黄河中游多沙粗沙区1997-2007年的水沙变化趋势及其成因

Trend of Sediment Yield in the Coarser Sediment-producing Area in the Middle Yellow River Basin in the Period 1997-2007 and the Formative Cause

基于近期水文泥沙和降水资料对黄河中游多沙粗沙区水沙变化进行了研究,结果表明,1997年以来,黄河中游多沙粗沙区的水沙变化出现了新的特点,产沙量变化趋势发生了改变,表现出明显的减小趋势。1986-1996年间,河口镇至龙门区间平均产沙量为5.08亿t/a;1997-2007年间,减小为2.33亿t/a,减小幅度达54.1%。在总体上来看,从1970年开始,河口镇至龙门区间产沙量具有明显的减小趋势。但在总体下降的趋势中,表现出次一级的变化趋势,即产沙量先下降,在1986年达到最低值后再增大,在1996年达到最大值后再减小。1998-2006年,产沙量与降水量之间的决定系数仅为0.017,二者不相关,产沙量的变化与降水量变化的关系不大。这说明,人类活动已经完全改变了产沙量与降水量关系,成为支配产沙过程的主导因素。1997年以后,水沙关系也发生了根本性的变化。1970年以来,河龙区间水土保持措施的减沙效益表现出复杂的变化,减沙效益先增大,达到峰值后减小,减小到最小值后再增大。人类活动对产沙量变化的贡献率呈增大趋势,1998-2006年间人类活动对产沙量变化的贡献率达到65%,年降水量变化对产沙量变化的贡献率减小为35%。1997年以来河口镇至龙门区间产沙量的大幅度减少,与水土保持措施的加强、大面积退耕还林草的实施、淤地坝建设力度的大大加强、大面积封禁治理的实施和大规模农村剩余劳动力的转移有密切的关系。

Based on hydrometric and precipitation data,a study has been made on the temperal variation in the drainage area between Hekouzhen and Longmen in the Yellow River basin. It has been found that the trend of sediment yield changed from increasing in the period 1986-1996 to decreasing in the period 1997-2007. During the period 1986-1996,the mean annual sediment yield was 5.08×108 t,but it decreased to 2.338×108 t for the period 1997-2007,decreased by 54.1%. As an overall tendency,the annual sediment yield in the Heliuzhen-Longmen area decreased,but there were some secondary trends. The annual sediment yield decreased to a minimum appeared in 1986,then increased to a maximum in 1996,followed by a decrease. For the period 1998-2006,the squared correlation coefficient r^2 between annual sediment yield and annual precipitation was as low as 0.017,indicating that the variation in sediment yield is not dependent on the variation in precipitation. This means that human activity has greatly changed the ＂normal＂ relationship between sediment yield and precipitation,and become the dominant control for sediment yield. After 1997,the relationship between annual sediment yield and annual runoff has also been totally changed. Since 1970,the sediment reduction rate （defined as the ratio of sediment reduction by soil and water conservation measures to the sediment yield in the supposed condition of unchanged precipitation） shows a complicated variation. It is increased to a maximum,then decreased to a minimum,followed by an increase. The contribution of human activity to the reduction in sediment yield showed an increasing trend. In the period 1998-2006,it was 65%,and the contribution of precipitation was only 35%. The marked decrease in sediment yield after 1997 can be explained by the enhanced soil and water conservation measures,the large-scale implementation of ＂replacing cultivated land by forestation＂ ,building of large number of check-dams for sediment trapping,putting the practice of ＂closing hillsides to facilitate affrestation＂ in large area,and large-scale transferring of surplus rural labors.