Spatio-temporal changes of wetness index in China from 1975 to 2004

Through research on variation of wetness index over a long term, it can be under stood how the dry-humid condition has changed under global warming, to make a better assessment on agroclimatic resources and rational agroclimatic zoning. Reference crop evapotranspiration (ET0) and new wetness index (w) were both calculated and analyzed according to Penman-Monteith scheme recommended by FAO with routine meteorological data from 616 weather stations in China from 1975 to 2004. In terms of grades of wetness index from Chinese Climate Classification Criterion, China is classified into severe arid, arid, semi-arid, semi-humid and humid zones, respectively, which are compared with the demarcations from rainfall-based indices in order to reveal the similarity and difference in association with the two schemes under current global climate warming. Besides, the spatio-temporal analysis of climate wetness over the country in the past thirty years was performed. The results show that the total ET0 in China ranges from 800 mm to 1600 mm on an annual basis from region to region, with the maximum (minimum) in Northwest(Northeast); P-M wetness index better expresses dry and humid conditions in China, in over-humid, over-dry and in Northeast region as well, in comparison with traditional climatic boundaries classified by isopluvials. The fluctuation in isopleth of wetness index changes among regions, more stable in the southwest, and the most unstable in the northwestern severe dry region; in the past thirty years, regions getting wet increased in number, but the degree and speed in getting arid were greater than those in getting wet, especially in the middle west part of China.