## 2 Materials and Methods
### Data
#### 2.2.2 Classification Groups  
Because of the similarity in temporal profiles of (catch) crop species [3], species with similar spectral and temporal characteristics were grouped together. The catch crop species are listed in Table 1, and other crops cultivated during winter are referred to as winter crops. The catch crop species were subdivided into three groups based on their plant morphology and phenology (Table 1) [34; 35]. The first group consists of leafy plant species; two example species in this group are white mustard and phacelia. This group is characterized by broad leaves and high aboveground biomass, properties that cause a high nitrogen uptake [34]. Plants in this group are very sensitive to frost damage and often do not survive winter. The second group consists of grass-like species with more narrow and upright leaves; two commonly used species are grass and oats (Table 1) [34; 35]. As opposed to the first group, grass-like crops are winter-hardy and have a slower nitrogen uptake throughout the season. Their aboveground growth is slower, but their root system is extensive, which contributes to reduced soil slaking and an increased build-up of soil organic carbon. Mixtures of any of the allowed species can be considered as a third group, where mixtures can either consist of only leafy species, only grass-like species, or a combination of species from both groups.  
The non-catch crop winter crop species were also grouped into three groups: (1) legumes, (2) winter cereals, and (3) other winter crops. Legumes are used as an agricultural practice to increase soil nitrogen and, therefore, are not used as a catch crop [34]. The winter cereals, winter barley (_Hordeum vulgarte_ L.) and winter wheat (_Triticum aestivum_ L.), were put into a separate group because farmers can use these crops as an 'equivalent measure' instead of sowing catch crops. The difference with sowing catch crops is that the winter cereals sown in the current year remain on the field as the main crop in the following year, so they are not removed from the field after the required period, as is the case with catch crops. Examples of the third group, other winter crops, are cash crops sown during the study period, for example, leek (_Allium ampeloprasum_ L.) and spinach (_Spinacia oleraca_ L.).  
Figure 3: Locations of the 6881 field observations in the study area: (**A**) colored by year of observation where the color indicates the year in which the winter year started (e.g., 2022 are observations in the winter year 2022–2023) and (**B**) the field observations located on an agricultural land use map. Land use: Department of Environment and Spatial Development (www.vlaanderen.be/datavindplaats (accessed on 22 November 2024)).  
To classify these six different groups, the approach of a classification hierarchy was taken, dividing the complex classification problem into progressive classifications for improved accuracy in the case of complex classes with a lot of intra-class variability [37; 38; 39]. A limited two-level hierarchy limits process complexity and training time, as separate classifiers were trained for each level (Figure 4A). On the first level, fields were classified on winter crop presence or absence, separating bare soil and crop residues from fields occupied by a winter crop. In the second level, sown fields were further classified into six groups: three catch crop groups (leafy, grass-like, and mixtures) and three non-catch crop groups (legumes, winter cereals, and other winter crops). The resulting label distribution shows that (1) the overall majority of the dataset consists of fields with catch or winter crops present, (2) most observations were collected in the winter years of 2020-2022, and (3) there is a label imbalance within the six crop groups (Figure 4B).