Reference Evapotranspiration Changes in the Haihe River Basin during Past 50 Years

In this paper, temporal trend of annual reference evapotranspiration (ET0) calculated using the FAO Penman-Monteith equation with the observed daily meteorological data at six stations (i.e. Datong, Weichang, Qinhuangdao, Tianjin, Beijing and Huimin) of Haihe River Basin, China, were detected with the help of parametric t-test and Mann-Kendall (MK) analysis. The six stations were divided in three different classes representing mountain (Datong and Weichang), continental (Beijing and Huimin) and coastal areas (Qinhuangdao and Tianjin), respectively. The result shows that there was a significant upward trend in ET0 at mountain area of the Haihe River Basin. On the contrary, a significant downward trend in ET0 can be found at coastal area. Moreover, the analyses of ET0 at continental area indicate that after 1960, ET0 of Beijing showed a sharp significant increase, while a moderate variation was presented for ET0 at Huimin.