Spatial And Temporal Trend Of Precipitation On The Loess Plateau During The Past 53 Years

The Loess Plateau in China is characterized with a vulnerable ecological system and severe soil erosion in the world. Understanding the trend of precipitation is critical for water resources management and soil erosion control in this region. Daily data at 100 meteorological stations in and around the Loess Plateau from 1957 to 2009 were used to analyze the spatial and temporal trend of precipitation and the return period of the extreme event. The results showed that:(1) For the interpolation of the average annual precipitation, yearly and monthly precipitation, both the KRIGING and TPS methods can reflect the spatial distribution of precipitation on the Loess Plateau. There is no significant difference for the precipitation interpolation precision between the KRIGING and TPS methods. The differences of the interpolation precision for both of the two methods at different time scales show nearly the same trend. The average A for the average annual precipitation is better than that of April about 14%, better than that of the annual precipitation and the July about 19%, 35% respectively.(2)The average annual precipitation of 1957~2009 is 434.4mm on Loess Plateau. The annual precipitation is between 300~400mm. The precipitation on Loess Plateau exhibits a great inter-annual variability. The maximum annual precipitation is about 1.3~2.3 times as the average annual precipitation, and is about 2~6.5 times as the minimum annual precipitation. The northwest precipitation is more variable than the southeast.The precipitation between months and seasons is uniform on Loess Plateau. The maximum monthly precipitation is distributed in July and August. In July and August, the precipitation is generally amount to 15~25% of annual precipitation. The seasonal precipitation mainly distributed in summer and autumn. The summer precipitation and autumn precipitation generally account for 50%~65% and 18%~32% of the annual total precipitation. The precipitation in flood period can be amount to 78%~92% of the annual total precipitation.On the spatial distribution, the area between the isohyets of <150mm,150~250mm,250~350mm,350~450mm, >550mm account for 1.22%,11.16%,14.12%,24.60%,27.58%,21.32% of the total Loess Plateau area。 (3)The average annual precipitation was detected to have a negative trend with 0.1 significance level on the Loess Plateau. The average decreasing rate was 1.15 mm a-1, which was about 0.26% of the average annual precipitation on the region. A high degree of variability of precipitation in the 1960s was observed than other decades. Only except for Winter, which showed an insignificant positive trend in 85% of the Loess Plateau, the average decreasing rate is 0.065mm/a. other seven time series, that are Spring, Summer, Autum, wet season, dry season, EFP(early flood period) and LFP(late flood period), were detected an insignificant negative trend in 70%, 72%, 83%, 84%, 90%, 56% and 89% of the total area, respectively. The average rate of the seasonal precipitation are -0.09mm/a,-0.57mm/a,-0.19mm/a, -0.80mm/a,-0.29mm/a,-0.45mm/a,-0.47mm/a.(4)The movement of 150, 250mm and 350 mm isohyests was found not evident between decades. While the 450mm and 550mm isohyets were found to move consistently to east 134km and south 213km from 1960s to 1990s, but retreat about 12.1km west and 54.8km north, respectively, from 1990s to 2000s. The area between the isohyets of 150~250mm,250~350mm,350~450mm increased by 0.67%,0.55%,2.66%,1.29% per decade. The area of the precipitation greater than 550mm is decreased by 5.12% per decade.(5) On the frequency spatial distribution patterns, the average days per decade for the M(Modrate precipitation), H(Heavy precipitation), R(Rainstorm), HR(Heavy Rainstorm) occurrence were respectively 72.6, 25.8, 4.9 and 0.4 on Loess Plateau. That means the average return period per decade for M, H, R, HR are 0.14a, 0.39a, 2.02a and 24.51a .The corresponding average decreasing rate of the M, H, R, HR were respectively 16.0%, 3.4%, 35.5% and 42.8%. The return period of the four precipitation classification changed from 0.13a, 0.39a, 1.82a, 19.80a in 1960s to 0.15a, 0.39a, 2.34a and 26.88a in 2000s.At Lanzhou, Xining stations in the west, trends of the frequency of R and HR was insignificant. At Xian and Changwu stations in the south, the frequency of R is detected decrease trend, but the HR frequency prsent increase trend. In the north, east and the middle area, the frequency decrease significant at Guyuan, Yanan, Etuokeqi and Huhehaote. But at Suide and Taiyuan stations, the frequency of HR is detected decrease trend, but the R frequency prsent increase trend.