Effects of Rice Straw Size on Flow Velocity and Rill Erosion: A Laboratory-Scale Experiment

The residues of rice production could be used as a mulch to reduce the effects of rill erosion on long and steep hillslopes. However, there is a need to identify the most effective size of this residue to apply as a countermeasure of rill erosion, exploring its effect on hydraulic variables. Several investigations have focused on the anti-erosive effects of other crop residues, while experiences on rice straw applications to reduce rill erosion are still lacking. To fill this gap, this study has measured the variability in flow velocity, stream power and the resulting soil loss in a rill covered by rice straw. Flume experiments simulating rill erosion have been carried out comparing soil loss among treatments with rice straw (dose of 3 tonnes ha−1 and lengths between 20 and 70, 80 and 130, or 140 and 190 mm) and a non-mulched control. Moreover, a multiple regression model that predicts soil loss for a rill cover with rice straw of a given length has been proposed. The application of rice straw reduced the soil loss by at least 20% compared to bare soils. The most suitable size of the applied straw was 90 to 130 mm, which reduces soil loss by 45%. Finer straw (20 to 70 mm) did not significantly improve the soil’s resistance to rill erosion. The beneficial effects of straw must be ascribed to the reduction in flow velocity due to the presence of straw, as shown by accurate power equations regressing the soil loss to this variable. In spite of some limitations (small experimental scale, local environmental conditions, and low incorporation level of the substrate), the results are useful for land managers and hydrologists for soil conservation in hillslopes subjected to intense rill erosion and with similar climatic and hydrological and geomorphological conditions as the case study.