range of 30 to 70 m, but it is relatively scattered. In addition, there is a small amount of them distributed in the area above 70 m in elevation, which is very fragmented. Figure 9a gives a better illustration of the three-dimensional spatial distribution of winter wheat. In addition, as shown in Figure 9b, the vast majority of wheat is distributed in areas with slopes less than 1. The main reason for this distribution is that the modern agriculture is based on mechanization from sowing, irrigation to harvesting and the monitoring of the growth process. Areas with large topographic relief not only restrict the operability of mechanization, but also are not conducive to irrigation. Therefore, altitude and topographic relief have an important influence on the distribution of winter wheat in Shandong.  
Figure 8: Spatial distribution of croplands in relation to its topography.  
Secondly, crop cultivation in Shandong Province is heavily influenced by irrigation resources as it is in the temperate monsoon zone, where precipitation is abundant but relatively concentrated. River irrigation is an important irrigation channel, and crop cultivation is often spread along rivers [66]. In different growth periods, abundant water has different yield-enhancing effects on winter wheat. As shown in Figure 10, multi-level rivers in Shandong provided abundant irrigation resources for crops. Winter wheat is more frequently distributed in the water basin areas dominated by first-order rivers in the west and north, and to a lesser extent near shorter-length rivers in the east. Therefore, winter wheat is more widely distributed and of higher farming quality in areas where water resources are abundant and the water supply is more economical and convenient.  
In general, the topography of Shandong is characterized by high relief in the central, is dominated by mountainous terrain, and has hilly terrain along the eastern coast; both of  
Figure 10: Spatial distribution of winter wheat in relation to multi-level rivers.  
Figure 9: Distribution of winter wheat in three-dimensional space, **(a)** refers to the distribution over elevation and **(b)** refers to the distribution over different slopes.  
these areas are unsuitable for the large-scale planting of winter wheat. The plains in the west and north are part of the three major watersheds, which are flat and wide, suitable for the large-scale cultivation and harvesting of crops, and have abundant irrigation resources, which are the main planting areas for food crops in Shandong.