## 2 Methods and Variables
### Selection and Description of the Study Variables  
The factors and respective variables used for explaining farmers' participation in PRP are presented in Table 1. We employ the three-category scheme of [PERSON] et al. [25], incorporating complex constructs for the first category and simpler variables for the other two. The first variable of the \"farmers' willingness\" category is Perceived Usefulness (PU). This construct measures the perception of farmers regarding the potential benefits that they will acquire from their participation in PRP. The variable has been extensively used under the Technology Acceptance Model (TAM) for modeling the attitude of farmers against new technological solutions [29]. As for research projects, the perceived usefulness has been considered in many of the studies on factors affecting the participation of farmers in ESs. [PERSON] and [PERSON] [21] state that farmers will be more easily engaged in ESs when they consider that their involvement will help them reach their targets as entrepreneurs, and for this it is vital that the ES providers showcase the benefits of their services for farmers. In addition, [PERSON] et al. [22] found that when farmers believed that their participation in an ES would improve their personal and professional skills and competences, they were more likely to participate in it. The construct in the present paper includes three items (PU 1-3) which capture the farmers' attitude against the usefulness of PRP and are measured by a Likert scale ranging from totally disagree (1) to totally agree (5) according to farmers level of agreement with the respective statements. The second construct (PU2) measures the trust of farmers to the providers of the PRP. Trust is defined as the degree on that \"one party is willing to rely on the actions of another party\" [30]. Trust has been used as a predictor of farmers' behavior in quite a few studies with ambiguous results, as its effect seems to be context-dependent and varying according to the type of institutions to whom trust is measured and the type of decisions to be considered [31; 32; 33; 34]. As for the ESs, [PERSON] and [PERSON] [35] showed that trust to service providers improved the acceptance of agri-environmental schemes among the Australian farmers. Finally, trust to the ES providers seems to influence the very likelihood of farmers to participate in them and the further utilization of ES outcomes [7; 21; 36]. The third construct (PU3) measures the motivation of respondents behind their engagement in farming activities and their overall belief over the scope of agriculture. As [PERSON] et al. [25] have shown, farmers' values and beliefs play an important role in shaping their behavior. Therefore, their participation in PRP may also be influenced by their beliefs for the role of agriculture and their motivations behind farming engagement [22]. The two items measure the basic motivation of farmers. Largest levels of agreement with the two statements reveal farmers who really enjoy farming and would not easily shift to other occupations. On the other hand, lowest agreement degree is associated with more profit-driven farmers.