Climate Change And Its Impacts On River Runoff And Sediment In Northeast China During The Past 50 Years

Northeast China is the region which is extremely sensitive to climate change, has the most vulnerable ecological environment in China. Frequent drought and flood disasters and soil erosion in Northeast China have been important factors which restrict the development of society and economy in recent years. Therefore, analyzing the change trend of climate change and assessing the effect of climate variability on water and sediment in detail can provide important information for disaster prevention and reduction, water resource management, agricultural production and ecological construction. Based on the daily series temperature, precipitation observations and the yearly series runoff and sediment load data over the period of 1958-2008, this study selected the Songhua River Basin and Northeast China as research sites, combing various research methods (non-parametric Mann-Kendall test, the estimated sen slope method, double mass curve and linear regression), to analyze the change trends and spatial distribution characteristics of meteorological and hydrological elements. The effects of climate change and human activities on runoff and sediment at different stations during different periods were assessed. The main conclusions are as follows:(1) The change trends, seasonal change, spatial distribution of average temperature and precipitation in the Songhua River Basin were analyzed. The average annual precipitation and temperature of the Songhua River Basin tended to decrease slightly and increase significantly during 1958-2008. The change range of average precipitation and temperature increased/decreased from southeast to northwest. The decreasing magnitude of average annual precipitation and precipitation in flood season were 0.81 mm/yr and 1.21 mm/yr. The average precipitation of winter and spring showed a upward trend with the change magnitude of 0.03 mm/yr and 0.13 mm/yr, while the average precipitation of summer and autumn repented a downward trend with the change magnitude of 0.56 mm/yr and0.45 mm/y. The average annual temperature which underwent abrupt change in 1990 has risen about 2.0℃during the past 51 years. The average temperature of winter increased at a faster rate (0.06℃/yr), whilst the average temperature of summer had the smallest increment—0.02℃/yr. The increasing magnitude of average annual temperature in the Songhua River Basin was much greater than the national and global level.(2) Based on the nine international extreme indices (pq90, px5d, pint, pxcdd, pnl90, txq90, tnq10, tnfd, txhw90), this study analyzed the spatial and temporal change trends of Northeast China. Results showed that the indices of extreme climate were spatially distributed with obvious gradients from the south to the north. Heavy rainfall threshold pq90, hot-day threshold txq90, cold-night threshold tnq10, greatest 5-day total rainfall px5d, simple daily rainfall intensity pint increased from north to south while the longest heatwave txhw90 and frost days tnfd decreased form north to south. Most stations had monotonic trends in extreme indices, nonetheless, the number of stations with significant trends varied greatly; only six or seven stations in precipitation-based indices had significant trends, but 23%, 68% of stations in extreme high temperature indices and over 90% of stations in extreme low temperature showed significant trends. The slight downward trends of frequency and intensity of extreme precipitation and the significant upward trend of drought indicator and extreme temperature indicated that extreme precipitation events did not change significantly, but tended to decrease slightly while the drought aggravated. Meanwhile, extreme temperature events became more severely and frequently and the length of winter shortened in Northeast China during the past 51 years.(3) Based on the measuresd data of the five main hydrological stations (Jiangqiao, Daben, Fuyu, Haerbin, and Jiamusi), the change trends of runoff and sediment were investigatived. Results showed that runoff of the five main stations presented a downward trend during 1958-2008. Among them, the decreasing trend of Daben, Haerbin and Jiamusi were significant at the 95% confidence level; the runoff of Jiamusi reduced in the rate of 5.50×108m3/yr, which is the greatest one. In the mean time, the sediment load of Daben, Fuyu and Haerbin showed significant decreasing trend while other two stations had insignificant increasing trend. The decreasing magnitude of sediment load at Harebin station was 8.55×104 t/yr, which was much larger than other stations.(4) The effects of climate change and human activeies on runoff and sediment in different stations of the Songhua River Basin during different periods were analyzed and assessed. In general, human activities play more important role in the evolutionary process of runoff and sediment, the percentage of affecting runoff and sediment load was 52.80% and 81.84%, respectively. In contrast, the changes of runoff and sediment were caused by climate change, with contribution being 47.20% for the former and 18.16% for the latter. The impact of climate change on the decrease of runoff has exceeded the impact of human activities gradually from the 1990s. The impact of human activities on sediment has been mitigated as time goes on, while climate change began to exert an increasing important influence on sediment load. In terms of regional variation, the runoff of middle and lower reaches of the Nenjiang River and the middle reaches of the main stream of the Songhua River were all greatly affected by climate change, and the influence extent of climate change on runoff tended to intensify as the years progressed. Conversely, human activities contributed 123.79% for the change of runoff in the second Songhua River and 156.32% for the lower reaches of the main stream of the Songhua River, which had greater impact than climate change. The impact of climate change and human activity on sediment load at different section is not completely corresponding exactly with the impact of the two on runoff. The impact of human activities on the change of sediment load exceeded that of climate change in the middle and lower reaches of the Nenjiang River and the lower researches of the Second Songhua River. The effect of human activities on the change of sediment load at the above three section were 94.09%, 145.74% and 86.36%. In the middle reaches of f the main stream of the Songhua River, climate change and human activities nearly had equal effect on the change of sediment load with the contribution of 52.77% and 47.23%. The sediment load in lower reaches of the main stream of the Songhua River was affected greatly by climate change in 1980s and 21st century, while human activitie play more important role for the change of sediment load in the 1990s, the percentage reach up to 96.86%.