Under the climate changes and human activities, large areas of lakes shrunk and wetlands degenerated. Evapotranspiration (ET) is the primary water loss in the water balance of closed-basin and lakes. However, ET is difficult to estimate, especially in the data-scarce or ungauged zones. For a closed lake without outflow, the inflow could be used to estimate the water shortage of ecological water demand. Based on meteorology data and the pan evaporation (ET_pan) data, Penman-Monteith and Hargreaves Model were used to estimate potential evapotranspiration (ET₀), which was used to estimate ET, and used the water balance model to assess the water inflow of Wolong Lake. PM model proposed the satisfactory estimation of ET₀ from Wolong Lake. The empirical formula of the net radiation was built based on the coefficient that derived from data fitting with function of the measured ET_pan and the estimated ET₀. The temperature was the dominant factor for the increment of the ET_pan. In the data-scarce or ungauged zones, ET₀ could be used to evaluate the evapotranspiration from the lake surface (ET_lake), instead of the ET_pan. The results found that the inflow of Wolong Lake could not meet the demand of the evaporation and water volume advancement.