## 4 Discussion
### Comparison of Existing Studies  
According to Table 4, the value of climate regulation in this study is generally higher than in other research results based on empirical values to estimate the cooling effect of paddy fields. For example, [PERSON] [56] estimated that the duration of persistent hot days in Suzhou in 2012 was 23 days, and the average daily evapotranspiration of paddy fields was about 6.89 mm/d. [PERSON] [44] and [PERSON] [62] estimated that the duration of air conditioning in Hunan Province in summer was 30 days, and the average daily use time was 6 h. The difference in empirical parameters directly leads to different evaluation results. In this study, according to the meteorological data of Tianjin, the hottest time is July to August. Therefore, the daily ET data of these two months were used to calculate the cooling effect value, which objectively reflects the climate regulation function of PFEs. The main reason for the low value of groundwater recharge in this study compared to most existing studies is that the price of agricultural water in this study area differs significantly from other study areas. For example, the price of water in Liu [56] and Li [36] were RMB 3.2/m\({}^{3}\) and RMB 0.67/m\({}^{3}\), respectively, while the price of agricultural water in this study area was RMB 0.24/m\({}^{3}\). In addition, previous researchers used a unified soil infiltration rate to calculate the groundwater recharge. However, soil quality differences between plots will lead to large errors in soil infiltration rate, which will affect the estimation accuracy of groundwater recharge. In this study, the amount of paddy field irrigation water plus precipitation minus evapotranspiration during the rice growth period was used to calculate the groundwater recharge. This method overcomes the error of soil water infiltration rates caused by soil types and texture differences in different plots or regions, and improves the estimation accuracy of groundwater recharge. There is large variability in the ESV per unit area of PFEs estimated by different researchers, and the main reasons are as follows: (1) the differences in economic development level, natural conditions, rice varieties and field management level between regions lead to different rice yield and cost input, which leads to large differences in some evaluation indexes; (2) price factors have a large impact on ESV, with differences in rice prices and agricultural water prices in different regions, while the combined effect of currency devaluation and other factors can lead to changes in ESV in different years; (3) inconsistencies in evaluation frameworks and methodologies have led to differences in the content and details of accounting by different researchers, such as [PERSON] [43] did not remove the cost of rice production from the value of supply services; (4) in some water-rich areas where agricultural water is free, and where groundwater replenishment in paddy ecosystems is real, the question of whether to calculate the ESV of this function or the appropriate value to be placed on the price of water for estimating it is a challenge for further research. The research on ESV evaluation originated from Europe and the United States, and has been unprecedentedly developed in China. Especially in recent years, Chinese scholars have accounted for more than 60% of the research on the ESV evaluation of PFEs. This is mainly because the Chinese government has paid more attention to ecological protection year by year, so that the ecological service value can be applied to specific work, such as policy formulation and performance evaluation. We focused on the comparison of research results of different scholars in China, and did not conduct a comparative analysis of research results outside China. This is mainly because different planting systems, planting costs and management methods in different countries make it difficult to conduct a comparative analysis of the ESV of PFEs between countries.