The Applicability and Modification of Takahashi Formula for Evaporation Estimation in Lhasa

Researches on the latent heat, the process of water balance and the dry climate of Tibetan Plateau are of great significance. But due to the limits of estimation methods, observation instruments and lack of stations, there is no long, accurate evaporation data on Tibetan Plateau. Therefore, it is important to choose a more efficient, simple formula to estimate evaporation. The data of the Lhasa can represent the average state of the entire Plateau. In order to calculate the evapotranspiration, several methods including Takahashi formula are adopted and the results are compared with data of the Asian Monsoon Experiment. The result shows that the calculated values are close by PM formula, Remanenko formula, Blaney-Criddle formula and Hargreaves formula. The values calculated by PM formula and Remanenko formula begin to increase from January, reaching the maximum in May, and then decreases. But for the values calculated by Blaney-Criddle formula and Hargreaves formula, the maximum appears in July, two months later. The values of Remanenko formula and PM formula are closest, so the Remanenko formula can be used for the area lack of data. The maximum of potential evapotranspiration appears in May, but the evaporation is smaller due to lack of water, though the sun radiation is strong. As the relative humidity increases with precipitation, the potential evapotranspiration decreases, while the actual evaporation reaches maximum in July. The value estimated by the Takahashi formula has large bias comparing with the observation, indicating that Takahashi formula is not suitable for Lhasa. The bias is greater when the temperature is higher, because the frozen soil, ice and snow thaw on Tibetan Plateau have added the uncertainties. Considering the direct proportion between the temperature and the gapbias, the temperature is divided into 5 grades: Lower than 0℃, 0—5℃, 5—10℃, 10—15℃ and higher than 15℃. Without changing the original coefficients of the Takahashi formula, a coefficient is introduced on the precipitation part, which leads to results closer to the observations. The maximum of actual evaporation appears in summer, reaching 100 mm or above, which is slightly less than the measured value of the Asian Monsoon Experiment, and the minimum appears in winter. The value calculated with the modified formula is significantly higher than the original formula, closer to the observed data. But the value in winter is significantly higher than the original value, slightly higher than the observation.