*2.1. Questionnaires*

Online questionnaires were sent to a random sample from members of the public of over 18 years old in Fujian Province. Snowball sampling was adopted to distribute online questionnaires to SHP owners and related governmen<sup>t</sup> administrators in Fujian Province with the help of the Fujian Province SHP Association by online links because it is recommended for use when samples are rare and difficult to find. There was no preference for selecting respondents in each group. Questionnaires were solely comprised of closed-ended questions with either response at the nominal level or binary response (see Table S1). The sample questions of single choice (A.) and multiple choices (B.) are shown as follows: (A.) Do you think it is necessary to implement environmental flows? (Yes/No); (B.) Who needs to bear the economic loss generated by implementing environmental flows? (The governmen<sup>t</sup> solely/The owners of SHPs/Electricity consumers by paying more for the electric bill/The governmen<sup>t</sup> and the owners/The governmen<sup>t</sup> and electricity consumers/The government, the owners, and electricity consumers).

As it is impossible to know how many times the online questionnaire links had been clicked, we were only able to filter invalid questionnaires by setting up reverse questions; if the obverse and reverse choices were selected at the same time, the questionnaire was considered invalid. The purpose of the study was presented before the questions to ensure each respondent was informed. After a pretest, the question template was re-evaluated; some questions were explained, and some were simplified. The number of questions posed to each target group was different (nine for SHP owners, 11 for governmen<sup>t</sup> administrators, and six for the general public). All of the questionnaires focused on the environmental impacts of SHPs, attitudes towards SHPs as green enterprises and the E-flows release, perspectives on paymen<sup>t</sup> for ecosystem services (PES) as a cost-sharing program, and the willingness to pay for E-flow implementation. The questions to governmen<sup>t</sup> administrators and owners also covered the conflicts and difficulties of E-flows implementation, average returns and electricity production losses and views on existing compensation policy.

A total of 513 owners, 58 governmen<sup>t</sup> administrators, and 667 members of public completed the questionnaires, with corresponding validity rates of 93% (478), 93% (55), and 90% (603), respectively. These high validity rates likely reflect the fact that all respondents volunteered to complete the questionnaires, i.e., people with a low willingness to respond would ignore the original links. The chi-square test was adopted to examine the differences in the choices of respondent groups, where *p* < 0.05 indicated a significant difference.

Yet, respondent accessibility has the potential to affect the response rate and prejudice the results, particularly in a survey targeted at a large area. Other survey limitations include gathering responses from those who did not actively participate.
