This study analyzed the influences of applications of two different evapotranspiration (ET) estimation methods on the ET and irrigation water requirements (IWR) for paddy rice, and water supply reliability of agricultural reservoirs in Korea. The traditional Modified Penman (MP) method and the recommended Penman-Monteith (PM) method were applied on 149 reservoirs located in Honam province. The past and current daily temperature and precipitation during rice-growing season were compared between 1987 to 2016.

The results obtained from this study are summarized as follows :

1. Increments in average temperature and precipitation during current decade, as compared to the past years, impacted the ET and IWR estimates. Higher ET was obtained for high temperature regions while lower IWR was obtained for regions with more precipitation and lower temperature.

2. The PM-based ET estimates were lower than the MP method. The differing reference evapotranspiration estimates produced by the two ET methods were more influential than the respective crop coefficients in determining the ET. Regardless of the MP and PM methods, ET increased in the all regions because of temperature rise.

3. The PM method estimated lower IWR from its MP-based counterparts depending on the respective ET results. For all the regions, despite the unequivocal ET increase, the IWR showed both increasing and deceasing tendency depending on the temperature and precipitation characteristics of the regions. For any two regions with similar increased temperature, the IWR inversely related to the precipitation.

4. PM method hence more reliable storage capacity of the reservoirs respect to water supply to paddy field. The results also showed that the influences of different ET methods applications on the water supply reliability of reservoirs are negligible for the cases of over four watershed ration or 600 mm unit reservoir storage, while significant variations of the results obtain from the applications between two ET methods for the opposite cases.

The results of this study provide basic information of applicability of PM method in estimation of agricultural water demand and assessment of water supply reliability of agricultural reservoirs. Since the field conditions such as conveyance losses, farming patterns and climate changes were not considered in this study, further studies considering field conditions should performed in order to adopt PM method in the field scale applications of the paddy farming in Korea.