Causes of decreased reference evapotranspiration and pan evaporation in the Jinghe River catchment, northern China

摘要

This study focused on the changes of reference evapotranspiration (ET_0) and pan evaporation (ET_(pan)) to study the impacts of climate change on the hydrological cycle in the Jinghe River catchment. Based on the Penman-Monteith equation, the ET_0 was calculated. The temporal trend and spatial distribution of ET_(0) and E_(pan measured with a 20-cm pan were examined at the 14 stations during 1957-2005. The effects of meteorological factors on the variation of ET_0 were determined by analyzing the trends in themselves with comparison between original climate and detrended climate scenarios and then their sensitivity to ET_0. Both the ET_0 and E_(pan) showed remarkable decreasing trends from 1957 to 2005 and their decreasing rate was 40.9 and 17.7 mm per 10 years, respectively. Trend analysis of meteorological factors exhibited that the reduction in ET_0 and ET_(pan) was principally caused by both significant decreases in wind speed and sunshine hours. Furthermore, the decreasing trend of ET_() was mainly dominated by the significant decrease in wind speed with high sensitivity, to a less extent, by the decrease in net radiation. Although relative humidity is one of the most sensitive variables, its effect on ET_0 was negligible because of its temporal constancy. The contribution of wind speed reduction to decreased ET_0 has increased from 50 to 76.1%, but net radiation, by contrast, decreased from 50 to 23.9%.