Temporal variation and multiscale correlation between runoff and sediment on sloping farmland under straw incorporation in the northern rocky mountainous area of China

Straw incorporation is a common nature-based tillage measure in agriculture. Straw incorporation to regulate soil erosion on sloping farmland has received increasing attention, and excavation of this physical process needs to focus not only on the total runoff sediment of rill erosion for different straw incorporation strategies, but also on the developmental process, especially the temporal non-stationary variations and multiscale correlations of runoff and sediment. In this study, concentrate flow tests were carried out, with treatments of maize straw length (< 3, 3 - 6 cm), incorporation depth (15, 20 cm), and straw amount (4 000, 8 000 kg ha(-1)). The ensemble empirical mode decomposition (EEMD) and time-dependent intrinsic correlation (TDIC) methods were used to elucidate the temporal variations and multiscale correlations of runoff and sediment in rill erosion. The results showed that: (1) runoff and sediment decreased significantly with increasing straw length and amount. (2) the high-frequency components of runoff and sediment time series had a mean period of 2.85 min, representing turbulence fluctuations, and the low-frequency components had a period similar to that of the erosion events (12.37 min). The trend on the long-time scale had a 67 % variance contribution for runoff, which remained the dominant rill erosion process, and 26 % for sediment. (3) 4 000 kg ha(-1) straw amounts may result in more frequent fluctuations in sediment transport and increased impacts of long time scale trend on runoff yield. Short straw has a large number and overlapping distribution, increasing the amplitude of sediment fluctuations. (4) the changes in surface micro-topography and hydrodynamics via straw incorporation resulted in correlations between runoff and sediment high-frequency fluctuations showing a dynamic inversion, while having stable and weak positive correlations at the event scale. (5) The 3 - 6 cm straw length, 20 cm incorporation depth, and 8 000 kg ha(-1) straw amount are the best straw incorporation strategies for sloping farmland in the northern rocky mountainous area of China, which have obvious soil and water conservation effects. These findings determine the unsteady change process and multi-timescale characteristics of runoff and sediment, help to reveal the law of rill erosion process, and formulate reasonable management measures for straw incorporation in sloping farmland.