Abstract:

By using different percentiles（larger than 75 or 95 percentiles）the frequency of precipitation days in the Yangtze River catchment from 1950 to 2000 was explored in this paper.The positive (increasing) trends of the yearly precipitation days at the 75th percentile appeared in most of the Yangtze River catchment，especially in the northern regions of upper and middle reaches of the catchment.The Sichuan basin was the only region with negative trends.For the 95th percentile，the negative trend regions were still mainly in Sichuan basin but extend to northern regions and there were also obviously increasing trend centers in middle and lower reaches of the Yangtze River catchment.On the decadal time scale，the most significant positive trends at the 75th percentiles were in the middle reaches in 1980s and 1990s.The lower reaches showed significant positive trends in 1980s.Those positive phases greatly contributed to the positive trends of the whole catchment during the last two decades.The most significant negative phase also occurred in the middle reaches during the earlier three decades.For this case，it seems that the precipitation days in the middle reaches are more sensitive to changes than other regions in the Yangtze River catchment.Take the Yangtze River catchment as a whole，a positive trend is very obvious for this percentile.For the 95th percentile，the trends between the middle and lower reaches of the Yangtze River catchment are coherent：after two decades (1960s and 1970s) of negative phases，the most significant increasing trend was shown in the last two decades.On the other side，the fluctuation of the precipitation days in the catchment of the upper reaches over the 50 years was somewhat smoother than for other regions.For the whole catchment，the precipitation days in the 1950s and in the 1990s were at positive phase that indicated the frequent heavier precipitation events in those periods.The precipitation days at the 75th percentile increase more obviously than that at the 95th percentile in the whole catchment compared the 75th with the 95th percentiles.This result indicated that an increasing precipitation in the recent two decades was mainly caused by excessive precipitation.The heavier precipitation events occurred mainly in the upper reaches in 1960s，in the middle and lower reaches in the 1950s，1980s and 1990s.According to the different trends at different percentiles，the variation of yearly precipitation days can be classified as the following types：Type 1：the increasing trend at 75th percentile but decreasing trend at 95th percentile.Type 2：decreasing trends of yearly precipitation days for both percentiles.Type 3：increasing trends of yearly precipitation days for both percentiles.

Key words: precipitation days, frequency, trends, the Yangtze River catchment〖ZK)〗〖WT〗〖LM〗