The larger the inner pipe diameter, the less friction there is during water transport. However, this increases the excavation hole area and pipe thickness, inducing more LCCO2 during construction. Then, there is a trade-off between construction and operation. In addition, the friction depends on the amount of water. To determine the diameter in this study, we assumed the case that the largest amount of water was transported, meaning all the PV electricity is distributed to seawater desalination. The reason for this is that the amount of water conveyed is controllable during operation, but the tunnel diameter cannot be readily changed. The amount of water required is expected to change with population fluctuations and seasons. Therefore, cases in which the maximum amount of water is transported are possible in practice. With this assumption, the tunnel diameter was determined to reduce the total amount of LCCO2 from the construction and the operation. The amount of LCCO2 emitted during the pipeline construction varies depending on the source of existing electricity. Thus, the internal diameters of the pipeline depend on the LCCO2 emission factor of the power source, and are approximately 1.4 m for natural gas and 1.6 m for coking coal, respectively.