| Exploring the relationship between rainfall and runoff in the small watersheds on the Loess Plateau is of great significance for flood control,soil erosion control,and vegetation restoration on the Loess Plateau.In this study,the Caijiachuan small watershed in Ji County,Shanxi Province,a typical residual gully region of the Loess Plateau,was selected as the research object,explore the relationship between runoff and sediment transport characteristics and rain patterns in different types of small watersheds.It provides references and basis for the prevention and control of soil erosion and the construction of hydrological models in small watersheds in the Loess Plateau.The important conclusions are as follows:?1?In the rainy season?May-October?of the Caijiachuan small watershed from 1990 to 2017,the rainfall was between 277.5mm and 696.0mm,with an average rainfall of 391.8mm.The rainfall concentrated in July and August,with rainfall of 110.0 mm and 91.3 mm,accounting for 51.24%of the rainfall.?2?The Caijiachuan small watershed from 2004 to 2018 had the most rainfall between 0 and 6hours,with 109 rainfalls.Erosive rainfall is divided into heavy rainfall,long-duration rainfall?Type A?,moderate rainfall?Type B?,and small rainfall?Type C?.The Type A rainfall has the least frequency?13fields?,the maximum rainfall?43.97mm?and the minimum rainfall intensity?1.52mm/h?.The rainfall of Type B rainfall is 24.18mm,the rainfall intensity is 1.97mm/h.Type C has the highest number of rains?136 fields?,the least rainfall?12.62mm?,and the highest rainfall intensity?6.34mm/h?.According to the location where rainfall concentratedly appears on the time axis,on the basis of the three types of rain classification,the process of all rainfall in the Caijiachuan small watershed is divided into pre-type rainfall?Type I,87 fields?and mid-type rainfall?II Type,53 fields?,late type rainfall?type III,42 fields?and uniform type rainfall?type IV,33 fields?.The average rainfall of the Caijiachuan small watershed is52.6mm,the average rainfall intensity is 5.26 mm/h,and the rainstorms from July to August accounted for 67.9%.pre-and mid-type storms are the main types of storms in the basin.?3?The type of rainfall has a significant impact on runoff.The runoff depth of Type A rainfall in each small watershed is 0.136mm?0.804mm,and the runoff depth of Type B rainfall is 0.160mm?0.712mm,which are significantly higher than the runoff depth of Type C rainfall?0.040mm?0.267mm?,The lag time of flood peaks formed by Type A and Type B rainfall is 0.88h?4.67h and 1.01?4.23h respectively,and the lag time of flood peaks formed by Type C rainfall is relatively short,0.83h?3.32h.The rainfall process has a significant impact on runoff.The runoff depths of the Caijiachuan small watershed under different rain conditions are:Type I?0.108mm?0.658mm?,Type II?0.078mm?0.682mm?,Type III?0.064mm?0.169mm?,Type??0.022mm?0.125mm?,the lag time of Hongfeng is respectively Type IV?3.95h?5.18h?>Type I?3.16h?4.08h?>Type II?2.68h?3.11h?>Type III?1.41h?3.00h?.?4?The runoff depth of the Caijiachuan small watershed from 2005 to 2015 in the rainy season was between 2.26mm and 12.70mm,with an average runoff depth of 5.71mm.The runoff depths of different types of small watersheds during the rainy season are farmlands and grasslands watershed?11.01mm?>agricultural watershed?10.98mm?>natural forests watershed?5.63mm?>enclosed watershed?4.97mm?>artificial forests and natural forests watershed?4.31mm?.?5?The modulus of rainfall and sediment transport in the Caijiachuan small watershed is0.002t/km2?20.878t/km2,the natural forests watershed is 0.002t/km2?4.473t/km2,the agricultural watershed is 0.003t/km2?134.587t/km2,the artificial forests watershed is 0.003t/km2?117.250t/km2,the enclosed watershed is 0.001t/km2?25.072t/km2,the artificial forests and natural forests watershed is0.001t/km2?98.547t/km2,the farmlands and grasslands watershed is 0.004t/km2?190.188t/km2.?6?The topographical features have a significant effect on runoff.Runoff depth is moderately related to the length of the watershed?correlation coefficient is-0.462?,and has a low intensity related to the watershed area and channel ratio drop?correlation coefficients are-0.351,0.361?.The dynamic change Di of forest land,grassland and agricultural land in Caijiachuan small watershed from 2005 to2015 is 0.008,0.039,and-0.103.The summer NDVI is between 0.72?0.77,and the correlation between runoff depth and NDVI is not significant?P>0.05?.In the landscape index,patch density?PD?,average shape index?SHAPE?MN?and perimeter area fractal dimension?PAFRAC?are related to runoff depth and flood peak flow with low intensity.5?15 days early impact on rainfall has a low correlation with the runoff depth of the basin?the correlation coefficient is 0.197?0.263,P<0.01?,and has little effect on the runoff depth.Rainfall,I30,I60,and rainfall kinetic energy E are strongly correlated with runoff depth and flood peak discharge?P<0.01?.?7?P and I30were selected as the characteristic factors of rainfall,and an empirical model of the depth of rainfall runoff under different rain patterns was constructed,When P and I30are small,the runoff depth of different rain types is type?>type?>type?>type?.With the increase of rainfall,the runoff depth of type III rainfall is gradually higher than that of type I and type II rainfall.With the increase of I30,the difference in runoff depth change of type I,type II and type III rainfalls gradually decreased. |
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