YUAN Yu-jiang, LI Jiang-feng, HU Ru-ji, LIU Chao-hai, JIAO Ke-qin, LI Zhong-qin
Correlation census shows that the correlation between the tree ring chronologies from the Ürümqi River Basin and precipitation from July to February is significant with the best single correlation coefficient of 0 74 and a significance level of 0.0001. From two tree ring residual chronologies of the West White Poplar Gully and Boerqin Gully, precipitation in the middle Tianshan Mountains in recent 350 a can be reconstructed, with an explanation variance of 62%. Many verifications, according to statistical parameters, independent precipitation data, historical climate records, glacial and other data, have shown that the reconstructed precipitation sequence for the recent 350 a is reliable. Analyzing the precipitation features indicates: 1) There were three wet periods, 1671(?)—1692, 1716—1794 and 1825—1866, with the reconstructed precipitation above the long term mean, and three arid periods, 1693—1715, 1795—1924 and 1867—1969, with the reconstructed precipitation below the long term mean;2) Three were two wet periods, 1716～1794 and 1825—1866, when the second and the third glacial terminal moraines formed before the Glacier No.1 at the headwaters of the Ürümqi River. According to a computation, annual precipitation during the 10 years when precipitation was maximum in the period of 1716—1794 was 534 1 mm, 59 mm more than now, with a precipitation anomaly of +12.5 %. This is consistent with the conclusion that precipitation in the strongest period in the Little Ice Age was above 50 mm more than now, which was drawn by Yao Tandong and Shi Yafeng who have calculated that from temperature. Annual precipitation of the maximum ten years precipitation during 1825—1866 was 504.6 mm, 30 mm more than now, with a precipitation anomaly of +6.3%; 3)During the 348 a, the reconstructed precipitation in the middle Tianshan Mountains from July to February appeared 18 wet years(accounting for 5.2%), 8 dry years(2.3%), 58 partial wet years(16.6%), 94 partial dry years (27.0%) and 170 normal years(48.9%); 4)In the recent 350 a, 1714 was the most wet year, with a precipitation, 507.2 mm, from July to February, 96 9% more than now(the mean was 257 5 mm during 1961—1993). 1787 was the most dry year, with a precipitation of 158.6 mm, 38.4% less than; 5)The variation of reconstructed precipitation from July to February has quasi periods of 5, 10, 53～54, 3.7 and 3.3 a; 6)There was a significant drying trend from 1716 to 1969; 7)The reconstructed precipitation had an abrupt change in 1831; 8)The series of reconstructed precipitation is well representational to that in the Ürümqi and Changji Prefectures on the north slopes of the Tianshan Mountains.
