从长期来看流域系统的侵蚀与产沙基本可以达到平衡,泥沙输移比约等于 1。但次降雨或分年度泥沙输移比有相当大的变幅,在短期内会经常存在泥沙的滞留和滞留的泥沙被重新侵蚀搬运,而出现泥沙输移比小于 1和大于 1的情况。这主要与降雨的空间分布和洪峰增减幅度及径流深度增减幅度密切相关。

The relationships between erosion and sediment yield, and the effect of rainfall runoff and silt laden runoff on sediment delivery ratio of drainage systems in the loess gully hilly areas are discussed based on data of Dali He drainage basin system. The characteristics of space time distributoin of delivery ratio in the basins of various scales and the power mechanism of single rainstorm event with flood due to increase and decrease of flood peak or runoff depth are illustrated by analyzing more annual single rainfall events. The results are as follows: (1) that the delivery ratio is close to 1 for long term (average value of more annual rainfall) while that estimated to be >1 or <1 for short term (single enent or annual rainfall) for various events in Dali He drainage basin. This phenomenon will frequently occur as a result of sediment retaining as well as re erosion and delivery of the retained sediment in a short period of time. (2) Sediment delivery ratio is closely related to the spatial distribution of rainfall magnitude of rise and fall of peak flood and that of runoff depth in the basin system. Changes of mechanism of delivery ratio ( D r ) can be explained by runoff depth ratio ( H b ) or rise of flow shear force ( τ ) with falling of runoff depth in the basin system. The flow shear force can be described as: τ=γ mhJ as a unit drainage basin τ=γ m(H b)hJ or as accretion of basin areas (3) Delivery ratio ( D r ) is closely related with runoff depth ratio ( H b ), single event rainfall distributing ratio ( P b ) and change amplitude of peak flood ratio ( H f ) in the basin system. The interrelation can be expressed as: D r=0 403P b 0 37 H b 1 066 H f 0 191 r=0 996 n=125 in Chabegou (4) Delivery ratio is close to 1 as H b is 1 due to even distribution of rainfall runoff; D r is >1 as H b >1 for flow kinetic energy increase and D r is <1 as H b <1 for flow kinetic energy decay.