Spatial and Temporal Characteristics of Reference Evapotranspiration Trends in the Haihe River Basin, China

In this study, the spatial and temporal patterns of trends for reference evapotranspiration (RET) at 34 meteorological stations (between 1957 and 2007) in the Haihe River basin, China, were analyzed using the Mann-Kendall (MK) test and the Sen’s method. To reveal the possible causes and main driving forces of the changing patterns of RET, the spatial distribution and temporal patterns of trends for four meteorological variables (i.e., temperature, wind speed, relative humidity, and sunshine duration) were examined for each station. In addition, partial relative analysis between RET and meteorological variables and a sensitivity analysis of RET to meteorological variables were conducted. The results show the following: First, the Haihe River basin is dominated by a significant decreasing MK trend in annual RET at >95% confidence level, which is observed at most stations in the eastern and southern areas of the basin. There are no observed trends or significant increasing MK trends in annual RET in the western plateau. For the intra-annual variability of RET, similar spatial-changing patterns are only observed in summer. Second, there are no obvious trends in RET from the late 1950s to the early 1970s for the whole basin. However, opposite trends of RET in the plateau area are identified after the 1970s in comparison with those in the plain and mountain area of the Haihe River basin. These two trends become significant in the late 1990s. Third, wind speed and sunshine duration are recognized as the two major driving forces for the decreasing trends in RET. Relative humidity is mainly responsible for the increasing trends in the western Haihe River basin during recent decades.