Land use and crop characteristics influence soil loss obviously,so that a quantitative evaluation of the effect of different
crops on soil loss(crop factor)is essential to land use and soil conservation planning.The effect of a given crop on soil erosion varies a great deal within the period from seeding to harvest because crop characteristics and canopy cover varies greatly in different seasons.Moreover,the distribution of the erosive rainfall within a year differs for different localities.Therefore,crop factor that takes both the protection of crop and rainfall erosion index into account is widely adapted in
soil loss prediction in many countries.Although some research on C (crop)factor have been reported in China,there were much disagreement between those C values;so no C values or related parameters is widely adapted in China so far.The main reason is that the present C values seldom include the influence of rainfall patterns and variety of crop coverage in a year;or crop stages were not defined clearly.Another reason is the compared fallow conditions are not consistent.The purpose of this study is to calculate soil loss ratio from different types of cropland to bare fallow land,on the basis of which crop factor on loess plateau is estimated. 108 plot year observation data from Tianshui,Gansu during 1945～1953(slope length 20m,slope width 5m,slope gradient 5°,8°,14°,17°,including lentil,winter wheat,buckwheat,maize intercropping with soybean grown by traditional tillage in rotation)and 48 plot year observation data from Ansai,Shaanxi during 1987～1992(slope length 20m,slope width 5m,slope gradient 25°,including buckwheat,potato,millet,soybean,which are grown by traditional tillage in rotation,fallow and two types of grass,sainfoin adsurgens)on loess plateau was analyzed in this study. In order to take account of variance of crop coverage,a crop year is divided into the following six crop stage periods according to crop coverage change with time:fallow(from plowing to preparing seedbed),seedbed(preparing seedbed to 10% coverage),established(10%～50% coverage),development(50%～75% coverage),mature(75%～harvest)and residue and stubble(harvest plowing).The length of each crop stage of specific crop is given in a table. Obtaining soil loss ratios(SLR)for individual crop stage periods are essential to derive the value of C factor.By analysis of 156 plot soil loss observations from two experimental field stations,soil loss ratios for each crop stage period of 7 crops are presented.In Ansai,soil consolidation caused the esodibility of fallow plots to decrease significantly after the only tillage in spring and regression analysis shows that monthly average erodibility(Soil Loss/ PI 30)decreases linearly within six months after tillage with the R 2=0.858.In order to keep the compared fallow conditions principally consistent,each soil loss was adjusted to the average level of the fallow plots by multiplying a modifying factor.Factor in the first month after tillage is 0 68;the second month,0.79;the third month,0.94;the fourth month,1.16;the fifth month,1.51;the six month,2.17.Then SLR of every stage of 4 crops was calculated.In Tianshui station,measured soil losses from cropped plots were compared with seedbed within 20 days after planting or plots two months after harvest(quasi fallow)because of no experimental plot kept fallow.When SLR was calculated,all chosen observation data from 27 rainfall events in each of which “quasi fallow plots” can be identified was grouped according to crop stage of different crops.For each rainfall event,12 plot observations were divided again into 4 sub groups according to slope.There is a “quasi fallow plots” among each sub group.The ratio of total soil loss from each crop stage to the total soil loss from corresponding “quasi fallow plots” with same slope is calculated as the SLR of that crop stage. On the bases of soil loss ratios presented here and erosion index dis