| Straw returned into farmland can increase soil organic matter while reducing air pollution caused by straw burning.Mixing the straw into the cultivated soil can change surface roughness,reduce the soil crust formation caused by rain drop and in turn affect the rainfall runoff process and hydrodynamic parameters.The flow velocity and hydrodynamic parameters,which characterize the hydrodynamic characteristics of slope,are important factors affecting soil erosion,runoff and sediment transport.Laboratory experiments were made to measure the velocities of water flow under four incorporation rates(0,2,4 and 8 ton/ha),three rainfall intensities(80,120 and 160 mm/h)and three slope gradients(10,15 and 20 degrees),on straw incorporation soil slopes.The electrolyte pulse method was used to measure the rainfall runoff velocity and hydrodynamic dispersion coefficient of straw incorporated slope,and the relevant hydrodynamic parameters(Reynolds number,Froude number and resistance coefficient)were calculated.The results show that the rainfall duration affects the flow hydrodynamic characteristics,but not change the flow regime.The change process of flow hydrodynamic characteristics can be divided into two stages with the increase of accumulated rainfall on bare soil slopes and straw incorporation(4 t/ha)slopes.Before the accumulated rainfall reached about to 100 mm,the flow velocity,Reynolds number and Froude number increased with the increase of accumulated rainfall,and the Darcy-Weisbach roughness coefficient decreased with the increase of accumulated rainfall;after 100 mm,the flow velocity and Froude number tend to be stable with the increase of accumulated rainfall,and the Reynolds number and DarcyWeisbach roughness coefficient increased with the increase of accumulated rainfall.The flow regime had little change with the increase of rainfall duration.On the bare soil slope,the flow was a supercritical laminar flow;on the 4 t/ha straw incorporation slope,the flow was a subcritical or critical laminar flow.The change range of time-dependent hydrodynamic parameters of straw incorporated slope was small,while that of bare soil slope was large.Based on the data of 100 mm cumulative rainfall,the effects of straw incorporation rate(0,2,4,8 t/ha),rainfall intensity and slope gradient on the runoff velocity and hydrodynamic parameters were analyzed.The results show that the flow velocity decreased exponentially with the increase of straw incorporation rate.The mean flow velocity ranged from 0.17 to 0.23 m/s over the bare soil slope,while the mean flow velocity ranged from 0.10 to 0.17 m/s,0.09 to 0.13 m/s,and 0.06 to 0.11 m/s over the soil slopes with straw incorporation rates of 2,4,8 t/ha,respectively.Compared with the flow velocity of 0 t/ha,the 2,4 and 8 t/ha straw incorporation rates reduced flow velocities by 28.44%,44.09% and 55.56%,respectively.Higher rainfall intensity and slope gradient caused increase in flow velocity.There was no significant difference(P<0.05)between two adjacent rainfall intensities and slope gradients.A nonlinear multiple regression model was used to fit the measured velocities,and the coefficient of determination is greater than 0.92,which shows that the fitting equation can predict well the effect of straw incorporation rate,rainfall intensity,and slope gradient on the flow velocity.Straw incorporation reduces the flow dynamics(Reynolds number,Froude number and hydrodynamic dispersion coefficient decreases),increases the slope resistance(resistance coefficient increases),changes the rainfall flow regime on slope farmland,and the straw incorporation rate of 4 t/ha and above makes the flow change from supercritical to subcritical.Hydrodynamic parameters were calculated based on the flow velocity and runoff discharge when the accumulated rainfall is 100 mm.The results show that straw incorporation had significant effect on the flow regime of rainfall runoff.With the increase of straw incorporation rate,Reynolds number and Froude number decreased,and the Darcy-Weisbach roughness coefficient increased.The flow regime changed from supercritical flow to critical and subcritical flow with straw incorporation of 4 t/ha and above.Straw incorporation reduced the hydrodynamic dispersion coefficient of water flow,the average hydrodynamic dispersion coefficient of 0,2,4,8 t/ha was 0.020,0.011,0.007,0.005 m2/s,respectively,but there was no significant difference(P<0.05)between two adjacent straw incorporation rates.On the one hand,straw incorporation can reduce the flow energy and make the flow stable;on the other hand,straw can increase the tortuosity of the flow path and the turbulence of the flow to a certain extent.Straw incorporation had a significant effect on the flow regime and resistance characteristics of slope,rainfall intensity,slope gradient and rainfall duration were also important factors.The results of the study can provide support for the study of the effect of returning straw to the field on the soil erosion of sloping land. |
