## 5 Discussion
### Management Strategies  
[PERSON] et al. [1] developed a NSP intensity calculation method in accordance with the requirements in the \"National Technical Guide for Verification of Water Environmental Capacity\", issued by the State Environmental Protection Administration of China, which has been widely used in the estimation of NSP in river basins [1; 29; 31; 32]. Now this method is quite developed. In this study, we have improved the method in principle, so that it better reflects the crop types and crop growing condition. We modified the coefficients according to crop types, environment, and crop growth quality. The method is thus applicable for NSP intensity calculations in different environments. This study improves the existing methods combined with remote sensing technology and takes the catchment area of the Yuecheng Reservoir as an example to estimate and map the NSP intensity on a large scale. This fully proves the great potential of using remote sensing technology in large-scale NSP estimation and mapping. In the future, we can also consider using more sufficient geographic data for larger-scale NSP estimation, so as to make the method more perfect and the results more reliable. Most NSP models only focus on the actual amount of non-point source pollutants in rivers but ignore the source and migration of pollution. This is not very useful for NSP control and management. NSP intensity mapping can reflect the capacity of land to generate pollutants. For cultivated land especially, crop types, planting mode, fertilization, and growth environment affect its contribution to NSP. NSP formation is a complex and ambiguous process. To efficiently manage NSP, various management measures and technologies have been developed. [PERSON] et al. proposed a \"4R\" strategy to more systematically and comprehensively manage NSP, which greatly improves control in agricultural applications [78]. The model is mainly composed of four steps: source reduction, process retention, nutrient reuse, and ecological restoration. Among these steps, source reduction is the key measure for mitigation of agricultural NSP [33]. This paper solved the problem of NSP source monitoring, which is a very key part of the \"4R\" strategy. NSP intensity map can make source control more targeted and strengthen management in places with high source intensity. Local governments can guide the planting mode of crops and the layout of farmland in conjunction with water quality. Poor agricultural management leads to soil erosion, as well as greater nitrogen and phosphorus losses in surface runoff, thus resulting in the formation of a large area of farmland NSP in the surrounding catchment [79]. The topic discussed in this paper is to extract crop information through remote sensing technology and then estimate the NSP intensity based on the contribution of different crops, which is a forward process. For management, it needs to be considered conversely. For the heavily polluted areas identified in the NSP intensity map, it is necessary to pay attention to the adjustment of crop planting mode and fertilization strategy to minimize the emission of pollutants. In addition to source control, the application of remote sensing in pollutant interception also has great potential. It can help design pollutant buffers, riparian, and so on [80]. It also plays a role in all aspects of NSP management. From the perspective of precision agriculture management, farmland grid management is a convenient and efficient management method which can break the constraints of administrative units and improve the efficiency of information retrieval/renewal [81; 82]. In this study, the source intensity was estimated by grid division, which improves the efficiency of cultivated land retrieval. In farmland management, the application amount of chemical fertilizer can be controlled hierarchically according to grids with different NPS intensity. In a grid with considerable source intensity, the application of chemical fertilizer would be reduced accordingly.