The Analysis Of Variation Characteristics And Cause Of Drought-wetness Over Tarim River Basin In Recent 50a

Applying the data of runoff, soil available water capacity and the transpiration of Penman-Monteith method, the index of PDSI was improved in the Tarim River basin. The spatial-temporal characteristics of drought-wetness in the Tarim River basin from 1961 to 2007 were analyzed. The multi-scale influences of the precipitation, runoff, and transpiration on drought-wetness were analyzed also, the contributions of these factors to PDSI index were given. The influences of the atmosphere circulation, NAO and North Polar Vortex to PDSI index were revealed, and the future variation of drought-wetness in the basin was predicted. The main conclusions are as follows:(1)In recent 50a, the wetting trend of annual drought-wetness is significant in the Tarim River basin, and it is fastest in east area, slowest in south areas, it is strongest in summer and weakest in spring. The period of recent 20a is the main period in which the trend is wetting in the basin. The abrupt changes of drought-wetness presented in year and season since 1961, the most abrupt change years are in 1986-1995, but in east subarea, the abrupt year is later than that of other subareas, it is about in late of 1990s, and it is earlier in north subarea, it is in early of 1980s.(2) In the Tarim River Basin, the Inter-annual variations of the drought-wetness is important to the violent change of drought-wetness in 1980s, while the influence of Inter-decadal variations on wetness is greater than that of Inter-annual variations since 1990s. The significant areas and strong areas of influence of the precipitation, transpiration and runoff accord with the significant areas and strong areas of oscillation of the drought-wetness at inter-annual scales and inter-decade scales.(3) The positive contribution of the transpiration, runoff and precipitation to drought-wetness increases since 1961, their annual contribution mean are -0.473,0.370 and 0.164, respectively. The increase of positive contribution of the precipitation, transpiration and runoff to drought-wetness results in the wetting of Tarim River Basin since 1980s. In recent 20a, the decrease of transpiration contributes most to wetting of the basin, while the increase of precipitation contributes most to the convert from drought to wetness of the basin in 1980s.(4) In the spring wet years, the abnormity of isobaric height field, wind field and water vapor field make the precipitation, runoff and transpiration of the basin increase, the summation of their contributions is positive, so the climate is wetter in the basin; In the summer wet years, the precipitation increases, while the variation of runoff and transpiration is small, so it is wetter in the basin also.In the spring high NAO index years, the abnormity of isobaric height field, wind field and water vapor field make the runoff and transpiration of the basin increase, and the precipitation decrease, the summation of their contributions is positive, so the climate is wet in the basin; In the summer high NAO index years, precipitation, runoff and transpiration decrease, the summation of their contributions is negative, so it is dry in the basin; In the spring and summer low NAO index years, the drought-wetness of the basin is contrary to that of high NAO index years,.In the spring high North Polar Vortex index years, the abnormity of isobaric height field, wind field and water vapor field make the runoff and transpiration increase, and the precipitation decrease, the summation of their contributions is negative, so the climate is dry in the basin, in the spring low North Polar Vortex index years, the precipitation increases, while the variations of runoff and transpiration are small, the summation of their contributions is positive, so it is wetter in the basin; in the summer high North Polar Vortex index years, the runoff, transpiration and the precipitation decrease, the summation of their contributions is negative, so it is dry in the basin; in the summer low North Polar Vortex index years, the drought-wetness of the basin is contrary to that of high North Polar Vortex index years.(5) According to the results of multi-model ensemble, the variations of the mean in 2001-2020 subtracting the mean in 1991-2000 in the basin in serres A1B are as follows:The air temperature increases in the Tarim River basin, the range is 0.4-0.8℃. The precipitation increases in the most area except in the west of west subarea and the northwest subarea, the decrease range is 0-4mm/a, the area where the precipitation increase most is in the south subarea, the range is 6-14mm/a. The drought-wetness of most area become arid except the small region in southwest and north of basin, the areas where the PDSI index decrease most are in the east subarea and west of west subarea, the most decreased value of PDSI index is 5.