Study On The Effects Of Soil Surface Conditions On The Soil Erosion Of Loess Sloping Farmland

Sloping farmland is an important cultivated land resource,especially in mountainous and hilly areas,where agricultural production largely depends on sloping farmland.Compared with the common farmland,sloping farmland usually has the disadvantages of large slope gradient,poor fertility,difficult irrigation and management,etc.,and is vulnerable to the threat of soil erosion.Soil surface conditions are important factor affecting the occurrence and development of soil erosion on sloping farmland.The types of field crops,the presence or absence of soil crust,and the fluctuation of the soil surface,etc.,all belong to the category of the soil surface conditions.At present,the effects of single factor of soil surface conditions such as field crops,soil crust and soil surface roughness on soil erosion have been well known,but the effects of different single factor combinations(Composite factor)on soil erosion are still unclear.In this study,maize and soybean,the common summer crops in the Loess Plateau region,were taken as the research objects.Through different slope treatments and artificial rainfall simulation experiments on runoff plots,the effects of single factor(e.g.,slope gradient,rainfall intensity,soil surface roughness,soil crust and field crops)and composite factor(different combinations of single factor)on the soil erosion of loess sloping farmland were studied.The main conclusions are shown as follows:1.The increase of slope gradient and rainfall intensity could reduce the time to runoff and increase runoff volume and sediment yield.The increase of slope gradient could also reduce the initial loss of rain,but the increase of rainfall intensity could increase the initial loss of rain.The relationship between time to runoff,initial loss of rain,runoff volume,sediment yield and slope gradient,rainfall intensity can be expressed by power functions.The increase of rainfall intensity could also reduce the critical slope gradient on which rill erosion first appears.In addition,rill length,rill density,rill erosion amount and rill erosion percentage all increased with the increase of slope gradient and rainfall intensity.2.Soil crusting could reduce the time to runoff and the initial loss of rain while surface roughness could increase the time to runoff and the initial loss of rain.The existence of soil crust could weaken the effects of soil surface roughness on the time to runoff and the initial loss of rain.Soil crust could increase the runoff volume and sediment yield on all tested slope gradients.20° was a critical slope gradient for the effects of soil surface roughness on runoff volume.Soil surface roughness could reduce runoff volume on slope gradients lower than 20° and increase runoff volume on slope gradient higher than 20°.15° was a critical slope gradient for the effects of soil surface roughness on sediment yield.Soil surface roughness could reduce sediment yield on slope gradients lower than 15°and increase sediment yield on slope gradient higher than 15°.The critical slope gradient for the effects of Crust & Roughness on runoff volume and sediment yield was between 10° and 15°,Crust & Roughness could reduce runoff volume and sediment yield on slope gradients lower than this critical slope gradient and increase reduce runoff volume and sediment yield on slope gradient higher than this critical slope gradient.Both soil crust and soil surface roughness could reduce the critical slope gradient on which rill erosion first appears.Compared with bare ground,rill length,rill density,rill erosion amount and rill erosion percentage could be increased by soil crust and soil surface roughness.Crust & Roughness could also increase rill length,rill density,rill erosion amount and rill erosion percentage,and this kind of effects was obviously greater than soil crust and soil surface roughness.3.Both maize and soybean could prolong the time to runoff,increase the initial loss of rain,and reduce runoff volume and sediment yield.These effects were gradually increased with the growth of corn and soybean,but weakened by the increase of slope gradient and rainfall intensity.Compared with slope gradient and rainfall intensity,vegetation coverage was the main factor affecting the effects of maize and soybean in prolonging time to runoff,increasing initial loss of rain,reducing runoff volume and sediment yield.For both maize and soybean,their performances in reducing runoff volume were better than that in reducing sediment yield.The effects of soybean in prolonging the time to runoff,increasing the initial loss of rainfall,and reducing runoff volume and sediment yield were obvious greater than that of maize.Under 80 mm/h rainfall intensity,maize could prolong the time to runoff and increase the initial loss of rainfall by 109%,and reduce runoff volume and sediment yield by 30% and 49%,respectively,in its whole growth process.Under 80 mm/h rainfall intensity,soybean could prolong the time to runoff and increase the initial loss of rainfall by 176%,and reduce runoff volume and sediment yield by 39% and 57%,respectively,in its whole growth process.4.The existence of soil crust will weaken the effects of field crops in prolonging the time to runoff,increasing the initial loss of rain,and reducing runoff volume and sediment yield.The practice of breaking soil crust could effectively eliminate the negative effect of soil crust.The practice of breaking soil crust and increasing soil surface roughness could further improve the effects of field crops in prolonging the time to runoff,increasing the initial loss of rain,and reducing runoff volume and sediment yield.On slope gradient of 3°,5°,10°,15° and 20°,the practice of breaking soil crust and increasing soil surface roughness was the most effective field management practice in prolonging the time to runoff,increasing the initial loss of rain,and reducing runoff volume.On slope gradient of 3°,5°,10°,15° and 20°,the practice of breaking soil crust and increasing soil surface roughness was the most effective field management practice in reducing sediment yield.But on 20° slope gradient,the practice of breaking soil crust and was the most effective field management practice in reducing sediment yield.5.The existing crop C-factor calculation models lack the parameters related to soil surface roughness and soil crust conditions,thus it cannot respond to the changes of soil surface roughness and soil crust.Introducing soil surface roughness and soil crust parameters into the C-factor calculation model is of great significance for improving the prediction accuracy of the C-factor calculation model.C-factor was significantly correlated with vegetation coverage,plant height,roughness index and crust thickness.According to the linear or nonlinear functional relationship between C-factor and vegetation coverage,plant height,roughness index and crust thickness,four types of C-factor calculation models were constructed,and they were: Linear function model,logarithmic function model,exponential function model and power & exponential function model.The fitting effect and prediction accuracy of exponential function model was the best among them.