Variations of the spring precipitation day numbers reconstructed from tree rings in the Urumqi River drainage,Tianshan Mts. over the last 370 years

The tree-ring cores of Tianshan spruces col lected from nine sites in the Urumqi River drainage of the middle Tianshan Mountains were used to establish three types of the tree-ring width chronologies over the last 370 years, using the international standard method of dendrochronology. Our study demonstrates that dendrochronology can be better used to reconstruct the number of the precipitation day than to reconstruct the precipitation amount in middle Tianshan Mountains. It is found that the residual chronology among the three tree-ring width chronologies has the best relationship with number of spring precipitation days from May 20 to June 8. The chronologies at Haxionggou B site and Zaierdegou site in the Urumiqi drainage have the highest correlation with the observed number of spring precipitation days at Daxigou meteorological station, and are used to reconstruct the spring precipitation days over the last 370 years in the drainage. The main significant decreasing trend of the number of the spring precipitation days occurred during 1665–1717, while the significant increasing trends happened during 1805–1841 and 1914–1943. The reconstructed series of the number of spring precipitation days has quasi-periodic variations of 3.3, 2.1, 2.5, 12.3 and 32.0 years with the dominated short periodical changes. The long cycle of 32 years is shown quite clearly in the 10-year smoothed sequence. The maximum spring precipitation days occurred mainly in the 1630s, 1840s and 1940s, while the lowest number of spring precipitation days for the 10-year average occurred in the 1710s.