## 5 Discussion  
### Different Contributions of Crops to NSP Intensity  
There are great differences in nutrient requirements, actual fertilization, fertilizer utilization rate, and fertilization node for different crops [64]. Taking winter wheat and summer maize in the study area as an example, winter wheat needs more nitrogen than summer maize but less phosphorus than summer maize [65; 66]. This directly affects the actual fertilization. To make crops grow better, farmers use different fertilization schemes for different crops. Fertilization schemes differ not only in the proportion of nutrients but also in the time nodes of fertilization. For winter wheat, it is necessary to apply starter fertilizer in October of each year and topdressing in the rejuvenation stage, jointing stage, and early filling stage. These growth stages of winter wheat in this study area are, respectively, in March, April, and May of the following year. For summer maize, it is necessary to apply starter fertilizer in June every year and topdressing in the great bell mouth stage and tasseling stage. The two growth stages of summer maize in this study area are July and August, respectively. The study area is in the same period of rain and heat. It is cold and rainy in winter but warm and rainy in summer. In October, March, and  
Figure 12: (**a**) Distribution map of NSP intensity estimated by the proposed method. (**b**) Distribution map of NSP intensity estimated by the traditional method. April, when winter wheat was fertilized, precipitation was less, while during the summer maize fertilization, precipitation was more. This indicates that not only the fertilization, crop fertilizer absorption rate, and other factors affect the generation of NSP intensity but also that the precipitation at the time of fertilization affects the loss of pollutants. If multiple cropping exists in the study area, crops are planted all year round and the frequency of fertilization increases significantly, which also affects the generation of NSP intensity [64]. The difference of NSP intensity caused by different crop types is only one aspect. For a large watershed, even for the same crop type, their growth situation is different due to their different growth environments [67]. Under certain conditions, the more dry matter fixed by photosynthesis, the better the growth of crops [51, 68], the better the absorption of fertilizer, and the fewer pollutants discharged into the environment. This is also an important factor in the estimation of NSP intensity. Different types of crops, different planting patterns, and different growth potential have different contributions to NSP. In addition, soil type, precipitation, slope, fertilization differences between different farmers, and other factors also affect the estimation of NSP intensity. Therefore, the mapping of NSP intensity has always been a difficult problem. [PERSON] et al. [1] proposed a traditional method for NSP intensity estimation. It uses a unified source intensity coefficient to estimate NSP intensity generated by farmland, without considering the effects of crop types or growth on it. The results of the NSP intensity estimation are shown in Figure 12b. Compared with the method in this paper, without fine crop classification and growth information, it cannot reflect the real NSP intensity. Compared with the NSP intensity estimated using the proposed method shown in Figure 13, the estimations were consistent with the overall trend. But there were considerable differences in the specific estimated quantities of each unit. This is because the method presented in this paper considers crop types and growth. For example, more fertilizer is applied in multiple cropping areas than in single cropping areas, so more NSP is generated. Therefore, the maximum NSP intensity throughout the study area estimated by the proposed method was more than it obtained by the traditional method. Figure 13 shows differences in the estimation of local NSP intensity.