Impact Of Human Activities On The Runoff And Sediment Loaded Change Of The Yan River Basin Based On The Similar Climate Condition

The impact of human activities on runoff and sediment loaded change is one of key topics worldwide. The runoff and sediment loaded change is effected by climate factors and land surface activities synthetically. Because soil characteristics, landform and natural vegetation changed very slowly naturally in the short term, the runoff and sediment loaded change are effected by human activities in the same climate (precipitation and potential evaporation) conditions according to the water balance principle and the hydrological characteristics of runoff generation over infiltration in Loess Plateau.Yan River Basin is located in the loess hilly gully region with frequent and large scale human activities such as soil and water conservation. A 50-year data series from 1952 to 2001 annual precipitation in the basin, runoff and sediment loaded at Ganguyi Hydrology Station of the Yan River and the meteorologic data of 15 weather stations within and near the basin is analyzed, and got the monthly potential evapotranspiration of Yan River Basin with FAO Penman-Monteith Equation. In the paired-periods with similar climate condition, the depth of precipitation are similar (difference <3%) , the processes of monthly precipitation change are similar (correlation significance of monthly data level <0. 05) and the potential evapotranspiration are similar. The impacts of human activities on runoff and sediment loaded change is analyzed based on the similar climate condition. The main findings are as follows:(1) Characteristics of runoff and sediment loaded of the Yan River;The annual runoff of the Yan River ranges from 1. 1×108 to 5. 0×108m3, the annual mean value being 2. 1×108m3 and the great coefficient of variation (CV 0. 339) among the dataset. The sediment loaded is from 0. 079×108 to 1. 8×108t, the annual mean value being 0. 46×108t and the coefficient of variation being much higher great (0. 756) . The distribution of monthly runoff and sediment loaded is very uneven within the year. The most sediment loaded (about 99. 6%) and runoff (66. 7%) is in the flood season from May to September respectively. There is a small peak of runoff change in March.(2) Characteristics of precipitation and evaporation;The annual precipitation of the Yan River Basin varies from 259. 9 to 853. 7mm, the annual mean value being 504. 8 mm and the coefficient of variation is 0. 253. The annual potential evapotranspiration various from 1729. 5 to 1964. 1mm, the annual mean value being 1825. 0 mm and the coefficient of variation is very small (0. 032) . The distribution of precipitation and potential evapotranspiration with the same year is also very uneven. The precipitation and potential evapotranspiration in the flood season (From May to September) accounts for 79. 3% and 68. 7% respectively. The spatial distribution of potential evapotranspiration is obvious that the low center is in the southwest of the Yan River Basin (the upper region of the Xichuan River) and shows an increase from the south-western to the north-eastern, the high values region is in the Yanchang County.(3) In the paired-periods with similar climatic conditions, the depth and process of precipitation and the potential evapotranspiration are both similar, and the number of the years the paired-periods involved is enough. It can be the precondition to analyze the impact of human activities on runoff and sediment loaded change of the Yan River Basin. The number of the years in the paired-periods with similar climatic conditions accounted for 90% of the total number of years in the Yan River Basin from 1952 to 2001. The average difference of annual precipitation in the paired-periods is 0. 88% (about 4. 4mm) , the average correlation coefficient of precipitation is 0. 7504 (Significance level 0. 05) . The difference between potential evapotranspiration is small enough that the precondition of runoff and sediment loaded in the paired-periods is reasonable.(4) The results show that there are clear stages of runoff and sediment loaded change influenced by human activities. When comparing with period from 1952 to 1970, the precipitation, runoff and sediment loaded reduced by 4. 5%, 22. 3% and 52. 4% respectively in the stage from 1971 to 1982, the impact of human activities plays a dominant role on runoff and sediment loaded change in this stage; In the stage from 1983 to 1992 the precipitation and runoff reduced by 2. 9% and 3. 0% respectively while the runoff increased by 1. 1% caused by human activities, so the decrease of rainfall is the main reason of runoff change. The human activity is the main factors which caused 29. 6% decline of sediment loaded; The precipitation, runoff and sediment loaded reduced by 14. 4%, 19. 7% and 41. 8% respectively while the runoff and sediment loaded increased by 10. 7% and 13. 0% respectively caused by human activities in the stage from 1993 to 2001, the decrease of rainfall is the main reason of runoff and sediment loaded reduction in this stage.This study compared and analyzed characteristics of changes in runoff and sediment loaded on the premise of selecting the strict similarity climate period, and can improve the objectivity and accuracy of evaluating process and results, thereby enriching theories and methods of research in the field such as climate change and environment effects of human activities.