| The arid index was calculated with Penman-Monteith method recommended by the FAO in 1990, based on climatic data of 93 meteorological stations in Yellow River Basin (YRB) authorized by the National Meteorological Bureau from 1957 to 2001, and the arid index distribution in Yellow River basin was also created through the kriging method.The result shows that the spatial distribution of arid index is strongly affected by climatic and geographical factors, and the arid index decreases from the north to the south.From 1957 to the end of 1990s, the trend of arid index was stable at first and then increased. At the same time, the seasonal variation of the arid index was obvious, which was winter >spring>autumn> summer.The maximum value of the arid index occurred in July, August and September in the upper, middle and lower reaches of the YRB respectively, and the minimum values were from November to the next March with a distinct regional variation. |