| Through the comparative study of the differences among the processes of hillslopeerosion and sediment yield in main water erosion regions in our country, will not only goodfor further understanding of the field of soil erosion laws, but also provide theoretical supportsfor the arrangement of hillslope soil and water conservation measurements in regions ofdifferent water erosion types. In this paper, the processes and mechanisms of hillslope soilerosion in three main water erosion regions (the Loess Plateau region, red soil low mountainhillyregion and purple soil hilly region), under different rainfall intensity (50, 75, and100mm/h) and slope gradients (5°-30°) was studied comparatively by adopting uniformresearching methodologies and experimental technologies, as well as the effects of rainfallintensity and slope gradient on the processes of hillslope erosion and sediment yield wereanalyzed. Besides, the developing features of rills and the evolution processes of erosion typeswere illustrated, and the dynamic mechanism of hillslope erosion and sediment yield were alsoanatomized. These were new developments in this field, and will provide theoreticalreferences for the establishment of soil erosion predicting model with pertinence and thearrangements of soil and water conservation measurements in our country. The mainconclusions are as follows:1. The hillslope rainfall and infiltration processes and runoff generation processes werestudied. The time of runoff generation tended to advance with the increasing of rainfallintensity and slope gradient. The order for the starting time of runoff generation was: purple,red soil and loess. The variation of slope infiltration rate along with time obey the law ofpower function, while the variation of slope runoff rate followed the law of logarithm function.The infiltration and runoff generation of purple soil hillslope and red soil hillslope entered tostable phases at about 10 minutes after the rainfall simulation began, while the loess hillslopeat about 30 minutes. The amount of runoff on the hillslopes of the three regions all decreasedwith the increase of slope gradient. It was found that the infiltration processes were closelyrelated to the crust formation and the evolution of erosion types on the hillslope. The runoffcoefficient of purple soil ranged from 0.78 to 0.93, red soil from 0.61 to 0.88, and loess from0.3 to 0.67.2. The sediment yielding processes of the main water erosion regions, and the effects ofrainfall intensity and hillslope gradient were comparatively analyzed. When the rainfallintensities were 50 mm/h and 75 mm/h and the hillslope gradients ranged from 5°-15°, or therainfall intensity was 100 mm/h and hillslope gradients ranged from 5°-10°, the red soilhillslope has the greatest sediment yield, and then the purple soil hillslope, loess hillslope was the least; under other experimental conditions, the order of sediment yield on the hillslopes ofthe three water erosion region was: purple soil hillslope>loess hillslope>red soil hillslope. Theresult indicated that hillslope gradients have important effects on soil erosion process in theloess and purple soil region. The sediment yield on loess and purple soil hillslopes reached themaximum value at the later period of the rainfall process, and the red soil hillslope at the initialstage. The sediment yield on the hillslopes of the three regions all increased with the increaseof rainfall intensity and hillslope gradient. The crucial slope gradient value of sediment yieldon the loess and purple soil hillslope appeared at 25°, but that of the red soil hillslope was notclear.3. The process of rill erosion developing in main water erosion regions and its influences onthe dynamic variation of sediment yielding were also illustrated. The time of rills formed wereshorten with the increase of rainfall intensity and hillslope gradient, as well as the rates ofheadward erosion. The headward erosion rates and sediment yielding rates of both loess andpurple soil were strong synchronism, which was weak for red soil. When the rainfall intensitywas 50 mm/h, rills on purple soil hillslopes appeared at the 26th min, and loess hillslopes at the40th min, while there was no clear rill appeared on the red soil hillslopes. When the rainfallintensity was 75 mm/h and 100 mm/h, rill on the red soil hillslopes appeared earlier than loesshillslopes and red soil hillslopes. The order of the slope length which were needed for theoccurring of headward erosion in the three water erosion regions was: purple hillslope>loesshillslope>red soil hillslope. The maximum value for the headward erosion on purple soilhillslopes and loess hillslopes were both18.5 cm/s, while red soil slopes was 10 cm/s.4. The average width and depth for rills on purple soil hillslopes were slightly wider anddeeper than that on loess hillslopes, and the rills on both soil hillslopes tended to form widerand deeper rills, the ranges of width were from 5.0 cm to 15.5 and 2.5 cm to 18.5 cm, and theranges of depth from 1.5 cm to 15 cm and 1.5 cm to 10.5 cm; while the rills formed on the redsoil hillslopes were always paralleled ones with smaller width and depth, which the width anddepth changed between 3.1 cm to 7.6 cm. Under the conditions in this study, the amount of rillerosion in the three water erosion regions all inceased with the increase of rainfall intensity orhillslope gradient. The proportion of rill erosion to total hillslope sediment yielding of was:10%-88.7% for loess, 37.1%-92.8% for purple and 14.7% - 80.7% for red soil, respectively.With the increase of rainfall intensity and hillslope gradient, the dominant erosion typeevoluted from sheet erosion to rill erosion. When the rainfall intenstiy were 50 and 75 mm/h,hillslope gradientwas 25°~30°, and rainfall intensity was 100 mm/h and hillslope gradient was15°~30°, the dominant erosion type on the loess hillslope was rill erosion; when the rainfallintenstiy was 50 mm/h, hillslope gradient was 5°~10°and rainfall intensity was 75 and 100mm/h, hillslope gradient was 10°~30°, rill erosion was the dominant erosion type on the purplesoil hillslopes ; when the rainfall intensity was 75 and 100 mm/h, hillslope gradient was20°~25°, the dominant erosion type on the red soil hillslopes was rill erosion.5. The flow types and characteristics of hydraulic parameter of the flows in the three watererosion regions were analyzed. The runoff rates in rills were influenced by rainfall intensity, hillslope gradient and the processes of generation and development of rills. And the runoffrates on hillslope followed the order: red soil>loess>purple soil. Under the conditions in thisstudy, the generation of rills, not only increased the amount of sediment yielding on thehillslope, but also caused the lateral overflowing of thin flow into the rills, which increased therunoff rate in rill , and this was more obvious on the purple soil hillslope. The runoff rateswithin the rills were 2.1 to 4.7 times greater than those of interrill on the purple soil hillslope,and were 1.1 to 2.3 on the loess hillslope, and 1.2 to 1.85 on the red soil hillslope.When therainfall intensity were 50 and 75 mm/h, the erosion amount increased sharply on both loesshillslopes and purple soil hillslopes, as the ratio of Reynolds of runoff within rills to that ofthin flow were 10 to 15; when the rainfall intensity was 100 mm/h, the sharp increase oferosion amount appeared as the ratio of Reynolds was 8.2 to 10.0. However, there was noobvious law on the red soil hillslope.6. The study discussed dynamics mechanism of sediment yielding driven by runoff erosionThe average specific energy in a cross section(E),was taken as the crutial dynamics for theoccurrence of hillslope erosion , the E value on the loess hillslopes.purple soil hillslope and red soil hillslope were E≥0.398 cm, E≥0.351 cm, and E≥0.883cm ,respectively. Moreover ,the sheer stress J was taken as the assitant crutial dynamics. The Jvalue on the loess hillslopes, purple soil hillslope and red soil hillslope were J≥5.44 Pa., J≥8.98 Pa and J≥3.87 Pa, respectively.7. this paper also estimated the soil erodibility K (t·hm2·h·hm-2·MJ-1·mm-1) The K value wasobtained by the Erosion and Productivity impact EPIC . The K for loess was 0.053, 0.058 forpurple soil and 0.032 for red soil, respectively. The amount of erosion caused by unit rainfall orby unit runoff followed order: purple soil hillslope>loess hillslope>red soil hillslope. Theaverage amount of loess eroded by per milimeter precipitation was 71.6 g/m2·mm-1, and that ofpurple soil and red soil was 155.8 and 40.7 g/m2·mm-1, respectively. While the average amountof loess eroded by per milimeter runoff was 161.5 g/m2·mm-1, and that of purple soil and redsoil was 171.75 and 64.1 g/m2·mm-1, respectively.
|
PDF Full Text Request