Impacts of cornstalk mulching buffer strip on rill erosion and its hydrodynamic character

Abstract: Cornstalk mulching buffer strip is an effective soil and water conservation practice on slope. To enlarge our knowledge about the effects and hydrodynamic mechanism of cornstalk buffer strip reducing soil erosion on loessial hillslope, this paper studied the impact of different cornstalk buffer strip position (5 and 7 m of slope length) on slope erosion process and its hydrodynamic characteristics under the typical high intensity erosive rainfall (100 mm/h) and the slope gradient that most rills happened (20o). Successive rainfall simulation experiments were carried out at rainfall simulation laboratory of the State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Yangling, from April to September in 2014. A slope adjustable pan (10 m long, 3 m wide, and 0.5 m deep) and a down sprinkler rainfall simulator system were included in the experiments. Loesial soil collected from 0 to 20 cm in the Ap horizon of a well-drained site in Ansai, Shaanxi Province, was packed according to natural soil structure on the Loess Plateau. Cornstalks collected in Yangling after harvest were settled on the different hillslope position based on rill developmental stage. 3 treatments including 1 control treatment and 2 mulching treatments were designed, and 3 successive 30-min rains were conducted in each treatment. The 1st rain was used to form rills on the hillslope with no buffer strip; cornstalk buffer strip was settled on the 5 m of slope length in both mulching treatments during the 2nd rain; while it was settled on the 5 and 7 m of slope length in mulching treatment I and II, respectively. Runoff and sediment samples were collected during successive rainfalls and rill morphology was measured after each rain. Rill flow hydraulic characters such as Reynold number, Froude number, Darcy-weisbach resistance coefficient, shear stress, unit power and unit energy of cross section were also calculated based on the measurement of runoff velocity and depth during the rains. The results showed that compared with the control, cornstalk buffer strip reduced total sediment yield and rill erosion amount by 27.2%-54.8% and 40.8%-59.2% respectively, and the contribution of rill erosion to total sediment yield was also reduced. In the aspect of rill morphology, cornstalk buffer strip reduced rill total surface area and rill average depth by 23.0%-32.0% and 6.7%-10.5%, respectively. At the late development stage of rills, buffer strip on the 7 m of slope length resulted in more sediment reduction and better rill morphology control effects. Studies on the individual rill showed that rill width appeared in a similar distribution every 1.2 m along slope length, which was shown after buffer strip was laid on the slope. Moreover, buffer strip also reduced the rill depth by accelerating the deposition process of sediment from upslope and protected the hillslope surface and rills below the buffer strip. Hydrodynamic analysis results showed that rill flow state extended from turbulence supercritical flow to laminar subcritical flow as a result of buffer strip. Compared with the rill flow above the cornstalk buffer, rill flow velocity was significantly decreased by 19.6%-21.9%, Reynold number and Froude number was decreased by 29.6%-37.9% and 8.3%-18.5% respectively, while Darcy-weisbach resistance coefficient was increased by 22.4%-43.3%. After rill flow going through the cornstalk buffer strip, shear stress, unit power and unit energy of cross section was reduced by 13.6%-21.5%, 20.0-21.0% and 9.5%-21.0% respectively, and the reduction of flow energy finally reduced the total sediment yield on hillslope. It can be drawn that cornstalk buffer strip can be settled on the hillslope surface every 5-7 m to decrease the runoff erosivity and finally decrease the rill erosion amount and total sediment yield, which can provide the valuble information for the settlement of soil conservation measures in similar terrain conditions.