黄土丘陵沟壑区典型流域产沙的降雨阈值变化

2000年以来,随着黄土高原林草植被大幅改善和大规模梯田建成,加之坝库拦截,黄土高原入黄沙量锐减,从而使黄土高原现状产沙情势的认知成为近年关注的热点。目前,增大林草和梯田覆盖状况可减少或遏制侵蚀产沙的观点已被广泛认可,但其在流域层面上的减沙机制仍有待深入认识。本文以黄土丘陵沟壑区30条面积较小的流域作为样本,以场次降雨为分析对象,以场次降雨的面雨量、最大1 h降雨量和降雨侵蚀力为降雨指标,以场次降雨的产沙强度≥500 t/km2为流域产沙的判断标准,探讨了林草梯田覆盖程度变化与可致流域产沙的降雨阈值之间的响应关系。研究表明,无论地貌类型如何,随着流域林草梯田覆盖程度的增大,流域产沙的降雨阈值均明显增加,其中黄土丘陵第1—3副区的降雨阈值与林草梯田有效覆盖率之间呈正向的指数函数关系。在相同下垫面下,黄土丘陵第1—3副区的降雨阈值差别很小;或因植被主要分布在梁茆,或因河道产沙占比较大,因此,砒砂岩区、黄土残塬区和黄土丘陵第5副区更易产沙,其降雨阈值明显小于第1—3副区。本成果不仅为诠释林草梯田减沙的机制提供了科学支撑,而且也可为黄河水沙情势的评价和入黄沙量预报提供重要参考。

Changes of rainfall threshold for sediment producing in the loess hilly and gully region of the Loess Plateau

Since 2000, with the significant improvement of vegetation, the large-scale construction and operation of terraces in the Loess Plateau, as well as the interception of reservoirs and check dams, the amount of sediment from the Loess Plateau into the Yellow River has decreased significantly and sharply, which made the current situation of sediment producing in the Loess Plateau become a hotspot in recent years. So far, the viewpoints have been accepted widely that the increase of vegetation-terrace coverage can restrain the sediment production, but its role mechanism still need to study further at watershed level. In this paper,30 small watersheds in the Loess Plateau were chosen as sample basins. Based on the observation data of rainfall events, the rainfall and maximum 1-hour rainfall and rainfall erosive power of individual rainfall event were selected as rainfall indexes, and the sediment yield intensity 500 t/km² was used as the criterion for determining sediment producing of the watersheds, then the response relationship between the change of vegetation-terrace coverage and the rainfall threshold that can lead to sediment producing in the watershed was discussed. The results reveal that the rainfall threshold that can lead to sediment producing in the watershed increases significantly with the increase of vegetation-terrace coverage, regardless of geomorphologic type. Among them, there is a positive exponential relationship between the rainfall threshold and the effective coverage of vegetation and terraces in the 1st to 3rd sub-regions of the loess hilly region. With the similar underlying surface, there is little difference between the rainfall threshold in the 1st to 3rd sub-regions of the loess hilly region, while the rainfall thresholds in the soft sandstone area, loess gully area, and the 5th sub-region of loess hilly area is significantly lower than that in the 1st to 3rd sub-regions, because of the sparse vegetation or because of the higher ability of sediment producing in the riverbed. This research can provide an important reference for explaining the mechanism of sediment decrease with vegetation-terrace coverage increasing, for evaluating the water and sediment conditions of the Yellow River,and for predicting the amount of sediment entering the Yellow River.