## 1 Introduction  
Non-point source pollution (NSP) refers to the environmental pollution without a fixed sewage outlet. The erosion of rainwater causes dissolved or solid pollutants from non-specific locations, along with surface runoff into receiving water, to create eutrophication and other water-based environmental problems. Compared with point source pollution, NSP is more difficult to measure and quantify, so it is also very difficult to study and manage. NSP intensity is used to quantify NSP and it refers to the total amount of pollutants discharged per unit area per unit time. It is related to the overall pollutant concentration. Greater source intensity leads to more serious pollution [1]. The differences of ground object types and geographical environment cause the obvious spatial distribution differences of NSP source intensity. NSP intensity's spatial expression represents the ability of land in each unit region to pollute the environment, which is highly significant for targeted NSP control and provides an important basis for rationally adjusting ground object types and structures. Farmland areas sensitive to NSP in the agricultural industry mainly comprise areas in which the cultivated land distribution is more complex. The spatial distribution of farmland NSP intensity is easily affected by crop type, crop condition, crop growth cycle, multiple croppings, and other factors. When one or several of these factors show spatial differences, the source intensity also changes. These factors usually influence the emission intensity of pollutants by causing different amounts of fertilizer used in and absorbed by the crops. For example, multiple cropping is widely used in Asia and Africa as a planting mode to improve the utilization efficiency of cultivated land [2]. When multiple cropping is performed in a field, two or more crop types are planted in different seasons of the year, which leads to an increase in the frequency of farmland fertilization. The resulting NSP intensity in this type of crop planting mode differs from the pollution intensity when a single crop is planted in a particular field. In addition, the amounts of fertilizer and growth cycles vary among crop types, which also affect the intensity of NSP [3]. Fertilization standards differ for each type of crop. Each crop's growing season also affects the location of fertilizer. For example, in areas with both rain and heat, or where more precipitation falls in summer, fertilizer loss is more likely to occur, while in places with less precipitation during winter, fertilizer is more likely to stay in situ, where crops absorb it. Crop growth also partially reflects fertilizer uptake and affects the intensity of NSP [4]. With all other conditions remaining the same, when a crop fixes more organic carbon through photosynthesis, it can grow better, absorb more fertilizer, and leave fewer pollutants in the environment. These complex situations have considerable impacts on the spatial distribution of NSP intensity. To accurately estimate NSP intensity, the accurate extraction of crop information is necessary. However, multiple factors (e.g., rapid changes in environmental conditions, the adjustment of government management policies, changes in market regulation, and farmers' willingness to report growth conditions) can cause crop configurations to significantly fluctuate over time [5]. National and/or local governments' regulatory policies cause multiple cropping indexes to continuously fluctuate. Crops' growing conditions also vary over time. All of these factors make it more challenging to estimate NSP intensity. Overall, this involves two main problems. One is the lack of a mapping model of NSP intensity specifically for farmland that considers complex conditions, while the other is a lack of dynamic spatial and temporal data for finding extensive crop information inside cultivated agricultural land. For the first problem, thus far, multiple NSP models have emerged in an attempt to provide more accurate pollution estimates.