基于时变矩BMA方法的赣江流域年输沙量变化归因分析

Attribution analysis of annual sediment load of Ganjiang River Basin using BMA based on time-varying moment models

分析河道泥沙影响因素并进行定量归因分析,对正确认识和评价流域水土流失治理措施有着重要意义。该研究通过对鄱阳湖赣江流域1958-2014年的年输沙量变化特征分析,引入降雨侵蚀、森林覆盖以及水库调控3个影响因素建立年输沙量时变矩模型,利用BMA方法融合4个时变矩模型模拟赣江年输沙量过程,并定量归因分析年输沙量变化中多因素的贡献程度。研究表明:1)以降雨侵蚀力、森林覆盖率和水库调控系数为主要物理影响因子构建的时变矩模型,可以较好地描述赣江年输沙量分布特征及时变过程,纳什效率系数达到0.93;2)BMA方法有效融合了多个时变矩模型的优点,年输沙量模拟的不确定性区间变小,通过遍历多因素情景组合可定量分离不同影响因素的贡献量及变化范围;3)相对于基准期(1958-1989年),变化期(1990-2014年)赣江流域年输沙量显著减少(P<0.05),主要受人类活动影响,与双累积曲线法和产沙函数法的定量归因结果相近。森林覆盖率和大中型水库调控对赣江流域年输沙量减少贡献量分别为414.87和261.45×10^(4)t/a,两者贡献率分别为59.72%、37.63%,而降雨侵蚀导致年输沙量增加约42.99×10^(4)t/a,其他因素减少年输沙量61.40×10^(4)t/a。对于赣江流域,持续加强流域森林资源的保护,强化大中型水库的调控能力,是提升流域水土流失治理能力的重要举措。研究结果可为流域水土流失综合治理提供科学参考。

Suspended sediment is a key driving factor in river evolution and ecosystem stability.However,there is a significant change in the annual suspended sediment loads in many basins under a current rapidly changing environment.Therefore,it is highly demanding to clarify the influencing factors and their attributions for the suspended sediment,thereby improving control measures of soil and water loss in basins.Taking the Ganjiang River in the Poyang Lake Basin of China as an example,the forest coverage rate increased by nearly 40%in the last 30 years,where there were 139 large and medium-sized reservoirs in 2014.In this study,the annual suspended sediment of the Ganjiang River Basin was firstly determined during the period of 1958-2014.Then,time-varying moment models were constructed for different types of probability distributions,where the factors of rainfall erosivity,forest coverage,and reservoir regulation were taken as explanatory variables.Finally,the Bayesian Model Averaging(BMA)was selected to combine four time-varying moment models for the simulation of annual suspended sediment load,and thereby to separate the attributions of different factors.The results were as follows:1)Rainfall erosivity,forest coverage,and reservoir regulation were all key factors affecting the suspended sediment load.The time-varying moment model was constructed using four probability distributions(Gamma,Pearson-Ⅲ,Weibull,Lognormal)considering the time-varying of location and scale parameters,where the rainfall erosivity(Re),forest coverage rate(Fc),and reservoir regulation coefficient(Rc)were utilized as physical covariables.The distribution characteristics and process changes of annual sediment load were reasonably described in the study areas,with a Nash-Sutcliffe Efficiency coefficient(NSE)of 0.93;2)BMA improved the simulation effect,while reduced model uncertainty,compared with the single time-varying moment model.The Root Mean Square Error(RMSE)and Relative Error(RE)of simulated value were 125.96×10^(4) t/a,and 11.41%,respectively.The Containing Ratio(CR)and average band-width(B)of uncertainty interval were 94.47%,and 481.38×10^(4 )t/a,respectively.A fixing-changing method was selected to perform the attribution analysis of suspended sediment load using BMA under various scenario combinations.3)The annual suspended sediment load showed a local trend in the study area from 1958 to 2014,with the change point year of 1989.There was a significant decreasing trend of annual suspended sediment load at the 5%significance level during the period from 1990 to 2014,due possibly to the increase of forest coverage during the regulation of large and medium-sized reservoirs.Specifically,the decreased sediment loads were 414.87×10^(4) and 261.45×10^(4) t/a,for the increase of forest coverage and reservoirs in 1958-1989,indicating a great contribution of 59.72%and 37.63%,respectively.The factor of rainfall erosivity increased the suspended sediment load by about 42.99×10^(4) t/a,whereas,the forest coverage and reservoir regulation caused the decrease of 61.40×10^(4)t/a.Furthermore,the decrease of suspended sediment load was actually caused by human activities,indicating better agreement with the conclusions obtained by the double cumulative curve and sediment yield function.The dynamic characteristics of the first and second moment were well elucidated in the observed samples using the BMA based on time-varying moment models,with an exact description for the mean value and variance.Therefore,the attribution analysis on the suspended sediment load was more objective and reliable in practice.Some recommendations were made here to strengthen the protection of forest resources,and the scientific regulation of large and medium-sized reservoirs,further to improve the control ability of water and soil loss in the Ganjiang River Basin.